Modeling Patterns and Controls of Food Web Structure in Saline Wetlands of a Rocky Mountain Basin

Semiarid intermountain basins of western North America contain many wetlands driven by flood irrigation from mountain snowpack. With decreasing snowpack and increasing urban water demands, maintaining these islands of productivity within otherwise dry shortgrass steppe requires understanding factors that support wetland food webs. In the Laramie Basin, Wyoming, USA the two main wetland types differ in consistency of freshwater inflow and salinity—oligosaline (0.5–5‰ TDS) wetlands dominated by the macroalgae Chara spp., and mesosaline (5–18‰) wetlands dominated by the submersed vascular plant Potamogeton pectinatus. Food web models indicate that production of epiphytic and benthic microalgae and bacteria was limiting to higher trophic levels. Production of benthic microalgae in more shaded sediments within Chara stands was only 11% of that in Potamogeton stands. However, total microalgal production was 32% greater in Chara stands because epiphyton production was 3 times higher. Nevertheless, macroinvertebrate production was 3.7 times higher in Potamogeton stands. Greater production of epiphytic (50% higher) and benthic (84% higher) bacteria fulfilled additional food web inputs required in mesosaline wetlands, which had 50% higher dissolved organic carbon (DOC) and 84% higher sediment organic carbon available to bacteria. Dispersal limitation of amphipods after more frequent drawdowns in mesosaline wetlands may have promoted much higher biomass of competing chironomid larvae. Thus, vegetation type and direct algivory appeared less important to the structure and production of invertebrate food webs than were sediment organic matter, freshwater inflow that dilutes DOC as bacterial food, and the frequency of drawdowns that inhibit taxa with limited dispersal capability.