SEASONAL HYDROLOGICAL EFFECTS ON SECONDARY PRODUCTION OF
HYDROPSYCHIDAE IN REACH 6, UPPER MISSISSIPPI RIVER
Introduction
Seasonal changes in a river are a way of life for organisms and can affect them in
many different ways. Thus, their life cycle develops a pattern with these changes. Larger
rivers have a smoother seasonal flood pattern than smaller rivers due to their larger
watershed areas that can absorb more water into their system than smaller rivers and
streams. Floodplain areas have different production rates due to the seasonal fluctuation
rates and can affect production at greater rates than other areas (Smock et al. 1992). This
is due to nutrients that are brought into use when floodplains are immersed with water.
Temperate habitats generally result in specific growth rates of organisms and the
production of populations that vary along with season and follow the temperature cycle
(Banse and Mosher 1980). Because of this, invertebrate production often differs between
habitats. Community composition and secondary production will differ among sites
within a large river in response to both physical characteristics of each habitat and the
types of organic matter present (Thorp and Delong 1994).
Increased velocity, substrate instability, and scouring of substrate surface are
commonly associated with floods and can harm benthic organisms. However, species
composition and relative abundance of many benthic communities remain relatively
constant through time and over multiple floods (Rempel et al. 1999). Modes of action of
the hydrologic disturbances on stream communities may work by erosion when habitats
are scoured by floods, or by desiccation when aquatic habitats decrease gradually and
organisms become stranded above the water surface (Feminella and Resh 1990). Flow
refugia reduce the loss of organisms during flooding and allow benthic populations to
thrive (Rempel et al. 1999).
Caddisfly larvae contribute significantly to zoobenthic biomass and the
processing of organic resources. Larvae, which are particularly responsive to instream
environmental gradients, are restricted to the permanently wetted stream perimeter.
Benthic habitat is reduced through alterations of natural-flow conditions during spring
and summer, but enhanced during fall and winter (Hauer and Stanford 1991). Georgian
and Thorp (1992) noted that large larvae feed selectively on high-quality prey food items
such as diatoms and drifting invertebrates. When there is an increase in density of
drifting invertebrates (Benke and Wallace 1997), increased food quality results (Benke
and Wallace 1997).
Hydropsychidae caddiflies, the focus of this study, are ideal organisms for life
history and production analyses. They generally have five distinct instars, which take 2-3
mon. to go from egg to adult emergence (Benke et al. 1984). Cohort production is the
accumulation of flows over that population-specific interval (Benke 1993). When an
aquatic animal population grows as a synchronous cohort in nature, mortality and
individual growth are easily incorporated into an estimate of secondary production using
a cohort technique (Benke et al. 1998).
Floodplain river instream primary production is a significant contribution to
secondary productivity (Thorp and Delong 1994). Secondary production is the most
comprehensive measure of success for a population because it is a composite of several
other components of success: density, biomass, individual growth rate, reproduction,
survivorship, and development time. Since production is directly related to consumption,
it represents a quantification of a population’s resource use in a given time interval
(Benke and Jacobi 1994). Thus, secondary production is a response variable that deals
with the formation of heterotrophic biomass through time. These measurements allow
one to determine the absolute and relative amounts of food resources consumed (Benke
1993). Also, production analysis provides a common link between populations and
ecosystems as it represents a measure of a population’s function at the community and
ecosystem level.
Populations, functional feeding groups, and consumers as a whole are dynamic
components of ecosystems and production is the ultimate dynamic variable. Biomass is
simply how much living tissue mass is present at a given time and production is a flow of
mass or energy, as observed by Benke (1993). The objective of this study was to
determine if river hydrology fluctuations affect secondary production of Hydropsychidae
in the Upper Mississippi River.
Methods
Samples were collected from two sites on the Upper Mississippi River in Reach 6,
near Winona, MN. Rock samples were taken from wing dams at each site. Site 1 was
located on the right bank near first light below dam 5A at river mile 727.9. Site 2 was
also on the right bank at river mile 726.3. Both sites are representative of the upper end
of a navigation reach, with hydrological patterns expected to follow near-historical
patterns (Sparks 1995).
Rock samples were collected approximately every 3 wks. Sample dates were
June 30, July 21, August 11, August 29, September 20, and October 11, 2000 (Figure 1).
A minimum of ten samples were collected on each sample date by removing five rocks
from each site on every sample date. Measurements of pH, depth, turbidity, and current
velocity were recorded for each sample to monitor any changes over the collecting period
that might affect our sampling of caddisfly larvae.
Upon collection, rock samples were placed in plastic bags with labels and filled
with 70% ethanol. Rocks were scrubbed in the laboratory initially by hand, then with a
brush to remove all invertebrates from the substrate. This material was rinsed through a
0.1-mm sieve and stored in a container with 70% ethanol. Rocks were measured by
length, height, and width for determination of surface area. Caddisfly larvae were sorted,
counted, and placed in vials, with ethanol, according to size (large or small) and species.
Larvae were split into two groups, small and large, because previous studies in the Large
River Studies Center had indicated two cohorts of Hydropsychidae exist in the Upper
Mississippi River.
Larval biomass was determined by using length-mass relationships, using
equations that predict mass as a power function of a linear dimension, particularly body
length and head width: M=aLb, where M is larvae mass (mg), L is any linear dimension
(mm), and a and b are constants as outlined by Benke et al. (1999).
Cohort production was calculated for the intervals between sampling dates: June
30-July 21, July 21-August 11, August 11-August29, August 29-September 20, and
September 20-October 11 (Appendix 1). Production was calculated using the increment
summation method using the equation P=N W, where P is production over a time
interval, N is the mean density of larvae, and W is the difference between final and initial
mean larval weight (Butler 1982).
Results
Discharge fluctuated throughout the study period (Figure 1). The spring 2000
flood did not match the historical average. Hydrological patterns were not at typical
times of the year. Mid-June through July showed a discharge higher than normal. After
this high discharge period, rates of production and density were, for the most part, at their
highest rates. Water temperatures typically fluctuated between 200 and 260C during the
summer months and went down to 100C for the last sample date in October.
Three species of Hydropsychidae were identified: Cheumatopsyche sp.,
Hydropsyche orris, and Potamyia flava. While two cohorts had been identified, overlap
during August-September made it difficult to separate them. Therefore, the focus became
changes in production of the size groups for each species. As shown in Figure 2, average
densities of large Cheumatopsyche sp. larvae were greatest at the beginning of August
and lowest during rest of months. Densities of large H. orris were lowest in June, then
increased July through August before decreasing in September-October. Large P. flava
densities were lowest in June and increased to about the same level for the rest of the
year. Figure 3 shows that the densities of small Cheumatopsyche sp. larvae were highest
during July-August and lowest in June. Small H. orris larvae densities were highest in
July and about the same during the other months except in September, when no small
larvae were found. Small P. flava larvae, which had lowest overall densities, followed a
pattern similar to Cheumatopsyche sp. larvae.
Production patterns were the greatest for both large and small H. orris larvae
during July-August interval (Figure 4). H. orris production at first increased until it
reached its climax then dropped off dramatically by the middle of August. No production
estimates were found during the August-September interval. Large P. flava larvae
production was highest in July-August and lowest throughout rest of the study period
(Figure 5). Small larvae production generally stayed low except for a slight increase in
September-October. Production for small Cheumatopsyche sp. larvae were lowest in
June, while reaching a maximum in July-August, before dropping in late August and
increasing in September (Figure 6). Large Cheumatopsyche sp. larvae production
increased from June and reached the highest rate in August, before dropping in
September and then again increasing by September-October.
Discussion
Hydropsychidae are known to utilize microhabitats with large stable substrate and
high water flow velocity. Current velocity and flow patterns would be expected to have a
direct influence on these filter feeders. Highest abundances are known to have occurred
on the upstream and top position of substrate, where the highest flows occur (Georgian
and Thorp 1992). Larval abundance can vary with depth, often as a function of oxygen
concentrations as affected substrate type and interstitial flow variability (Smock et al.
1992). Hydrological events can reduce the density of caddisfly larvae occupying
substrate in a particular area. Increases and decreases of densities can be partially
attributed to this; however, densities of small caddisfly larvae increased July through
August due to the hatching of new eggs. Decreases after August are probably due to the
size change when small larvae were then categorized as large, thus accounting for the
dramatic increase in numbers of large larvae in August. Decrease of large larvae
densities at the end of August is due to emergence. This typically takes place over late
July, August, and September (Beckett 1982).
Secondary production of larvae increases July through August, generally for all
three species and both sizes. It is even more pronounced in large larvae production. This
is due to emergence preparation of large larvae. Declines in small larvae are due to
mortality as larvae and losses during aestivation and pupation (Whiles et al. 1999).
While in large larvae, loss is due greatly to preparation for emergence and actual
emergence of adults.
Differences in substrate stability affect productivity. A positive relationship
exists between productivity and water flow. A high level of secondary production
requires a nearby center of primary production, which is during the summer/fall low-flow
period, rather than long-distance transport of organic matter from upstream sources via
the main channel (Thorp and Delong 1994). Thus, production is the lowest during
periods of high discharge in the spring and greatest during periods of low discharge in
late summer. At times when there is more food in the water and less disturbance,
production can increase to its maximum. Increases in secondary production in this study
did occur when river discharge rose. Overall, production was lowest during high
discharge (June-July) and highest during receding and low-flow conditions (JulyAugust).
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Figure Headings
Figure 1: Average historical daily discharge (cfs) compared to 2000-year daily discharge
of Upper Mississippi River, near Winona, MN (* denotes sample dates).
Figure 2: Densities (number/m2) of large Cheumatopsyche sp., Hydropsyche orris, and
Potamyia flava, from Reach 6. Bar denotes + 1 SE.
Figure 3: Densities (number/m2) of small Cheumatopsyche sp., Hydropsyche orris, and
Potamyia flava, from Reach 6, Upper Mississippi River on each sampling date, near
Winona, MN, 2000. Rocks collected from wing dams. Bar denotes + 1 SE.
Figure 4: Secondary production (mg/m2/d) of large and small Hydropsyche orris.
Figure 5: Secondary production (mg/m2/d) of large and small Potamyia flava from
Reach 6, Upper Mississippi River, near Winona, MN, 2000. Sampling dates were from
June through October 2000.
Figure 6: Secondary production (mg/m2/d) of large and small Cheumatopsyche sp. from
Reach 6, Upper Mississippi River, near Winona, MN, 2000. Sampling dates were from
June through October 2000.