An Ecopath/Ecosim analysis of an estuarine food web: seasonal energy flow and response to river-flow related perturbations

Estuaries are often the receiving basins for major river systems which makes them vulnerable to anthropogenic influences. I used Ecopath and Ecosim to investigate the structure of the Weeks Bay, Alabama, USA food web, and the responses of the food web to bottom-up perturbations. Four season-specific steady-state Ecopath models were developed and used to compare the production, biomass, consumption, biomass flows, and higher order indices of ecosystem functioning of the Weeks Bay food web under winter, spring, summer, and fall conditions. The season-specific Ecopath snapshots indicated that the structure of the Weeks Bay food web was resilient. Winter had the lowest biomass and production and heavy reliance on detritus, leading to high biomass and production in spring and summer, culminating in the most complex food web in fall. Ecosim simulations were performed that examined the responses of the Weeks Bay food web to single pulses of high and low flow years, more extreme seasonal variation of river flow, and interannual variation in river flow (two high-flow years followed by four low-flow years). Baseline simulations produced cyclic behavior at the lowest trophic levels that was gradually dampened as one moved to higher trophic levels of the food web. Lower trophic level groups showed the largest responses and quickest recovery from single pulses of high flow and low flow, while upper trophic level groups showed generally the smallest responses and longest recovery. More extreme within-year variation in river flow caused exaggeration of the seasonal responses predicted under baseline conditions, with little or no change in the average long-term biomass of groups. Interannual variation in high and low flow was mimicked in the biomasses of lower trophic level groups, with some of the upper trophic level groups responding with small, yet consistent, net changes in their long-term average biomass. Additional simulations showed that net changes in biomass were due to the magnitude, number, and temporal arrangement of high- and low-flow years. Results suggest that changes in nutrient influxes or precipitation patterns can affect the variation and long-term average biomass of key groups of the Weeks Bay food web