Understanding Ecological Responses to Hydrologic Alteration in Streams and Rivers III
Presiding: B Maiolini, Museo Tridentino di Scienze Naturali; B Helms, Auburn University
NB31E-01 08:30h
River and Wetland Food Webs in Australia's Wet-Dry Tropics: General Principles and Implications for Management.
The tropical rivers of northern Australia are internationally recognised for their high ecological and cultural values. They have largely unmodified flow regimes and are comparatively free of the impacts associated with intensive land use. However, there is growing demand for agricultural development and existing pressures, such as weeds and feral animals, threaten their ecological integrity. Using the international literature to provide a conceptual framework and drawing on limited published and unpublished data on rivers in northern Australia, we have derived five general principles about food webs and related ecosystem processes that both characterise tropical rivers of northern Australia and have important implications for their management. These are: (1) Seasonal hydrology is a strong driver of ecosystem processes and food web structure; (2) Hydrological connectivity is largely intact and underpins important terrestrial-aquatic food web subsidies; (3) River and wetland food webs are strongly dependent on algal production; (4) A few common macroconsumers species have a strong influence on benthic food webs; (5) Omnivory is widespread and food chains are short. These principles have implications for the management and protection of tropical rivers and wetlands of northern Australia and provide a framework for the formation of testable hypotheses in future research programs.
NB31E-02 08:45h
A rapid bioassessment of stream diversion effects
Ecologists recognize that flow alterations imposed by dams impair water quality and many ecosystem processes. However, there is little empirical evidence addressing the ecological effects of stream dewatering through flow diversion. Intensive, multi-year sampling of macroinvertebrates and periphyton from streams in a single subalpine basin in Colorado has shown reduced abundance and diversity of macroinvertebrates and increased periphyton production in diverted reaches. This research suggests that ecosystem structure and function can be affected by diversion-mediated environmental changes. Our rapid bioassessment investigated the generality of the macroinvertebrate response to diversion. Following established EPA methods, we sampled 15 diverted streams across a range of watershed areas and diversion operation categories throughout Colorado and southern Wyoming over 4 days in September, 2004. Scoring of habitat variables demonstrated that diversion severely reduced habitat quality at all sites, compared to upstream reference sites, irrespective of the operational category of diversion. Macroinvertebrate abundance, richness, and evenness were reduced in poor-condition habitats. These results suggest that diversions of all types have the potential to depress basal food web members and thus likely diminish production of higher trophic levels in both the stream and adjacent riparian zone.
NB31E-03 09:00h
The Influence of Water Level Management on Hydrology, Benthic Organic Matter, and Invertebrates in Backwaters of a Mississippi River Navigation Pool.
The USACE manages a 1-2ft. drawdown in the lower reaches of Mississippi River Pool 25 to stimulate moist soil vegetation production. The availability of mid-pool backwaters is relatively high and the 1-2ft drawdowns result in only ~20%-25% losses of off-channel habitat. In contrast, lower-pool reaches undergo off-channel habitat reductions of ~30%-60% and experience extensive dry periods (~60d/y). We examined the influence of these hydrologic differences on invertebrates and organic matter during 2001-2003. Stovepipe cores and zooplankton samples were collected seasonally at three mid-pool and four lower-pool backwater sites. Benthic organic matter was temporally stable at mid-pool, but lower-pool values were variable because of fluctuations in CPOM (P=0.005). Lower-pool habitats had generally higher macroinvertebrate taxa richness, with significantly higher values in spring 2002 and fall 2003 (P<0.05). Multivoltine taxa (e.g., Chironomidae, P=0.02) dominated lower-pool habitats in spring, but had higher relative abundance and biomass at mid-pool in fall. Abundance and biomass of larger, univoltine taxa such as Hexagenia were higher in mid-pool habitats (P=0.01). Zooplankton abundance was not significantly different between pool reaches, but some taxa (e.g., Cyclopoida) were more abundant in the lower pool (P=0.03). Results show water level management in this navigation pool influences backwater energy resources and communities.
NB31E-04 09:15h
Effects of Variable Flows on Invertebrate Drift in Two Spring Runs in Comal Springs, Texas, USA
Comal Springs, TX, the largest spring in the western U.S., experiences anthropogenic flow reductions. Drift sampling evaluated the effects of fluctuating flow on quantitative patterns and taxonomic composition of invertebrate drift. The composition, abundance, and seasonal patterns of drift varied between runs. Invertebrate drift was greater (2x) at night than by day and marked crepuscular peaks in drift activity, common in streams throughout the world, were observed in only one spring run. Multiple regression analysis of the total drift rate and drift density in each spring run was performed with water depth, current velocity, and Julian day as independent variables. Significant relationships were found in spring run 1, but not in spring run 3. Simple linear correlations between drift of individual taxa in each spring run and the three independent variables showed significant negative relationships to current velocity and water depth, while drift rate and density of several taxa in spring run 3 showed significant positive relationships to current velocity. This indicates differences in habitat and organism ecology between the spring runs may cause different responses to variable flows. Such insects may be used as indicators of hydrological change and used to predict critical flow levels and aquifer management targets.
NB31E-05 09:30h
Diel Drift Patterns and Spatio-temporal Distribution of Macroinvertebrates in the Blanco River, Texas: A Groundwater Dominated Stream Subject to Intermittent Flow
The Blanco River is a relatively pristine karst stream in central Texas and designated a conservation target by The Nature Conservancy. It is fed primarily by groundwater in the upper reaches and dominated by runoff and intermittency downstream. The spatial and temporal structure of macroinvertebrates in the Blanco River was assessed with seasonal Hess and d-net samples during 2003-2004 and three diel drift samples from May to October 2004. Our downstream site showed a 47% drop in diversity, but comparable abundances to up- and mid-stream sites. Ephemeropteran and trichopteran taxa (e.g. Tricorythodes and Cheumatopsyche) comprised about 60% of drift and benthic samples alike, however, non-insect taxa were nearly absent from the drift. Some taxa not present in the benthic samples were present in the drift. Post-dusk and pre-dawn peaks in diel drift were discerned. No strong seasonal patterns were detected which may be attributable to an unusually wet year and asynchronous, multivoltinous life cycles associated with mild seasonality in subtropical regions. The Blanco River's historically variable hydrological regime may be further exacerbated by long-term flow alteration associated with increasing anthropogenic development and could alter the composition and distribution of macroinvertebrate assemblages.
NB31E-06 09:45h
Evaluating ecological effects of small-scale agricultural diversions on stream flow in Coastal California
In the absence of summer precipitation, grape growers in the California wine country often pump water directly from the stream for frost protection, irrigation, and heat protection. Managers may not be able to evaluate the impacts of the many small diversions on flow or anadromous salmonid habitat because of the uncertainty of human water needs. Building on previous research to predict diversion impacts on streamflow, we monitored flow in Franz Creek (a third-order stream draining 40 km2 in Sonoma County) to evaluate our model, and then measured water coverage in riffles and pools to quantify the change in salmonid habitat that these diversions would cause. Our model reasonably predicted flow decreases from diversion for frost protection: flow decreased by up to 80% on each cold morning, causing 50% riffle loss. Though it was not indicated in our model, the loss of habitat on hot summer days (suggesting diversion for heat protection) was also dramatic: levels dropped suddenly, reducing volume in intermittent pools by over 50%. Both changes in flow led to sudden reductions in habitat, suggesting that diversions are a major impediment to salmonid restoration in the region.