Influence of Ground and Surface Water Relations on Algal
Influence of Ground and Surface Water
Relations on Algal Communities in the Rio
Grande Wild & Scenic River
EDWARDS AQUIFER RESEARCH & DATA CENTER
Stephen D. Porter and Glenn Longley
R1-11
October 2011
Cover photo: Rio Grande Wild & Scenic River segment (Rio Grande near
Boquillas, Mexico)
Influence of Ground and Surface Water
Relations on Algal Communities in the Rio
Grande Wild & Scenic River
By
Stephen D. Porter and Glenn Longley
Glenn Longley, PhD; Director
In cooperation with National Park Service, Chihuahuan Desert Network
Cooperative Agreement H1200050003
Kirsten Gallo, PhD; Inventory & Monitoring Network Coordinator
R1-11
October 2011
Table of Contents
List of Figures
Figure 1. Location of study area and TCEQ-USGS river gages in the Rio Grande basin. ............ 2
i
Figure 11. Abundance of three common benthic diatom species in the Rio Grande................... 15
Figure 13. Locations of major spring complexes along the Rio Grande Wild & Scenic River. ... 17
Figure 14. Water temperature variability in the Rio Grande during June 2010-May 2011. ......... 18
Figure 16. Specific conductance variability in the Rio Grande during June 2010-May 2011. ..... 19
Figure 17. Dissolved oxygen variability in the Rio Grande during June 2010-May 2011. .......... 19
Figure 19. Rates of metabolism in the Rio Grande above Presidio, June 2010-May 2011. ......... 21
Figure 20. Rates of metabolism in the Rio Grande below Presidio, June 2010-May 2011. ......... 22
Figure 21. Rates of metabolism in the Rio Grande at Castolon, June 2010-May 2011. ............... 22
Figure 22. Rates of metabolism in the Rio Grande at Rio Grande Village, June 2010-May 2011.
Figure 23. Rates of metabolism in the Rio Grande at Foster Ranch, June 2010-May 2011......... 23
List of Tables
ii
Introduction
Draining one of the largest river basins in the United States, the Rio Grande originates in southern Colorado and flows through much of New Mexico before entering Texas near El Paso
(Fig. 1). Large amounts of water diversion and extraction for agricultural and domestic uses in
New Mexico and the urban areas of El Paso, Texas and Ciudad Juarez, Mexico have significantly reduced streamflow in the “forgotten reach” of the Rio Grande, from Fort Quitman downstream to Presido, Texas (U.S. Army Corps of Engineers 2008; Wong et al. 2007). So much water is diverted from the Rio Grande before it reaches the outskirts of El Paso that the channel typically is dry between El Paso and Presidio (NPS 2004; 2006). During low-flow seasons, and particularly during extended periods of drought, the Rio Grande downstream from
Presido is functionally disconnected from its original sources of water (snow melt in the southern
Rocky Mountains and irrigation-return flows). Consequently, the quantity and quality of water down river from Presidio is controlled essentially by precipitation and water-use practices in the
Rio Conchos basin in the State of Chihuahua, Mexico. The Rio Conchos enters the Rio Grande near the towns of Presidio, Texas and Ojinaga, Mexico. The Rio Conchos drains 68,387 square kilometers (km
2
) of largely montane and semi-arid land (Fig. 1). About 39,592 hectares (396 km
2
) of agricultural land is irrigated in the basin. Agricultural return flows to the Rio Conchos and its tributaries influence water-quality conditions in the river. The Rio Conchos provides about 80 percent of the average annual flow to the Rio Grande below Presidio (Porter et al.
2009), and nearly 100 percent of flow during extended periods of drought.
Scenic and environmental resources along the Rio Grande below the Rio Conchos confluence have become threatened as a result of water-quality degradation. Water quality issues in the Rio
Grande have been reported by International Boundary and Water Commission (IBWC) 1997,
2004; Texas Natural Resource Conservation Commission (TNRCC) 1992, 1997, 2002; National
Park Service (NPS) 1995a, 1995b; Smith and Alexander 1985; Smith et al. 1982; Lambert et al.
2008, and others. Porter et al. (2009) reported increasing trends of nutrients (e.g. dissolved orthophosphorus) and salinity (specific conductance, chloride concentrations, etc.) in the Rio
Grande during the past 30 years that are commensurate with increases in agricultural activity in the region. There are no published reports describing the abundance and composition of algal communities or algal processes in the Rio Grande. The influence of nutrient enrichment from regional agricultural (and other human) activities on eutrophication and organic enrichment in the Rio Grande is not well understood.
The Chihuahuan Desert Inventory and Monitoring Network (CHDN) is one of 32 networks in the
NPS charged with developing long-term natural resource monitoring plans for their park units.
Big Bend National Park and the Rio Grande Wild and Scenic River are two of the seven national parks, monuments, recreation areas, and historic sites within the CHDN. As part of a cooperative agreement between the CHDN and the Edwards Aquifer Research and Data Center
(H1200050003), Texas State University (EARDC), algae samples were collected from the Rio
Grande following an extended period of low-flow conditions. Continuous monitoring data also were analyzed to evaluate inter-annual changes in water quality and to estimate rates of river metabolism.
1
Figure 1. Location of study area and IBWC and TCEQ-USGS river gages in the Rio Grande.
This report was prepared to describe algal relations with trophic and salinity conditions in the
Rio Grande and the natural and human factors that influence those conditions along a 450kilometer (km) river segment that includes the boundaries of Big Bend National Park and the Rio
Grande Wild and Scenic River. This is the second of two CHDN reports prepared under the cooperative agreement. The first report (Porter et al. 2009) provided a historical perspective of water quantity and quality conditions in the Rio Grande; this report is available at: http://gatodocs.its.txstate.edu/eardc/newsletter/CHDN_TrendsFinal.pdf or from http://eardc.txstate.edu.
Purpose and Scope
The purpose of this report is to describe phytoplankton (algal seston) and periphyton (benthic algae) assemblages and rates of metabolism (cf. primary productivity and community respiration) in the Rio Grande, from the Texas Commission on Environmental Quality (TCEQ)-
U.S. Geological Survey (USGS) continuous monitor at the IBWC river gage below the Rio
Conchos confluence near Presidio, Texas to the gage at Foster Ranch near Langtry, Texas.
Changes in algal-community conditions and processes along this 450-km section of the Rio
2
Grande are discussed in relation to gradients of salinity, organic enrichment, and trophic condition. The influence of ground-water discharges (riparian springs) along the boundaries of
Big Bend National Park and the Rio Grande Wild and Scenic River segment is discussed relative to changes in algal-species composition and improvements in water quality.
Acknowledgments
We would like to acknowledge the support of Kirsten Gallo, CHDN coordinator, and Jeffrey
Bennett, research coordinator at Big Bend National Park, for their interest in Rio Grande research and support in funding this study. Rick Bradbury, Big Bend National Park volunteer, provided assistance with field work, and Cary Carman, USGS, San Angelo, Texas assisted with obtaining water-quality data from the gaging stations. We thank Cliff Regensburg, IBWC, Del
Rio, Texas for providing access to the Rio Grande at Foster Ranch; Philip Dickerson, Texas
Parks & Wildlife, Midland, Texas for providing access to the Rio Grande at Maravillas Creek; and Jack Defoe, Lajitas Facilities and Water Plant Manager, for providing access to the Rio
Grande below Lajitas, Texas. We thank Christine Kolbe (TCEQ, Austin, Texas) and Elizabeth
Verdecchia (IBWC, El Paso, Texas) for collecting benthic algal samples in the Rio Grande Wild
& Scenic River segment during March 2009. Colleague reviews by Rene Barker, Kirsten Gallo,
Jeffrey Bennett, Elizabeth Verdecchia and Leslie Grijalva (IBWC, Texas Clean Rivers Program) improved the quality of this report, and their efforts are much appreciated.
Summary
Algae samples were collected from 12 locations along the Rio Grande during April 2011 following an extended period of low-flow conditions. Sampling was initiated in the Rio
Grande below Presidio, Texas, an IBWC river gaging station below the Rio Conchos confluence, and continued 450 km down river to the gaging station at Foster Ranch, near
Langtry, Texas. This segment of the Rio Grande forms the southern boundaries of Big
Bend National Park and the Rio Grande Wild & Scenic River, as well as Big Bend Ranch
State Park and Black Gap Wildlife Management Area. The abundance and species composition of algal communities was used to assess trophic conditions, organic enrichment, and longitudinal changes in water quality
Continuous-monitoring data (discharge, water temperature, pH, specific conductance, and dissolved oxygen (DO)) were compiled from five Rio Grande gaging stations and analyzed to evaluate inter-annual changes in water quality. Rates of river metabolism, indicators of algal productivity (Pmax) and community respiration (Rmax), were calculated based on changes of DO concentrations during the day (increasing rates of change) and during the night (decreasing rates of change).
The Rio Grande exhibits a gradient of salinity, nutrient, and organic enrichment extending from the Rio Conchos confluence down river to Foster Ranch. During lowflow conditions, the upper 110-km segment of the river (from Presidio to Castolon) was characterized by dense growths of benthic algae, large abundances of algal seston, and high rates of metabolism. Algal communities in the upper segment of the Rio Grande were dominated by brackish-water species, notably centric diatoms that are indicative of
3
eutrophic to hypereutrophic conditions.
The middle 155-km segment of the river, from Castolon to La Linda, was a transition zone where the abundance of algae and rates of metabolism were moderate and waterquality conditions were changing, as indicated by decreases of specific conductance and pH and increases in water temperature. Algal communities in the middle segment of the
Rio Grande were dominated by planktonic green, blue-green, and diatom taxa indicative of mesotrophic or eutrophic conditions, and halophilic, freshwater benthic diatoms.
The lower 180-km segment of the Rio Grande, from La Linda to Foster Ranch, was a zone of improving water quality resulting from considerable discharges of relatively higher-quality ground water from the Edwards-Trinity (Plateau) aquifer in the U.S. and, presumably, aquifers associated with the Del Carmen and Glen Rose Formations in
Mexico. The abundance of algae was average compared with streams and rivers in the
U.S., generally, and rates of metabolism were low. Benthic algal communities were dominated by filamentous red algae, benthic diatoms, and nitrogen-fixing algae indicative of mesotrophic conditions and relatively low concentrations of dissolved nitrogen.
The predominance of red algae in a large, turbid river such as the Rio Grande was unusual. These algae commonly occur in shaded, headwater streams, springs or seeps.
Sources of red algae in the Rio Grande most likely are from spring complexes along the lower segment of the river. Many common diatom species in the lower Rio Grande segment are found typically in springs, streams, and rivers that drain the limestone terrain of the Edwards Plateau province. The abundance of these species in the Wild and Scenic segment of the Rio Grande indicates improved water quality relative to conditions found upriver. Changes in the major-ion chemistry of groundwater along the lower segment, from a sodium-sulfate to a calcium-bicarbonate type, are consistent with changes in algal community structure.
Environmental and Hydrogeologic Setting
The study area is located in the Southern Deserts ecoregion (Omernik 1986), an arid landscape in the Trans-Pecos region of Texas. Annual precipitation in the region ranges from as little as 20 centimeters (cm) in the low-lying alluvial plains of the desert to more than 50 cm in the Chisos
Mountains that completely enclose central parts of Big Bend National Park, where elevations exceed 2,377 meters above mean sea level. Average daily evaporation generally is greater than precipitation, ranging from 170 to 260 cm per year (Bennett and Finch 2002). Major surfacewater resources in the region include the Rio Grande and intermittent tributary streams such as
Alamito, Terlingua, and Tornillo Creeks.
The Rio Conchos typically supplies the largest percentage of Rio Grande flows allocated by
Mexico in accordance with the 1944 Treaty between the U.S. and Mexico. The total annual flow of the Rio Conchos averaged 737,000 acre-feet through the 1980s, more than five times the flow of the Rio Grande measured upstream from the Rio Conchos confluence (Blackstun et al. 1998).
Three water-storage reservoirs (La Boquilla, La Colina, and Luis L. Leon) control flow in the
Rio Conchos, approximately 405, 393, and 183 river kilometers upstream, respectively, from the
Rio Grande confluence.
4
The Rio Grande forms the southern boundary of several parks, recreational and wildlifemanagement areas that, collectively, include 486,837 hectares (1,203,000 acres) in Texas. Big
Bend National Park, with more than 1,250 square miles (323,748 hectares), and the Rio Grande
Wild & Scenic River, a 196-mile (316 km) section of the Rio Grande from near the southern tip of Big Bend National Park (Mariscal Canyon) to the Terrell-Val Verde county line (about 32 km upstream from Langtry, Texas) are managed by the National Park Service. Big Bend Ranch
State Park, the largest state park in Texas with more than 300,000 acres (121,406 hectares) and
Black Gap Wildlife Management Area, with about 103,000 acres (41,683 hectares) are managed by the Texas Parks & Wildlife Department. Lands to the east of Black Gap Wildlife
Management Area are privately owned, and public access to the river is restricted to canoeists and rafters. Float trips down the Rio Grande are a popular recreational activity.
The Rio Grande was designated by Congress as a Wild and Scenic River in 1978. U.S. rivers with this designation are to be preserved in their free-flowing condition, and their associated ecosystems are to be actively protected in their natural state. The designation for the Rio Grande came in recognition of the ecological importance of the riparian and canyon habitats within the free-flowing section of the Rio Grande that borders Big Bend National Park. Downstream from the eastern boundary of Big Bend National Park, the Rio Grande enters a system of desert canyons 134 km long. Numerous springs and seeps along the Wild & Scenic River section contribute substantial discharges of ground water to the river, increasing the quantity of water transported by the river and improving water quality (Bennett and Cutillo 2007; Bennett et al.
2009; Porter et al. 2009; Brauch et al. 2010).
The geology of the region qualifies as some of the most complex in Texas (LBG-Guyton 2001).
After being inundated by ancient seas for more than a billion years, the region was subjected to tectonic uplift and crustal buckling that folded, faulted, and fractured the older marine strata.
Following sporadic episodes of volcanic activity, a system of bolsons (downfaulted basins between adjacent mountains) developed. Material eroded from the adjacent mountains was redistributed as basin-fill deposition within the bolsons. The net effects of this geologic activity serve to characterize the hydrogeologic framework of aquifers in the study area (condensed from
Porter et al. 2009).
The Tertiary Volcanics (Igneous) aquifer, a system of hydrologically-disconnected groundwater bodies, is characterized by fractures, crevices, and vesicular zones within upper parts of congealed lava flows that form most of this aquifer. The tops and bottoms of the lava layers generally are the most permeable because they typically comprise highly fractured rubble zones caused by the rapid cooling of molten lava. Because central parts of the lava flows cooled relatively slowly, they remain comparatively dense and, therefore, exhibit less permeability.
Water quality of the Volcanics aquifer is characterized by low concentrations of dissolved solids, indicating that atmospheric precipitation is transmitted relatively rapidly from the land surface to the aquifer. Recharge also occurs from rainfall that infiltrates the coarse-grained alluvial fans that skirt many of the mountain flanks. Because of the hydrologically-disconnected nature of the
Volcanics aquifer, the quantity and quality of groundwater that might be discharged to the Rio
Grande are poorly understood. No significant gains in Rio Grande discharge were reported from
Tertiary Volcanics aquifers in recent (2011) studies by NPS hydrologists (Bennett 2011); however, those studies were focused on locations in the Wild and Scenic River segment down
5
river from Big Bend National Park.
Carbonate (limestone and dolomitic) rocks in the region form water-bearing zones classified by the Texas Water Development Board as the “Upper Cretaceous Series” or what is known locally as the “Cretaceous Limestone” or “Santa Elena” aquifer (Maxwell et al. 1967). The Del Carmen and Glen Rose Formations probably are the sources of most spring discharges into the Rio
Grande from Mexico (Bennett and Cutillo 2007; Bennett et al. 2009). The stratigraphic equivalents of the Cretaceous strata that comprise the Edwards-Trinity aquifer system north of the Rio Grande also are major contributors of groundwater inflow to the river and adjacent springs. The southern extension of the Edwards-Trinity aquifer (Barker and Ardis 1996) underlies much of the Big Bend and Black Gap wildlife management areas, extending westward at least as far as Lajitas, Texas (Far West Texas Water Planning Group 2006). Regional potentiometric maps indicate that the Rio Grande in southern Terrell and Val Verde Counties is a discharge area for the Edwards-Trinity and equivalent aquifers on both sides of the international border (Barker and Ardis 1996; Boghici 2004; Bennett et al. 2009).
Steep groundwater gradients toward the river from both sides have sustained an outflow of groundwater through a complex of hot and cold-water springs (Boghici 2004; Brauch et al.
2010), in addition to diffuse, upward discharge to the river through permeable parts of the Rio
Grande river bed (i.e. the hyporheic zone). Results of recent (2011) hydrologic studies along the
Wild and Scenic River segment indicated an increase of 324 cubic feet per second (cfs) in the flow of the Rio Grande between Solis Landing and Langtry, Texas (Bennett 2011). Spring discharges along the Wild and Scenic River segment can account for as much as two-thirds of the flow in the Rio Grande near Langtry, and groundwater discharges to the river were estimated to range between 142,000 and 192,000 acre feet (1.75—2.37 * 10
11
Liters), annually (Bennett et al. 2009; Bennett 2011).
Ground water, including that which issues through the land surface as spring or seep flow, provides the most reliable water supply for the region (MacNish et al. 1996). Much of the aesthetic beauty and biological diversity within the parks and along downstream reaches of the
Rio Grande Wild and Scenic River depends on the temporal and spatial distribution of these springs and seeps. The permanency of groundwater discharge from springs and seeps along the
Rio Grande is dependant on the long-term stability of these aquifers.
Methods
Water Quality and Discharge
Water chemistry and streamflow data were retrieved from the records of five TCEQ Rio Grande continuous-monitoring stations managed by the USGS shown in figure 1
(http://waterdata.usgs.gov/tx/nwis/uv/?site_no= the 8-digit USGS station ID and http://www.tceq.texas.gov/waterquality/monitoring/swqm_realtime.html). The data were reviewed with quality control procedures (e.g. Wagner et al. 2006), and mean daily values were calculated for water temperature, pH, specific conductance, dissolved oxygen (DO), and discharge based on 96 measurements recorded at 15-minute intervals at the monitoring stations.
Specific conductance also was measured at the time of algal sampling using a Cole-Palmer
6
Model 1481-60 conductivity meter. Graphs of dissolved-oxygen concentrations during 24-hour periods (diel DO curves) were examined to determine periods of time when increases in DO concentrations (0800 through 1700) and decreases in DO concentrations (midnight through
0600) both were relatively linear.
River Metabolism
Increases of DO concentrations in rivers during daylight hours generally result from photosynthetic processes of algae and (or) other submerged aquatic vegetation, whereas decreases of DO during periods of darkness commonly are attributed to oxygen demand from respiratory process of river microorganisms (e.g. bacteria, fungi, and algae). The decreasing slope of the DO concentration gradient between midnight and 6 am (Fig. 2) is proportional to the rate of respiration in the river. High rates of respiration can result in low dissolved-oxygen concentrations leading to adverse water-quality conditions and fish kills. Rmax (the estimated maximum rate of respiration) was calculated for each 24-hour period by subtracting the DO concentration recorded at 0600 from the DO value recorded at the beginning of the period
(midnight), then dividing by 6 hours. Similarly, the increasing slope of the DO concentration gradient during the day is proportional to the rate of primary production in the river (Fig. 2).
Pmax (the estimated maximum net rate of productivity) was calculated for each 24-hour period by subtracting the DO concentration recorded at 0800 from the DO value recorded at 1700, then dividing by 9 hours. Pmax values less than zero and Rmax values greater than zero were reset to zero in the data set. Pmax and Rmax are reported in units of milligrams of oxygen per Liter per hour (mg O
2
/L/hr), which is equivalent to units of grams of oxygen per cubic meter per hour (g
O
2
/m
3
/hr).
Figure 2. Estimating rates of river metabolism.
Estimates of river metabolism do not account for physical exchanges of oxygen between the river and atmosphere (e.g. Wilcock 1982; Marzolf et al. 1994; Melching & Flores 1999) or other
7
factors (e.g. light, time of travel, etc.) considered in more formal studies of metabolism in flowing waters. Thus, these estimates of river metabolism are likely to be conservative.
Assuming that physical conditions that control reaeration processes among Rio Grande locations are similar (or with low variance), the Pmax-Rmax approach can be useful for assessing nutrient and organic enrichment as a system process in comparison with estimates of river condition (e.g. water chemistry; biological integrity measures). This approach has been used in studies of
Midwestern U.S. agricultural stream systems (Sorenson et al. 1999; Porter 2000), as well as
Western U.S. rangeland drainages such as the Yellowstone River basin (Peterson et al. 2000).
Algal Communities
Quantitative samples of algae suspended in the water column (phytoplankton or algal seston) and algae attached to submerged rocks in the river (periphyton or benthic algae) were collected from
12 Rio Grande locations during low-flow conditions of April 4-12, 2011 (Fig. 3).
Figure 3. Locations of Rio Grande study sites.
The study sites correspond with the approximate locations of Texas Commission on
Environmental Quality (TCEQ) monitoring stations listed in the appendix of this report.
8
Sample-collection procedures followed USGS National Water-Quality Assessment (NAWQA) protocols described by Porter et al. (1993) and Moulton et al. (2002). Algal seston samples were obtained by collecting 1.8 Liter subsurface water samples at the thalweg of the river channel and preserving samples with 47 milliliters (mL) of concentrated, buffered formalin. Benthic algal samples were collected from five randomly selected cobbles in riffle areas at each location using the foil template method described in the NAWQA protocols. Samples were preserved with sufficient concentrated buffered formalin to constitute a 4 percent concentration in the sample.
Samples were placed in boxes that excluded exposure to light and returned to the EARDC laboratory for processing. Foil templates, representing the surface area from which benthic algae were collected, were rinsed of sediment and allowed to dry at ambient temperature. The mass of the foil templates was measured with an analytical balance and mass-area relations were estimated by linear regression.
Benthic algal samples also were collected from three sites in the Rio Grande Lower Canyons area that were not accessible during the 2011 field study (Fig. 3, Sites 1-3). These samples were collected during March 2009 using the same protocol as described above (Christine Kolbe, Texas
Commission on Environmental Quality, written communication; Porter 2010).
Algal seston samples were concentrated by sedimentation and placed into 20 mL scintillation vials. The concentrated sample subsequently was diluted to result in a suitable number of algal cells within a microscope field of view for identification and enumeration. A 0.1 mL aliquot of the diluted algal suspension was pipetted into a Palmer-Maloney counting cell, and 600 (or more) algal cells were identified and enumerated from each sample at a magnification of 400X.
Quantitative diatom slides were prepared by pipetting an appropriate dilution of the sample on to glass cover slips, allowing the subsample to dry on the cover slip at ambient temperatures and incinerating the organic matter in the sample by heating on a hot plate at maximum temperature for one hour. Permanent diatom slides were prepared by mounting cover slips on microscope slides using Naphrax© media. Diatom valves were identified and enumerated (minimum of 600 valves) at 1000X magnification. Results from the Palmer-Maloney and diatom-slide counts were harmonized and reported in units of cells/mL.
Filamentous algae and clumps of algae in benthic-algal samples were cut into small pieces with scissors before homogenizing the sample with a small, plastic mixer and withdrawing aliquots for analyses of diatoms and soft algae (algae exclusive of diatoms). The subsample for soft algae was diluted to a concentration suitable for examination. Counts of “macroalgae” in the sample
(i.e. Cladophora ) were made with a Sedgwick-Rafter counting cell. All algal filaments in the counting cell were enumerated. Average volume of the cells (biovolume) was calculated by measuring the length and diameter of 10 or more representative cells. Total biovolume of
Cladophora at each site was calculated by multiplying the abundance of cells (cells per square centimeter, cells/cm 2 ) by the average biovolume of one cell in the sample (cubic micrometers,
m 3 ). Units of mm 3 /cm 2 were converted to cm 3 /m 2 (proportional to grams per square meter, g/m
2
) by dividing mm
3
/cm
2
by 10
8 . Counts of “microalgae” in the sample were made with a
Palmer-Maloney cell as described above. Sample aliquots for diatom analyses were cleaned of organic matter using the hydrogen peroxide and potassium dichromate method (van der Werff
1955), and permanent diatom slides were prepared and processed as described above. Results from the three enumeration methods were harmonized and reported in units of cells/cm
2
.
9
Algal nomenclature is in accordance with Wehr & Sheath (2003). Diatom species and varieties are based on taxonomic keys by Krammer & Lange-Bertalot (2000-08); blue-green algal taxonomy is consistent with Komárek & Anagnostidis (2008a; 2008b); and Kristiansen & Preisig
(2007) was consulted for synurophyte taxonomy. Digital images were made of all species encountered during the study and are available upon request.
Algal Indicators of Nutrients, Salinity, and Organic
Enrichment
Algal abundance (standing crop) and species composition in the Rio Grande are influenced by nutrient enrichment and salinity, the effects of which are most apparent below the Rio Conchos confluence, extending down river for nearly 200 km to near the southern tip of Big Bend
National Park near Solis Landing. Large populations of small (4
m – 10
m diameter) centric diatoms were predominant in benthic and seston algal samples down river from the Rio Conchos confluence (Figs. 4 and 5). Cyclotella atomus, Stephanocyclus (formerly Cyclotella ) meneghiniana , and two species of Stephanodiscus ( S. hantzschii and S. minutulus ) accounted for between 35% and 68% of benthic algal cells and between 47% and 76% of algal seston cells at
Rio Grande locations upstream from the river gage at Castolon (Fig. 3). These centric diatom species typically are found in eutrophic to hypereutrophic waters with high concentrations of organic (total) nitrogen and phosphorus (Porter 2007; Potapova & Charles 2007). Cyclotella atomus and Stephanocyclus meneghiniana , the most common centric diatoms, also are brackishwater species (van Dam et al. 1994; Porter 2007). The abundance of these species, in addition to benthic brackish-water diatoms such as Craticula halophila, Fragilaria fasiculata, Navicula exilis, Navicula gregaria, Navicula veneta, Nitzschia constricta , and Surirella brebissonii
(Tables 1 and 2), is consistent with specific conductance values measured at the time of sample collection (Fig. 6).
Figure 4. Abundance of benthic algae in the Rio Grande.
10
Figure 5. Abundance of indicator taxa in algal seston from the Rio Grande.
Figure 6. Specific conductance values in the Rio Grande during early April 2011.
Total cell counts of algal seston (Fig. 7) were consistent with eutrophic conditions in the Rio
Grande from Presidio to Boquillas Canyon, a distance of about 220 km. Although cell counts of benthic, filamentous-green algae ( Cladophora glomerata ) were relatively small (8,000—17,000 cells per square centimeter (cells/cm 2 )) in proportion to diatoms, the biovolume (biomass) of this species was large throughout a 100-Km segment of the Rio Grande below the Rio Conchos confluence (Fig. 8). Cladophora biovolume ranged from 18 cubic centimeters per square meter
(cm 3 /m 2 ) below Presidio to 35 cm 3 /m 2 at Colorado Canyon; biovolume was 26 cm 3 /m 2 below
Lajitas, near the western boundary of Big Bend National Park. These values are proportional to
11
units of ash-free dry mass (grams per square meter (g/m
2
)) and are indicative of eutrophic waterquality conditions (cf. Biggs 1996).
Dense growths of Cladophora are a source of organic enrichment in the Rio Grande that could adversely affect dissolved-oxygen conditions following periods of detachment (e.g. scouring of algae from rocks; hydrologic disturbance), concentration of algal biomass at low-gradient locations down river, and subsequent senescence and decomposition of the biomass. Although
Cladophora “mats” can provide beneficial habitat for macroinvertebrates and larval fish, the material collected from the Rio Grande was found to contain considerable amounts of sediment deposited (or filtered) from the water column, likely reducing microhabitat quality and influencing oxidation-reduction properties near the base of the algal mats.
Figure 7. Abundance of algal seston in the Rio Grande.
Figure 8. Biovolume of Cladophora glomerata in the Rio Grande.
12
The relative abundance of other green algal taxa in the Rio Grande varied from about 12 percent to 21 percent of seston samples at locations upstream from the Castolon gage, increasing to a range of 43 to 66 percent of the community downstream from this location (Figs. 5 and 9; Table
1). Green algae were dominated by phytoplankton species typically found in lentic waters (e.g. lakes, wetlands, etc.) such as Desmodesmus abundans, D. quadricauda, Scenedesmus acuminatus , and Ankistrodesmus falcatus . These are eutrophic species with high optima for total nitrogen and phosphorus concentrations (Potapova 2005). The number of species and abundance of these taxa were largest in algal samples collected between Johnson Ranch and Boquillas
Canyon, suggesting changes in water quality and (possibly) larger percentages of lentic habitat
(e.g. pools) along this segment of the river. Increases in the abundance of these taxa with decreases in water conductivity along down-river segments of the Rio Grande may indicate that they are intolerant of the relatively elevated salinity conditions found upriver (e.g. specific conductance > 2,000
S/cm). Similar changes in benthic diatom indicators of salinity were found along the middle and lower segments of the river.
Figure 9. Relative abundance of algal seston in the Rio Grande.
Benthic diatom communities in the Rio Grande were dominated by Achnanthidium minutissimum , a cosmopolitan species typically found attached to Cladophora (i.e. epiphytic), with a broad range of tolerance to salinity and nutrient enrichment (Porter 2007). Many diatom species found in the Rio Grande are motile taxa (e.g. Navicula, Nitzschia, and Surirella ) that can move through sediments to position their cells near the top layers of deposited sediment where light does not limit metabolic processes. These taxa generally are associated with fine sediments in depositional microhabitats or on rocks covered with sediment deposited from the water column. The abundance of these taxa has been proposed as a “siltation index” for streams and rivers (e.g. Barbour et al. 1999). By contrast, the centric diatoms discussed previously typically are found suspended in lakes, reservoirs, or large rivers (planktonic) or in loose association with other algal assemblages (metaphytic). The source of centric diatoms in the benthic algal
13
communities most likely is from cells accumulated from the seston by deposition or filtration through the dense mats of Cladophora .
Figure 10. Relative abundance of benthic algae in the Rio Grande.
The proportion of benthic (pennate) diatoms relative to centric diatoms in the Rio Grande increased with distance from the Rio Conchos confluence (Fig. 10); moreover, the number and abundance of tolerant diatom species decreased and the number and abundance of sensitive or facultative diatom species increased (Table 2). For example, the abundance of Brachysira vitrea,
Denticula kützingii , Fragilaria nanana , and Nitzschia microcephala increased downstream from the Castolon gage location, about 129 km from the Rio Conchos confluence (Fig. 11). Those species are considerably less tolerant of salinity and nutrient enrichment (van Dam et al. 1994;
Porter 2007) than the predominant benthic diatom species at upstream locations. The abundance of nitrogen-fixing diatoms (e.g. Denticula ) in the Rio Grande downstream from Solis Landing may indicate low concentrations of dissolved nitrogen at the time samples were collected.
Nitrogen-fixing algae are capable of converting atmospheric nitrogen (N
2
) to ammonia nitrogen as a source of nutrition, thereby out-competing other algal species in streams with low ambient nitrogen concentrations (Geitler 1977; Wehr & Sheath 2003). The abundance of Calothrix , a nitrogen-fixing, blue-green algal taxon, also increased downstream from Solis Landing (Table 2).
The proportion of cyanobacteria (blue-green algae) generally was larger in seston than benthic algal samples (Figs. 9 and 10). Glaucospira sp. (formerly Spirulina laxa ) was found at all Rio
Grande locations and was the only blue-green algal species found in the upper segment of the study area. Glaucospira is a eutrophic taxon (Porter 2007) with high optima for organic nitrogen
(Potapova 2005), but is not a nuisance species. The abundance and number of blue-green algal species increased downstream from the Castolon gage (Fig. 5; Tables 1 and 2). These taxa (e.g.
Calothrix, Chroococcus , and Schizothrix , plus several other genera in the family
Oscillatoriaceae) are common species that form biofilms on rocks and other submerged surfaces
14
in flowing waters (spring discharges, streams, and rivers). The occurrence of these taxa at locations upstream from the Castolon gage may be limited by the relatively excessive sediment deposition noted previously.
Figure 11. Abundance of three common benthic diatom species in the Rio Grande.
The abundance of filamentous red algae in the Rio Grande increased downstream from Solis
Landing and was largest at Foster Ranch where cell counts of Audouinella hermanii and
Batrachospermum spp. accounted for over 60 percent of the benthic algal community (Fig. 10;
Table 2). Although it was not possible to sample algae from the Rio Grande at locations between
Maravillas Creek and Foster Ranch during 2011 because of logistical issues, benthic samples collected during March 2009 contained algal species similar to those found at the Lower
Canyons sites in 2011 (Fig. 3; Table 3). Batrachospermum dominated benthic algal biovolume
(about 1 cm
3
/m
2
) in the Rio Grande below Bullis Fold (Fig. 3, site 3) during 2009 (Porter 2010).
Batrachospermum is found typically in spring discharges and cool, shaded, headwater streams with good water quality (Wehr & Sheath 2003), and predominance of this taxon in a large, turbid river is unusual. Qualitative algal samples collected from spring discharges near Bullis Fold during 2009 also were dominated by Batrachospermum . The riparian springs along this segment of the Rio Grande are the likely source of red algae (and perhaps other algal species) to the river.
Maintenance of large populations of Batrachospermum in the Lower Canyons segment of the
Rio Grande may be attributable to improved water quality and other factors associated with spring discharges from the Edwards-Trinity and Santa Elena Formation aquifers.
15
Rio Grande Hydrology, Water Quality, and Metabolism
Algal sampling was preceded by six months of low-flow conditions in the Rio Grande. Although discharge conditions during summer 2010 were comparable with historical data, flow at the
Castolon and Rio Grande Village gages (Fig. 3) decreased appreciably during fall 2010, remained relatively stable during the winter, and then continued to decrease during spring 2011
(Fig. 12). Low discharge was a result of severe drought conditions in the region; no rainfall had been reported at Big Bend National Park since September 2010 (Rick Bradbury, National Park
Service volunteer, personal communication). Differences of discharge values recorded at Rio
Grande Village compared with those at Castolon provide an estimate of the contribution of groundwater discharges along this segment of the river. During relatively stable low-flow conditions (Fig. 12; November 2010-February 2011); daily mean discharge at Rio Grande
Village exceeded values at Castolon by about 44 cubic feet per second (cfs). By the end of June
2011, the difference in discharge between gaging stations had increased to about 55 cfs. Figure
12 shows that the inflow of groundwater along this 100 km river segment accounted for a large percentage of the flow in the Rio Grande during low-flow conditions at this location. These estimates of flow augmentation are consistent with historical accounts of gains ranging from 30 cfs to 60 cfs in this segment of the river (Texas Board of Water Engineers 1960; Porter et al.
2009). Locations of selected spring complexes along the middle and lower segments of the Rio
Grande study area are shown in Figure 13. Influence of ground-water inflow from the Edwards-
Trinity aquifer is shown by green shading of the landscape in Figure 13
Figure 12. Rio Grande flows at Castolon and Rio Grande Village during 2010-11.
.
Gains in river flow associated with groundwater inflows are not uniform along the Rio Grande
(Slade et al. 2002; Porter et al. 2009). About 75 to 80 percent of the annual mean discharge in the Rio Grande below Presidio originates in the Rio Conchos basin, whereas the remaining 20 percent comes from the Rio Grande above Presidio. Historical (1918-58) estimated gains in
16
annual mean discharge from below the Rio Conchos confluence to Johnson Ranch, a distance of about 155 km, were about 60 cfs. Because of the influence of several major tributary basins (e.g.
Alamito Creek; Terlingua Creek) along this segment of the Rio Grande, we assume that these gains are associated primarily with tributary surface-water inflows and that the potential influence from groundwater discharge is minimal. Seepage runs conducted by the U.S.
Geological Survey and National Park Service during winter 2006 indicated gains of 52 cfs near
Rio Grande Village and 134 cfs near the Maravillas Creek confluence (Outlaw Flats Spring
Complex; Brauch et al. 2010). Historical estimated gains in the Lower Canyons segment of the
Rio Grande ranged from 100 cfs at Stillwell Crossing to 220 cfs at Reagan Canyon (refer to
Porter et al. 2009), indicating considerable groundwater augmentation to Rio Grande flows through the Lower Canyons. These gains are relatively consistent with those of more recent data, ranging from gains of 159 cfs (2001) to 251 cfs (2005), reported by Brauch et al. (2010).
Rio Grande Village
Castolon
Map from Bennett (2007)
Figure 13. Locations of major spring complexes along the Rio Grande Wild & Scenic River.
Seasonal and longitudinal variability in water-quality conditions were evaluated at five Rio
Grande continuous-monitoring sites: Above Presidio; Below Presidio; Castolon; Rio Grande
Village; and Foster Ranch (Figures 1 and 3). Daily mean values (based on 96 measurements recorded at 15-minute intervals) for water temperature, pH, specific conductance, and DO are shown in Figures 14-17, beginning June 1, 2010 and ending May 31, 2011. Mean water temperature was similar among sites during a period of seasonally-average flow (Figs. 12 and 14,
June-October 2010); however, the daily mean temperature generally increased with distance downstream from the Rio Conchos confluence during the low-flow period (November 2010-May
2011). Water temperature at the Rio Grande Village site was consistently higher than at other sites, presumably influenced by groundwater discharges from hot spring complexes located upstream from Rio Grande Village. Daily mean values for pH and DO generally were lower at
Rio Grande Village than at other sites, particularly during the low-flow period (Figs. 15 and 17).
Differences among these constituents in the river at Rio Grande Village may indicate changes in
17
water-quality conditions that are associated with local ground-water inflows. The sources and conditions under which these groundwater discharges are produced are not well understood.
Figure 14. Water temperature variability in the Rio Grande during June 2010-May 2011.
Figure 15. pH variability in the Rio Grande during June 2010-May 2011.
18
Figure 16. Specific conductance variability in the Rio Grande during June 2010-May 2011.
Figure 17. Dissolved oxygen variability in the Rio Grande during June 2010-May 2011.
19
Daily mean values for specific conductance generally were lower during seasonally-average flow conditions than during periods of reduced flow (Figure 16). Specific conductance in the Rio
Grande generally decreased, longitudinally, from Presidio to Foster Ranch. Short-term decreases of specific conductance values in the Rio Grande below Presidio (Fig. 16; February and April
2011) may have resulted from reservoir releases in the Rio Conchos basin that would tend to dilute the high-conductance values observed in the Rio Grande above Presidio. Specific conductance in the upper segment of the Rio Grande (from Presidio to Castolon) generally remained high during the low-flow period, whereas mean values decreased at Rio Grande
Village and Foster Ranch during periods of lower flow (Fig. 16). Conductance values measured during early April (Figure 6) were about 30 percent lower than those reported by Bennett (2011) in late February 2011, consistent with continuous-monitoring data for Rio Grande Village and
Foster Ranch (Figure 16). Decreases in conductance values during this time period are likely associated with dilution as the relative contribution of groundwater discharge to flows in Rio
Grande increases. Relatively low conductance values in the Rio Grande at Foster Ranch are the result of dilution by water from the Edwards-Trinity, Del Carmen, and Glen Rose Formation aquifers issuing from spring discharges along the Lower Canyons segment.
Daily mean DO concentrations frequently were lower at Rio Grande Village and Rio Grande above Presidio than at other locations (Fig. 17). Dissolved-oxygen concentrations in the Rio
Grande below Presidio (and Castolon during winter) exceeded concentrations in the Rio Grande above Presidio, reflecting better water-quality conditions in the Rio Conchos than in the Rio
Grande upstream from the junction of the two rivers. Another factor that could result in relatively higher DO concentrations below Presidio and at Castolon is the abundance and productivity of algal seston and (or) benthic algae in the 110-km segment between those locations.
Estimates of river metabolism were calculated for each day at the five continuous-monitoring stations based on increasing rates of DO concentrations during the day (0800-1700 hr; Pmax) and decreasing rates of DO during the night (midnight-0600 hr; Rmax) (refer to Methods section and Figure 2). Pmax values (positive) are proportional to net rates of primary productivity; increases of DO concentrations primarily are a result of photosynthetic processes of algae that produce oxygen, whereas Rmax (negative values) is an estimate of respiration processes
(primarily by bacteria, algae, and (or) fungi) that consume oxygen from the water, resulting in decreases of DO concentrations during the night. Gross primary productivity can be estimated by subtracting Rmax from Pmax (cf. Odum 1956).
River metabolism values increased with the abundance of algae in the Rio Grande. Pmax values varied from zero to 1.73 g O
2
/m
3
/hr; median values were relatively high in the Rio Grande below
Presidio (0.35 g O
2
/m
3
/hr) and at Castolon (0.29 g O
2
/m
3
/hr), and lowest in the Rio Grande at
Foster Ranch (0.06 g O
2
/m 3 /hr). Rmax values varied from zero to – 1.08 g O
2
/m values were largest (ca. – 0.12 g O
2
/m
3
3 /hr; median
/hr) in the Rio Grande below Presidio, Castolon and Rio
Grande Village, whereas Rmax was relatively low (– 0.02 g O
2
/m
3
/hr) at Foster Ranch (Fig. 18).
Rates of metabolism were larger in the Rio Grande below Presidio than above Presidio, reflecting the influence of the Rio Conchos on water-quality conditions below the confluence with the Rio Grande and (or) wastewater discharges from the Ojinaga-Presidio urban areas.
Median Pmax values declined between Presidio and Rio Grande Village (Fig. 18), whereas median Rmax values were similar along this segment of the Rio Grande. Larger-than-expected
20
rates of oxygen demand, relative to net productivity, at Rio Grande Village could be responsible for the lower-than-expected DO concentrations at this site. River metabolism values for the Rio
Grande at Foster Ranch could be regarded as “reference conditions,” where the influence of nutrient and organic enrichment is minimal in comparison with other Rio Grande locations.
Figure 18. Comparison of median Pmax and Rmax values in the Rio Grande.
Figure 19. Rates of metabolism in the Rio Grande above Presidio, June 2010-May 2011.
21
Figure 20. Rates of metabolism in the Rio Grande below Presidio, June 2010-May 2011.
Figure 21. Rates of metabolism in the Rio Grande at Castolon, June 2010-May 2011.
22
Figure 22. Rates of metabolism in the Rio Grande at Rio Grande Village, June 2010-May 2011.
Figure 23. Rates of metabolism in the Rio Grande at Foster Ranch, June 2010-May 2011.
23
Rates of metabolism generally were larger during low-flow conditions (winter and spring) than during the seasonally higher flow conditions in the Rio Grande during June-September 2010
(Figs. 19-23). Metabolism rates began to increase during low-flow conditions in late fall 2010, with largest values occurring during spring (e.g Fig. 20) or winter (Fig. 21). Large Pmax and
Rmax pulses in the Rio Grande below Presidio during March-April 2011 (Fig. 20) may have been preceded by changes in water quality following reservoir releases in the Rio Conchos basin
(as suggested by reductions in specific conductance at this site; Figure 16). A large pulse in river metabolism occurred in the Rio Grande at Castolon during late December through January
(Fig. 21). Although considerable metabolic activity was occurring upriver (below Presidio) during this period, Pmax and Rmax values at Castolon were among the largest observed and cannot be explained solely on the basis of water-quality conditions upriver. Reasons for this pulse in metabolism are not well understood; however, it is possible that human activities more proximate to Castolon (e.g. fertilization of the Lajitas golf course during the fall and (or) increases in the number of visitors over the Christmas holidays) may have resulted in nutrient enrichment of the river that stimulated the growth of algae between Lajitas and Castolon
(approximately 37 km).
Rates of metabolism were relatively low in the Rio Grande at Rio Grande Village and Foster
Ranch during the winter but began to increase with temperature and day length during spring
2011 (Figs. 22 and 23). Pmax and Rmax values at Rio Grande Village continued to increase through May 2011, when values exceeded those at sites near Presidio and Castolon. Despite increases of respiration rates at Rio Grande Village during March-May 2011, daily mean DO concentrations continued to increase during the period (Fig. 17), whereas DO concentrations near
Presidio and Castolon declined. Metabolism values in the Rio Grande at Foster Ranch were relatively low throughout the year and did not exhibit seasonal periodicity (Fig. 23).
Surface and Ground Water Relations with Algae and Water
Quality
The Rio Grande exhibits a gradient of salinity, nutrient, and organic enrichment from the Rio
Conchos confluence to Foster Ranch (Segment 2306 of the Rio Grande; Texas Commission on
Environmental Quality). During low-flow conditions, the upper 110-km segment of the river
(from below the Rio Conchos confluence to Castolon) is characterized by dense growths of benthic algae, large abundances of algal seston, and high rates of metabolism. Algal communities in the upper segment of the Rio Grande were dominated by brackish-water species, notably centric diatoms that are indicative of eutrophic to hypereutrophic conditions. Organic enrichment of the upper segment of the river is demonstrated by high algal productivity that is stimulated by nutrient enrichment from non-point sources (primarily irrigated agriculture) in the
Rio Conchos and Rio Grande basins. Pmax and Rmax values were similar to those observed in
Midwestern agricultural streams (Sorenson et al. 1999). Low-flow conditions enhance water clarity (e.g. less sediment transport) and low river stage provides additional benthic habitat where the availability of light is favorable for the growth of algae. Transport of autochthonous sources of carbon (the algae) to downstream locations following hydrologic disturbances (e.g. pulses in river flow) may adversely affect dissolved- oxygen concentrations following microbial respiration and decomposition of transported algal biomass.
24
The middle 155-km segment of the river, from Castolon to La Linda, is a transition zone where the abundance of algae and rates of metabolism were moderate and water-quality conditions were changing, as indicated by decreases in specific conductance and pH and increases in water temperature. Algal communities in the middle segment of the river were dominated by planktonic green, blue-green, and diatom taxa indicative of mesotrophic or eutrophic conditions, and halophilic (but intolerant of high salinity) freshwater diatoms. The relative abundance of planktonic algae may reflect slower river velocity and greater incidence of pools and other lentic habitats in the river segment, particularly during low-flow conditions. Those habitat conditions are common in low-gradient reaches of streams and rivers. Figure 24 shows the elevation of the locations where algal samples were collected. Relatively little change in elevation can be observed along the 52-km reach of the Rio Grande between Rio Grande Village and La Linda.
Figure 24. Elevations of sites where algal samples were collected in the Rio Grande.
Groundwater discharges in the middle segment of the river contribute a significant proportion of
Rio Grande flows, particularly during periods of low flow. Discharges from the Gambusia Hot
Springs Complex (Fig. 13), located between Solis and Rio Grande Village, are characterized by higher temperature (ca. 38 o
C) and lower specific conductance (ca. 1,500
S/cm) than values observed at the Castolon gage (Bennett 2007; Brauch et al. 2010). Relatively little is known about concentrations of other water-quality constituents in these hot-spring discharges; however, the higher-than-expected water temperature and lower-than-expected pH values found in the Rio
Grande at Rio Grande Village are indicative of changes in water quality in the middle segment.
It is possible that the persistently low DO concentrations observed at the Rio Grande Village gage are (at least partially) associated with higher water temperature and (or) oxygen-demanding processes associated with the chemical quality of groundwater inflows. According to Bennett et al. (2009), the source of groundwater is the Edwards-Trinity (Plateau) Aquifer; however, the processes resulting in hot-springs discharges are poorly understood.
25
The lower 180-km segment of the Rio Grande, from La Linda to Foster Ranch is a zone of improving water quality, relative to conditions in the upper Rio Grande segment, resulting from considerable discharges of ground water along the segment. The abundance of algae was typical for U.S. streams and rivers (unpublished USGS-NAWQA data), and rates of metabolism were low. Benthic algal communities were dominated by filamentous red algae, benthic diatoms, and nitrogen-fixing algae indicative of mesotrophic conditions and relatively low concentrations of dissolved nitrogen. Specific conductance values measured during the study decreased from
1,830
S/cm at La Linda to 1,030
S/cm at Foster Ranch. According to Brauch et al. (2010), specific conductance in groundwater discharges varied from 850
S/cm in the Outlaw Flats
Spring Complex (Glen Rose Limestone) near Maravillas Creek to 476
S/cm in the Madison
Fold Spring Complex (Glen Rose Limestone), downstream from the location of the three sites where benthic algae were collected during 2009 (refer to Figures 3 and 13).
The predominance of red algae ( Batrachospermum and Audouinella ) in a large, turbid river such as the Rio Grande is unusual. These taxa commonly occur in small, shaded, headwater streams and springs. Batrachospermum also was abundant in spring outflows near the locations where algae were collected during 2009 (Porter 2010). The most-likely sources of the colonization and growth of Batrachospermum in the Rio Grande are the riparian spring complexes along the lower segment of the river. Many common diatom taxa in the lower Rio Grande segment (e.g.
Achnanthidium, Brachysira, Denticula, and Encyonemopsis ) are common residents of limestone springs and streams in the Edwards Plateau province (S. Porter, unpublished data), and the abundance of these taxa in the Rio Grande suggests water-quality conditions similar to those found in that geochemical setting. This hypothesis is consistent with reported changes in majorion chemistry, from a sodium sulfate (Na-SO
4
) type to a calcium bicarbonate (Ca-HCO
3
) type of groundwater between the Las Palmas and Madison Fold spring complexes (Figure 13; Brauch et al. 2010). Longitudinal differences in the degree of ground- and surface-water interactions largely account for the gradient of salinity, nutrients, and organic enrichment in the Rio Grande.
The abundance and species composition of algal communities in the river respond along those gradients.
Conclusion
Algal communities in the Rio Grande are influenced by the interaction of impaired surface-water quality with groundwater discharges of relatively higher-quality waters that issue from riparian springs along the Wild and Scenic River segment. Algal-species indicators of salinity and eutrophication were predominant along a 220 Km segment of the Rio Grande down river from the Rio Conchos confluence. Dense growths of filamentous algae and high rates of river metabolism along this segment provide evidence of organic enrichment and degraded water quality. During low-flow conditions of the Rio Grande during (and prior to) this investigation, spring discharges along the river diluted concentrations of dissolved salts and nutrients in the surface water, thereby improving water quality. Increases of Rio Grande discharge down river from Solis Landing in Big Bend National Park are consistent with increases in the volume of groundwater discharges along the segment, resulting in a gradient of improving water-quality conditions. Rates of river metabolism and the abundance of algal-species indicators of salinity and nutrient enrichment decreased with increases in groundwater contributions to the river.
Algal species found commonly in small streams and springs in the Edwards Plateau province
26
became a major component of benthic-algal communities in the Lower Canyons area of the Rio
Grande. We conclude that groundwater discharges to the Rio Grande improve water quality and the condition of biological communities in the river during low-flow conditions. Maintenance of the quality and quantity of spring discharges to the Rio Grande could be critical for ensuring ecological integrity in the Wild and Scenic River segment.
The biological and water-quality condition of the Rio Grande could be better understood by employing a “multiple lines of evidence” approach to assessment and monitoring such as advocated by the USGS National Water Quality Assessment (NAWQA) and USEPA
Environmental Monitoring and Assessment Program (EMAP). Information from biological communities (algae, macroinvertebrates, and fish), aquatic habitat conditions, and water chemistry are integrated into an overall assessment of the quality of the resource. Although algal data for the Rio Grande are limited to this report, unpublished results from macroinvertebrate and fishery investigations conducted by governmental and academic institutions could be compiled, analyzed with multivariate and multi-metric procedures, and synthesized into an overall assessment of river quality. We encourage the National Park Service and other institutions with interest in enhancing the quality of the Rio Grande to include assessments of algal communities in conjunction with water-quality and biological monitoring because of the direct relation between algal-community structure and water chemistry and the importance of algae as a food resource for macroinvertebrates and fish (e.g. Rio Grande silvery minnow).
Value could be added to existing continuous-monitoring data by calculating and monitoring rates of river metabolism (e.g. Pmax and Rmax described in this report) as an indicator of organic enrichment and microbial processes.
Additional biological research could be considered along the Lower Canyons area of the Rio
Grande Wild and Scenic River, where water-quality conditions appear to be improving as a result of riparian spring discharges. Ideally, this research would include a “multiple lines of evidence” approach during low-flow conditions when the river is accessible by rafting. Results from recent hydrologic investigations could be used to estimate the percentage of Rio Grande flow gains attributable to groundwater inflow from spring complexes. Site selection for biological, water quality, and habitat assessments could be based on a gradient of groundwater contributions to river flow rather than a series of fixed-interval sampling locations. Results from this research could improve understanding of ecological conditions and processes in the Rio
Grande such as: (1) does the quality of macroinvertebrate and fish communities improve with increases in the percentage of groundwater in the Rio Grande, as demonstrated for algal communities; (2) do discharges from spring complexes contribute critical riparian habitats for enhancing biological diversity in the Rio Grande; and (3) how do biological communities in the
Rio Grande respond to antecedent hydrologic conditions, for example, extended periods of drought or flooding.
27
Literature Cited
Barbour, M.T., J. Gerritsen, B.D. Snyder, and J.B. Stribling. 1999. Rapid bioassessment protocols for use in wadeable streams and rivers: Periphyton, benthic macroinvertebrates and fish, 2 nd
edition. EPA 841-B-99-002. U.S. Environmental Protection Agency, Washington,
D.C.
Barker, R.A. and A.F. Ardis. 1996. Hydrogeologic framework of the Edwards-Trinity aquifer system, west-central Texas: U.S. Geological Survey Professional Paper 1421-B, 61 p.
Bennett, J.B., and S.T. Finch. 2002. Concepts of ground-water recharge in the Trans-Pecos
Region, Texas: Geological Society of America South-Central Section 36 th Annual Meeting
(11-12 April 2002), Sul Ross State University, Alpine, Texas, Abstract.
Bennett, J.B. 2007. Regional groundwater resources important to Big Bend
National Park and the Rio Grande Wild and Scenic River: National Park
Service Memo to Files, Science and Resource Management, Big Bend National Park, 5 p.
Bennett, J, and P. Cutillo. 2007. Geologic and geochemical characterization of thermal springs along the Rio Grande in Big Bend National Park and the Rio Grande Wild and Scenic River:
Geological Society of America Annual Meeting (28-31 October 2007), Denver, Colorado,
Geological Society of America Abstracts with Programs , 39 (6):510.
Bennett, J., K. Urbanczyk, B. Brauch, B. Schwartz, and W.C.P. Shanks. 2009. The influence of springs on discharge and river water chemistry in the Lower Canyons, Rio Grande Wild and
Scenic River: Geological Society of America Annual Meeting (18-21 October 2009),
Portland, Oregon, Geological Society of America Abstracts with Programs , 41 (7):246.
Bennett, J. 2011. Trip report for gain loss survey of ground water and spring contributions to flow of the Rio Grande, February and March of 2011: U.S. Department of the Interior,
National Park Service, written communication to William E. Wellman, Superintendent, Big
Bend National Park, 15 p.
Biggs, B.J.F. 1996. Patterns in benthic algae of streams, in Stevenson, R.J., M.L. Bothwell, and
R.L. Lowe, eds., Algal Ecology. Freshwater benthic ecosystems: Academic Press, San
Diego, California, p. 31-56.
Blackstun, D., L. Woosley, and M. Flora. 1998. Water-resources issues in the Rio Grande—Rio
Conchos to Amistad Reservoir Subarea: U.S. Department of the Interior, U.S.—Mexico
Border Field Coordinating Committee, Fact Sheet 3.
Boghici, R. 2004. Hydrogeology of Edwards-Trinity Aquifer of Texas and Coahuila in the
Border Region (Chapter 4), in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds.,
Aquifers of the Edwards Plateau: Texas Water Development Board Report No. 360, p. 91-
114.
28
Brauch, B., J. Bennett, K. Urbanczyk, and B. Schwartz. 2010. Spring studies in the Lower
Canyons of the Rio Grande, Rio Grande Wild and Scenic River, Texas: Geological Society of America Annual Meeting (11-13 April 2010), Geological Society of America Abstracts with Programs , 42 (2):76.
Far West Texas Water Planning Group. 2006. Far West Texas Water Plan: unpublished document prepared for the Texas Water Development Board, 573 p.
Geitler, L. 1977. Zur entwicklungsgeschichte der epithemiaceen Epithemia, Rhopalodia , und
Denticula und ihre vermutlich symbiotischen spharoidkorper: Plant Systematics and
Evolution 128 :259-275.
IBWC (International Boundary and Water Commission). 1997. Second phase of the binational study regarding the presence of toxic substances in the Rio Grande/Río Bravo and its tributaries along the boundar portion between the United States and Mexico. Final Report. El
Paso, Texas.
IBWC (International Boundary and Water Commission). 2004. Third phase of the binational study regarding the presence of toxic substances in the upper portion of the Rio Grande/Río
Bravo between the United States and Mexico. Final report, June 2004. El Paso, Texas.
Komárek, J., and K. Anagnostidis. 2008a. Süsswasserflora von Mitteleuropa, Volume 19/1:
Cyanoprokaryota, Part 1: Chroococcales: Spektrum Akademischer Verlag GmbH,
Heidelberg, Germany, 548 p.
Komárek, J., and K. Anagnostidis. 2008b. Süsswasserflora von Mitteleuropa, Volume 19/1:
Cyanoprokaryota, Part 2: Oscillatoriales: Spektrum Akademischer Verlag GmbH,
Heidelberg, Germany, 759 p.
Kristiansen, J., and Preisig, H.R. 2007. Süsswasserflora von Mitteleuropa, Volume 1/2:
Chrysophyte and Haptophyte Algae, Part 2: Synurophyceae: Spektrum Akademischer Verlag
GmbH, Heidelberg, Germany, 252 p.
Krammer, K., and H. Lange-Bertalot. 2000. Süsswasserflora von Mitteleuropa, Volume 2/5:
Bacillariophyceae, Part 5: English and French translation of the keys: Spektrum
Akademischer Verlag GmbH, Heidelberg, Germany, 311 p.
Krammer, K., and H. Lange-Bertalot. 2004. Süsswasserflora von Mitteleuropa, Volume 2/4:
Bacillariophyceae, Part 4: Achnanthaceae, Kritische Ergänzungen zu
Achnanthes s. l.,
Navicula s. str., Gomphonema : Elsevier GmbH, Munich, Germany, 468 p.
Krammer, K., and H. Lange-Bertalot. 2007a. Süsswasserflora von Mitteleuropa, Volume 2/1:
Bacillariophyceae, Part 1: Naviculaceae: Elsevier GmbH, Munich, Germany, 876 p.
Krammer, K., and H. Lange-Bertalot. 2007b. Süsswasserflora von Mitteleuropa, Volume 2/2:
Bacillariophyceae, Part 2: Bacillariaceae, Epithemiaceae, Surirellaceae: Elsevier GmbH,
Munich, Germany, 611 p.
29
Krammer, K., and H. Lange-Bertalot. 2008. Süsswasserflora von Mitteleuropa, Volume 2/3:
Bacillariophyceae, Part 3: Centrales, Fragilariaceae, Eunotiaceae: Spektrum Akademischer
Verlag GmbH, Heidelberg, Germany, 599 p.
Lambert, R.B., C.M. Kolbe, and W. Belzer. 2008. Quality of water and sediment in streams affected by historical mining and quality of mine tailings, in the Rio Grande/Río Bravo basin,
Big Bend area of the United States and Mexico, 2002. U.S. Geological Survey Scientific
Investigations Report 2008-5032.
LBG-Guyton. 2001. Igneous aquifer system of Brewster, Jeff Davis, and Presidio Counties,
Texas: Draft report prepared for the Far West Texas Regional Water Planning Group by
LBG-Guyton Associates, with Water Prospecting and Resource Consulting, LLc and Sul
Ross University, 42 p.
Mace, R.E., W.F. Mullican III, and E.S. Angle (eds.). 2001. Aquifers of west Texas. Texas
Water Development Board Report 356, 272 p., Austin, Texas.
MacNish, R.D., L.J. Lacher, C.M. Fleming, and M.D. Flora. 1996. Water resource management plan, Big Bend National Park, Texas: Department of Hydrology and Water Resources,
University of Arizona, Tucson, 394 p.
Marzolf, E.R., P.J. Mulholland, and A.D. Steinman. 1994. Improvements to the diurnal upstream-downstream dissolved oxygen change technique for determining whole-stream metabolism in small streams: Canadian Journal of Fisheries and Aquatic Sciences 51 :1591-
1599.
Maxwell, R.A., J.T. Lonsdale, R.T. Hazzard, and J.A. Wilson. 1967. Geology of Big Bend
National Park, Brewster County, Texas: The University of Texas at Austin, Bureau of
Economic Geology, Publication 6711, 320 p.
Melching, C.S., and H.E. Flores. 1999. Reaeration equations derived from U.S. Geological
Survey database: Journal of Environmental Engineering 125 (5):407-414.
Moulton, S.R., II, J.G. Kennen, R.M. Goldstein, and J.A. Hambrook. 2002. Revised protocols for sampling algal, invertebrate, and fish communities as part of the National Water-Quality
Assessment Program: U.S. Geological Survey Open-File Report 02-150, 75 p.
NPS (National Park Service). 1995. Baseline water quality data inventory and analysis. Big Bend
National Park. Technical Report NPS/NRWRD/NRTR-95/51. U.S. Department of the
Interior, National Park Service, Washington, D.C.
NPS (National Park Service). 2004. Rio Grande Wild and Scenic River. Final general management plan/Environmental impact statement, Brewster and Terrell Counties, Texas:
U.S. Department of the Interior, National Park Service, Denver, Colorado, 166 p.
NPS (National Park Service). 2006. Big Bend. The flow of the Rio Grande. U.S. Department of the Interior, National Park Service, Washington, D.C., http://www.nps.gov/bibe/naturescience/waterflow.htm.
30
Odum, H.T. 1956. Primary production in flowing waters: Limnology and Oceanography
1 (2):102-117.
Omernik, J.M. 1986. Ecoregions of the United States: Annals of the Association of American
Geographers 77 (1):118-125, Map Supplement.
Peterson, D.A., S.D. Porter, and S.M. Kinsey. 2001. Chemical and biological indicators of nutrient enrichment in the Yellowstone River basin, Montana and Wyoming, August 2000:
Study design and preliminary results: U.S. Geological Survey Water-Resources Report 01-
4238.
Porter, S.D., T.F. Cuffney, M.R. Meador, and M.E. Gurtz. 1993. Methods for collecting algal samples as part of the National Water-Quality Assessment Program: U.S. Geological Survey
Open-File Report 93-409, 39 p.
Porter, S.D. 2000. Upper Midwest river systems—algal and nutrient conditions in streams and rivers in the upper Midwest region during seasonal low-flow conditions. in EPA-822-B-00-
002, Nutrient Criteria Technical Guidance Manual, Rivers and Streams. U.S Environmental
Protection Agency, Washington, D.C.
Porter, S.D. 2007. Algal Attributes: An autecological classification of algal taxa collected by the
National Water-Quality Assessment Program: U.S. Geological Survey Data Series 329, 18 p.
+ digital data base.
Porter, S.D. 2010. Rio Grande Algae: Preliminary benthic algal results from three locations in the Lower Canyons of the Wild & Scenic River (March 2009): Edwards Aquifer Research &
Data Center, Interim report to Jeff Bennett, Big Bend National Park, July 1, 2010, 4 p.
Porter, S.D., R.A. Barker, R.M. Slade, Jr., and G. Longley. 2009. Historical perspective of surface water and groundwater resources in the Chihuahuan Desert Network, National Park
Service: Edwards Aquifer Research & Data Center Report R1-09, Texas State University,
San Marcos, Texas, 112 p.
Potapova, M. 2005. Relationships of soft-bodied algae to water-quality and habitat characteristics in U.S. rivers: Analysis of NAWQA national data set: Academy of Natural
Sciences of Philadelphia, Patrick Center Report 05-08, http://diatom.acnatsci.org/autecology
Potapova, M., and D.F. Charles. 2007. Diatom metrics for monitoring eutrophication in rivers of the United States: Ecological Indicators 7 :48-70.
Round, F.E. 1984. The ecology of algae: Cambridge University Press, U.K., 653 p.
Slade, R.M., Jr., J.T. Bentley, and D. Michaud. 2002. Results of streamflow gain-loss studies in
Texas, with emphasis on gains from and losses to major and minor aquifers. U.S. Geological
Survey Open-File Report 02-068, Austin, Texas.
31
Smith, R.A., R.M. Hirsch, and J.R. Slack. 1982. A study of trends in total phosphorus measurements at NASQAN stations. U.S. Geological Survey Water-Supply Paper 2190,
Washington, D.C.
Smith, R.A. and R.B. Alexander. 1985. Trends in concentrations of dissolved solids, suspended sediments, phosphorus, and inorganic nitrogen at U.S. Geological Survey National Stream
Quality Accounting Stations. In National Water Summary 1984, U.S. Geological Survey
Water-Supply Paper 2275, Washington, D.C.
Sorenson, S.K., S.D. Porter, K.K.B. Akers, M.A. Harris, S.J. Kalkhoff, K.E. Lee, L.R. Roberts, and P.J. Terrio. 1999. Water quality and habitat conditions in upper Midwest streams relative to riparian vegetation and soil characteristics, August 1997: Study design, methods, and data:
U.S. Geological Survey Open-File Report 99-202.
Texas Board of Water Engineers. 1960. Channel gain and loss investigations, Texas streams,
1918-1958: Texas Board of Water Engineers Bulletin 5807-D.
TNRCC (Texas Natural Resource Conservation Commission). 1992. Binational study regarding the presence of toxic substances in the Rio Grande/Río Bravo and its tributaries along the boundary portion between the United States and Mexico. Texas Natural Resource
Conservation Commission. Austin, Texas.
TNRCC (Texas Natural Resource Conservation Commission). 1997. Second phase of the binational study regarding the presence of toxic substances in the Rio Grande/Río Bravo and its tributaries along the boundary portion between the United States and Mexico. Texas
Natural Resource Conservation Commission. Austin, Texas.
TNRCC (Texas Natural Resource Conservation Commission). 2002. Phase III of the binational study regarding the presence of toxic substances in the upper portion of the Rio Grande/Río
Bravo. Texas Natural Resource Conservation Commission. Austin, Texas.
U.S. Army Corps of Engineers. 2008. Forgotten reach of the Rio Grande, Fort Quitman to
Presidio, Texas. Section 729. U.S. Army Corps of Engineers, Albuquerque, New Mexico. van Dam, H., A. Mertens, and J. Sinkeldam. 1994. A coded checklist and ecological indicator values of freshwater diatoms from the Netherlands: Netherlands Journal of Aquatic Ecology
28 :117-133.
Van der Werff, A. 1955. A new method of concentrating and cleaning diatoms and other organisms: Int. Ver. Theor. Angew. Limnol. Verh.: 12 :276-277.
Van Metre, P.C., B.J. Mahler, and E. Callender. 1997. Water-quality trends in the Rio
Grande/Río Bravo basin using sediment cores from reservoirs. U.S. Geological Survey Fact
Sheet 221-96.
32
Wagner, R.J., R.W. Boulger, Jr., C.J. Oblinger, and B.A. Smith. 2006. Guidelines and standard procedures for continuous water-quality monitors—Station operation, record computation, and data reporting: U.S. Geological Survey Techniques and Methods 1-D3.
(http://pubs.water.usgs.gov/tm1d3)
Wehr, J.D., and R.G. Sheath. 2003. Freshwater algae of North America: Academic Press, San
Diego, California, 918 p.
Wong, C.M., C.E. Williams, J. Pittock, U. Collier, and P. Schelle. 2007. World’s top 10 rivers at risk. World Wildlife Federation, Gland, Switzerland.
Wilcock, R.J. 1982. Simple predictive equations for calculating stream reaeration rate coefficients: New Zealand Journal of Science 25 :53-56.
33
Table 1. Abundance of algae in seston samples from the Rio Grande, April 2011, in cells per milliliter.
34
Table 1. (cont’d) Abundance of algae in seston samples from the Rio Grande, April 2011, in cells per milliliter.
35
Table 1. (cont’d) Abundance of algae in seston samples from the Rio Grande, April 2011, in cells per milliliter.
36
Table 2. Abundance of algae in benthic samples from the Rio Grande, April 2011, in cells per square centimeter.
37
Table 2. (cont’d) Abundance of algae in benthic samples from the Rio Grande, April 2011, in cells per square centimeter.
38
Table 2. (cont’d) Abundance of algae in benthic samples from the Rio Grande, April 2011, in cells per square centimeter.
39
Table 3. Abundance of algae in benthic samples collected from the Rio Grande, March 2009, in cells per square centimeter.
40
Table 3. (cont’d) Abundance of algae in benthic samples from the Rio Grande, March 2009, in cells per square centimeter.
41
Appendix. Correspondence of algal-study sites with approximate locations of Texas Commission on Environmental Quality (TCEQ) Rio Grande monitoring locations.
Algae Site Name
-- Rio Grande above Presidio, TX
TCEQ Site ID TCEQ Gage ID
13230 CAMS 757
A
B
D
E
--
F
G
H
I
J
L
1
2
Rio Grande below Presidio, TX
Rio Grande near Colorado Canyon
Rio Grande at Lajitas Resort
Rio Grande at Santa Elena Canyon
Rio Grande at Castolon, TX
Rio Grande at Johnson Ranch
Rio Grande at Solis Landing
Rio Grande at Rio Grande Village
Rio Grande above Boquillas Canyon
Rio Grande at La Linda, MX
Rio Grande at Foster Ranch
Rio Grande at Taylor’s Farm
Rio Grande above Silber Canyon
13229
16862
18441
13228
18482
13227
18534
16730
16276
13225
13223
20623
20625
CAMS 758
CAMS 720
CAMS 721
CAMS 759
42
Plate 1. Common centric diatoms in the Rio Grande.
43
Plate 2. Common green algal species in the Rio Grande.
44
Plate 3. Common pennate diatoms in the Rio Grande.
45
Plate 4. Common pennate diatoms and blue-green algae in the Rio Grande.
46
Plate 5. Common red algae in the Rio Grande
47
Back cover photo: Rio Grande upper segment (above Lajitas, Texas)
48
49
