Janet Garcia
REU 2014
The Effects of Sediment Disturbance on Macroinvertebrate Communities at the
Confluence of the Rio Grande and Rio Salado
ABSTRACT
Unlike the sandy composition of the Rio Grande’s streambed, one of its
tributaries, the Rio Salado, has a mixture of varying sizes of sediments. These then get
deposited into the Rio Grande, creating a confluence area of the two streams. In addition
to sand, there is gravel and cobbles mixed in. The confluence creates a heterogeneous
habitat for benthic macroinvertebrate communities. This naturally attracts high diversity
of macroinvertebrates because there are various niches to be filled. However, as sediment
is disturbed by flow, so are the macroinvertebrate communities. My hypothesis is that
there will be less macroinvertebrate diversity in areas of high sediment disturbance, since
these are not stable areas for food sources or shelter. To test this, I had three project sites
that were along the confluence. One was upper-confluence, the second at the confluence,
and the last was downstream. At these sites I collected four sediment samples from each.
These were dried, painted, and placed in baskets to be positioned back in the river at the
same location from which they were taken. After 24 hours the baskets will be collected.
We will assess sediment disturbance by measuring the proportion of painted sediment
that left the basket and the new (un-painted) sediments that entered the basket. However,
due to river water levels, the sediment disturbance experiment was performed artificially.
In addition to observing the sediment disturbance, a 1 cm core sample of the sediment
was taken to analyze the diversity of aquatic macroinvertebrate communities and another
set to measure periphyton. My results supported that the site with the overall highest taxa
diversity was the downstream site and there was no significant difference between the
sites when looking at the biotic index. There appeared to be a trend in the chlorophyll a
amounts which suggested that the downstream site had the most and there was no
apparent trend for sediment lost. We conclude that there is a correlation between the type
of test site and the taxa diversity richness.
INTRODUCTION
Tributary confluences in river ecosystems are important locations since they
provide high biological diversity known as “hot spots” (Rice et al. 2006). The main
contribution to the importance of confluences is that the recruitment of water and
sediment from tributaries can create alterations that increase the physical heterogeneity in
the main channel (Rice et al. 2006). As substrate heterogeneity increases, so does taxa
diversity (Rice et al. 2001). Thus various niches are created, ready to be exploited by
diverse species. Tributaries deposit sediments into the main channel with high energy.
The types of sediments that are deposited are usually large like rocks, cobbles, and
stones. Depending on particle surface roughness, this will affect periphyton growth and
detritus retention, which in turn influences food availability for insects and higher trophic
levels (Rice et al. 2001). Also, high water discharge can cause coarse sediment
disturbance and agitate periphyton and invertebrate communities (Schwendel et al. 2011).
Periphyton is a mixture of algae, cyanobacteria, microbes and detritus, which is usually
found on underwater surfaces. In general there are two types of macroinvertebrates
regarding movement or lack thereof, sessile and motile. Sediment disturbance creates a
zonation pattern where sessile organisms are excluded from dynamic sediments and
motile organisms are favored (Hinchey et al. 2006). These organisms are affected when
they experience sediment disturbance directly and when the nearby periphyton is also
disturbed.
The Rio Grande and Rio Salado confluence provides us with a tributary-main
stream system. Different from the Rio Grande, the Rio Salado has a heterogeneous mix
of sediments. Not only does it have sand but it also has gravel, rocks, cobbles, and stones.
The goal of this study is to explore how the heterogeneous habitat of the confluence
affects benthic macroinvertebrate communities. Macroinvertebrates live in water for all
or most of their life, making them great bioindicators of lotic ecosystem health. These
critters also provide lotic ecosystems with nutrient cycling of important ecological
materials as well as add to the overall river biodiversity (Mac Nally et al. 2011). They are
an intermediate link between primary producers, primary consumers and higher predators
(Malmqvist 2002).
MATERIALS AND METHODS
My project location is along the confluence of the Rio Salado and Rio Grande
within the boundaries of the Sevilleta National Wildlife Refuge which is situated at
approximately 34°16'21.27"N 106°51'29.44"W. Within this research area, there are three
testing locations: upper edge of the confluence, middle of the confluence, and
downstream. At each of the testing locations, three different samples were taken. There
was a sediment, macroinvertebrate, and periphyton sample taken. To first determine
where we would take sediment samples from, velocity measurements were taken to find
areas of similar velocity and thus similar sediment disturbance would also be seen here.
Four replicate sediment samples were taken from each of the three sites about 5m apart to
total 12 sediment samples. Next, we took macroinvertebrate samples from each of the 12
sediment sites. Four 1cm sediment cores were taken from each site using a 60ml syringe.
Right after collection, ethanol was poured on the samples to preserve them. These
samples were then taken to the laboratory where they were placed on a 250m sieve and
sifted through with water to find and pull out the macroinvertebrates. Once all the
macroinvertebrates were separated from the sediment, they were sorted and identified and
placed in site-specific vials. Lastly, periphyton samples were also taken from the same 12
sites as the macroinvertebrates. Three 2ml cores were taken from the sites using the same
syringe. Samples were placed in 50ml centrifuge tubes and these were wrapped in
aluminum foil right after collection as a means of stopping photosynthesis. The
periphyton samples were placed in the freezer to preserve them and later taken to the
University of New Mexico. There, ethanol was poured into the tube samples to extract
the chlorophyll and were ran through a spectrophotometer to get a reading on chlorophyll
a amounts.
The sediment samples were taken to the laboratory where various procedures
followed. They were dried under the hot New Mexican sun, weighed, and placed through
a sieve column that shook for three minutes. The sieve column separated the sediment
sample into grain sizes. The various grain sizes were weighed to get proportions of the
sediment composition. After, each sample was spray-painted bright orange to provide a
strong contrast with the natural river sediments. Originally, the sediment disturbance
experiment was to be executed by placing the painted sediments in baskets back into the
river from the same location where they were collected from. This was to be done to
assess sediment movement. After 24 hours we wanted to see what proportion of the
painted sediment had flowed out of the basket. However, towards the end of the month of
June 2014 pre-monsoon rains came and elevated the height of the river. This made it too
deep and thus unsafe to enter and place the baskets. In spite of this, the experiment was
conducted artificially at the Sevilleta Long Term Ecological Research Field Station. Here
I nestled the painted sediment baskets in a rectangular container that contained a sand
matrix. A garden hose was attached to the larger container and was used to fill it up and
act as our “river.” The hose knob was marked off in three areas as a way to standardize
three velocities ranging from low to high as a means to mimic what we had found in the
river. The low velocity was used for all upper-confluence samples, medium velocity for
confluence samples, and high velocity for downstream samples. Before the water was
turned on, a before picture was taken of the painted sediment. Each sample was placed in
the matrix one by one and the water hose ran for 30 minutes at the respective site
velocity. After the trial was done, a picture was taken again of the sample to measure
proportion of the sediment that left the sample. ImageJ, a computer program was used to
do a qualitative image analysis on the photos to measure sediment disturbance. This was
later presented as the percentage of sediment lost at each of the sites.
RESULTS
Macroinvertebrates
The overall abundance of macroinvertebrates at each of the research sites seems
to indicate that there is a trend. It appears that abundance is increasing on a positive slope
from the upper-confluence site to the downstream site (Fig. 1). However, once an
ANOVA test was run on this data, it showed that there was no significant difference
between the sites affecting the macroinvertebrate abundance. Taxa diversity was assessed
using Shannon-Weiner’s diversity index which accounts for species richness and
evenness. Again, there appeared to be a trend alluding that the downstream had the most
diversity (Fig. 2). An ANOVA test was run on this data, which gave us a p-value of
0.021, which is less than 0.5 meaning that there was a significant difference. We are 95%
sure that the downstream site in fact is more diverse than the other two sites. In total,
eight different taxa were found and they were classified into six families, one phylum,
and one subclass. Families: Chironomidae, Leptohyphidae, Ceratopogonidae,
Simuliidae, Hydropsychidae, Baetidae; Phylum: Nematomorpha; Subclass: Oligochaeta.
The sites were predominantly the most abundant with members of the family
Chironomidae and the least with Baetidae (Fig. 3). The biotic index is a scale that
measures the quality of a habitat based on the types of organisms that live in it. The most
tolerable are given a score of 10 and the most sensitive are given a score of 0. When
evaluating the biotic score of the sites, we found that there was no significant difference
between the sites and the biotic score their organisms were given (Fig. 4).
Periphyton
To measure periphyton, we looked at the quantity of chlorophyll a in each of the
samples. After the periphyton samples were run through the spectrophotometer, the
results were just as I had expected. Although no statistical tests were run on this data, we
see a clear trend (Fig. 5). It appears that the downstream site has the highest amount of
chlorophyll a with 23.3 mg/m2 and the upper-confluence site only had 8.7 mg/m2.
Sediment disturbance
There appears to be no actual trend between the sites. The upper-confluence site
had a mean 32.5% of sediment lost and the confluence had 31.8% (Fig. 6). This closeness
in percentages leads us to conclude that there might have been various factors affecting
the data collection while the experiment was running and the experiment design. It was
expected to observe that the upper site would have the most disturbance and the
downstream site to have the least.
DISCUSSION
Macroinvertebrates
We first see that there is no significant difference between the research sites when
assessing overall macroinvertebrate abundance. This suggests that the location where
they are found along the confluence does not have an affect on their amount. However
their quantities could be due to other factors such as sediment coarseness, periphyton
availability, and sediment disturbance. The taxa diversity among the sites was
significantly greater at the downstream locations. Downstream of the confluence is where
a “…combination of coarse sediment inputs (gravel and cobble size)…” is found
(Svendsen et al. 2009). Luttenton and Baisden (2006) mention that their findings on
greater taxa diversity being on larger substrata is consistent with that of Patrick’s (1967,
1976) results on his studies. It makes sense that more diversity can be supported on larger
sediments because they don’t experience as much disturbance as sand would and
therefore provide a stable habitat. The upper-confluence site is predominantly made up of
sand and thus only the most tolerable organisms can survive there. In this case, it was the
chironomids. Luttenton and Baisden (2006) state that sediment abrasion [Horner et al.
1990] is a common mechanism by which periphyton communities are disturbed. This
means that sand acts as “sand paper” when it is disturbed by water flow and periphyton
gets scraped off resulting in no stable food sources.
Periphyton
The trend suggests that the downstream test location was the one with the most
chlorophyll a amounts. This came as no surprise since we just discussed how in the
downstream area there are larger/heavier sediment particles and therefore don’t tumble
with water flow, preventing most periphyton from being scraped off. Death (1996)
mentions in his study that periphyton biomass increased with decreasing disturbance
frequency. This supports my results and also tells us that disturbance decreases from the
upper-confluence site to the downstream site.
Sediment disturbance
This experiment gave us an unclear trend. We can’t really extract any logistic data
from the graph; however, there are possible factors to having an unclear trend. This
experiment design had never been done before, so naturally there were various mishaps.
Although the hose was positioned in such a way to evenly water the catching container,
holes were still created in the sand matrix, potentially disrupting sediment movement and
water flow. Also, the ImageJ software that was used to qualitatively assess the photos for
sediment lost could have been a factor. In this program, the individual uses their best
judgment on recreating the contrast of the photo on the software, leaving a lot of room for
error. However, what I would’ve expected to see was that the sandy upper-confluence
site to have the most disturbance and the downstream site to have the least since it has
mainly large and heavy sediments.
In the future, it would be interesting to gather these same samples again, however,
post-monsoon season to compare them to this pre-monsoon data. With rainfall, comes
more water to fill and overflow rivers and these in turn have a higher water discharge.
With higher energy in the rivers, this can move more sediment and especially those that
are larger and heavier. We would expect to observe disturbance to the downstream areas
that were not seen during the pre-monsoon season. Following monsoon season, we could
also look at how the macroinvertebrate community recovers after a greater disturbance.
There are various mechanisms that allow these biota to survive or recover from natural
disturbances (Palmer 1992).
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Appendix
Fig. 1 Macroinvertebrate abundance by site
Fig. 2 Macroinvertebrate diversity by site
Fig. 3 Taxa distribution among the sites
Fig. 4 Biotic index of the macroinvertebrates by site
Fig. 5 Chlorophyll a amounts at each of the sites
Fig. 6 Percent of sediment lost at each site