Understanding Ecological Responses to Hydrologic Alteration in Streams and Rivers II
Presiding: M Gurtz, U.S. Geological Survey; M Freeman, U.S. Geological Survey
NB24D-01 15:30h
Reintroduction of flooding and change in denitrification rates on a leveed Midwestern floodplain
We examined the response of soil denitrification to a floodplain restoration project that reintroduced controlled flooding to a previously isolated 900 hectare site adjacent to the Baraboo River, Wisconsin. Monthly June-August for one year prior and two years post restoration we measured denitrification in static cores (SC ΜgN2O-N/kgsoil-hr) and potential denitrification in carbon and nitrate amended soil slurry samples (DEA ΜgN2O-N/kgsoil-hr). Rates showed high temporal and spatial variability. Individual sample SC rates ranged widely (0.00 to 16.7) with a mean value of 1.10 (SD = 3.02). DEA rates were in the same general range (0.00 to 15.0) but with a slightly higher mean (mean 1.41, comparison p<0.05). Denitrification was not highly correlated with soil nutrient content. However, the difference in rates between SC and DEA samples pre-restoration was higher than post-restoration, suggesting that denitrification was less limited after the reintroduction of flooding. Vegetation type and history of flooding were better predictors of denitrification than current soil water content. Despite large variability in measured rates, these results demonstrate that there is potential for denitrification to remove significant amounts nitrate from river systems. Management of water levels to provide adequate exchange between river and floodplain is important to this process.
NB24D-02 15:45h
Establishing Minimum Flow Requirements Based on Benthic Vegetation: What are Some Issues Related to Identifying Quantity of Inflow and Tools Used to Quantify Ecosystem Response?
Establishing minimum flow requirements in aquatic ecosystems is one way to stipulate controls on water withdrawals in a watershed. The basis of the determination is to identify the amount of flow needed to sustain a threshold ecological function. To develop minimum flow criteria an understanding of ecological response in relation to flow is essential. Several steps are needed including: (1) identification of important resources and ecological functions, (2) compilation of available information, (3) determination of historical conditions, (4) establishment of technical relationships between inflow and resources, and (5) identification of numeric criteria that reflect the threshold at which resources are harmed. The process is interdisciplinary requiring the integration of hydrologic and ecologic principles with quantitative assessments. The tools used quantify the ecological response and key questions related to how the quantity of flow influences the ecosystem are examined by comparing minimum flow determination in two different aquatic systems in South Florida. Each system is characterized by substantial hydrologic alteration. The first, the Caloosahatchee River is a riverine system, located on the southwest coast of Florida. The second, the Everglades- Florida Bay ecotone, is a wetland mangrove ecosystem, located on the southern tip of the Florida peninsula. In both cases freshwater submerged aquatic vegetation (Vallisneria americana or Ruppia maritima), located in areas of the saltwater- freshwater interface has been identified as a basis for minimum flow criteria. The integration of field studies, laboratory studies, and literature review was required. From this information we developed ecological modeling tools to quantify and predict plant growth in response to varying environmental variables. Coupled with hydrologic modeling tools questions relating to the quantity and timing of flow and ecological consequences in relation to normal variability are addressed.
NB24D-03 16:00h
Do spates determine species? The influence of altered hydrology on stream fish assemblages.
We investigated the influence of short-term hydrologic regimes on fish assemblages in 18 watersheds (500-2500 ha, Lower Piedmont ecoregion) along an urbanization gradient north of Columbus, Georgia, USA. We monitored stream hydrology continuously with pressure-transducers for 1 y and also quantified fish and habitat variables in representative 100-m reaches in each watershed. We characterized each stream's hydrograph using a suite of 44 metrics, broadly characterized as Magnitude, Frequency, Duration, Flashiness, or Predictability measures. Fish species richness, diversity, health, proportion of lithophilic spawners (sensitive guild) and overall assemblage integrity (IBI) declined with an increasing number of spates (Frequency). In addition, the proportion of sunfish species, primarily Lepomis auritus and L. macrochirus, increased with increasing numbers of spates, suggesting that hydrologically altered streams are numerically dominated by tolerant species. Overall, Frequency measures were the most informative of all hydrological predictors, suggesting spate frequency is potentially a strong driver of fish assemblage structure. Interestingly, not all watersheds exhibiting a high number of spates were highly urbanized, thus factors other than % impervious surface, which was highest in the urban watersheds, influence the hydrologic regime. Overall, our data suggest a functional connection between land use change, altered stream hydrology, and shifts in fish assemblages.
NB24D-04 16:15h
The independent and interactive effects of reduced streamflow and fine sediment deposition on macroinvertebrate community structure and function.
Fine sediment deposition is a major cause of benthic habitat degradation and macroinvertebrate community impairment in stream ecosystems. Reduction in streamflow caused by human extraction of water can have dramatic effects on sediment transport dynamics, often resulting in an increase in fine sediment deposition downstream of the diversion structure. While reduced streamflow and fine sediment deposition are both known to affect benthic communities, the independent and interactive effects of these habitat alterations have not been addressed. This research is aimed at decoupling these effects. In autumn, 2004, colonization boxes containing 5 different levels (percent, by mass) of fine sediments were implanted in a gravel-bed mountain stream and exposed to 4 different levels of streamflow (15, 33, 67, 100 percent of ambient). Aquatic macroinvertebrate colonization was measured after 4 and 8 weeks. We predicted that community structure and diversity would become impaired as fine sediments increased and streamflow decreased. Moreover, we hypothesized that particular species traits such as rheophily, habit, feeding type, and respiration mode would show sensitive and differential responses to flow and fine sediment treatments. Results from this research are expected to provide insight into the mechanistic causes of benthic community impairment associated with water diversions in mountain streams.
NB24D-05 16:30h
Relations between Flow Regime and Invertebrate Communities in Upland Rivers of the Latrobe Basin, Victoria, Australia.
There has been very little success in establishing quantitative relations between flow regime and invertebrate community composition in Australian rivers. Habitat template theory proposes that species with similar traits occupy a similar habitat type. This means that communities may be characterised by their traits when investigating relations with environmental variables such as flow. Indeed the dominance of certain traits may be more predictable than abundance of individual taxa. Unfortunately the traits of many aquatic communities, such as aquatic invertebrates in Australia, are poorly known. This paper presents an approach to establishing a relation between flow and these communities drawing on habitat template theory. Biological indices reflecting the composition of invertebrate community are developed based on an assumed trait response to hydraulic variables (Froude number and Reynolds number) at the scale of individual invertebrate samples for eight sites in the upper Latrobe basin in south-east Australia. Component weights are derived for each taxa to describe their response to hydraulic conditions. The derived indices are tested using independent data and found to predict the responses of invertebrate community at the reach-scale to changes in flow through time. Component weights for each taxa are discussed in relation to known traits of some taxa.
NB24D-06 INVITED 16:45h
Panel Discussion: Understanding Ecological Responses to Hydrologic Alteration in Streams and Rivers
Land managers are confronted with increasing demands to tap flowing (lotic) aquatic systems, and the surficial aquifers that feed them, for agricultural, industrial, and domestic uses. Concurrently, aquatic resource managers are being asked to specify controls on water withdrawals and diversions to sustain the functions of associated ecosystems. The functions of "healthy" rivers and streams include processing of organic matter and nutrients, maintenance of banks and riparian function, and dissipation of floods, as well as sustained biological diversity and production. Resource managers require specific information on ecosystem responses to hydrologic alteration and enhanced decision-support tools to resolve issues related to water use and habitat protection. This panel discussion will address priority research needs in flow management.