We examined the effects of hydrology and nutrients on the food webs of ten coastal wetlands on Lake Superior, using published stable isotope food web data for three wetlands and original data from seven additional systems in order to span regional hydrologic and nutrient enrichment gradients. We used a dual-source isotope mixing model to estimate the proportion of carbon in fish that originated from planktonic versus periphytic invertebrates, and we related carbon source to 1) nutrient enrichment, 2) hydraulic residence time, and 3) an index of nutrient loading that incorporates residence time and nutrient concentrations. There was no relationship between nutrient enrichment and the proportion of planktonic versus periphytic C in fish. Proportion of planktonic C in fish increased significantly with hydraulic residence time (F = 5.68, R2 = 0.42, p = 0.044). Riverine wetlands generally had lowest proportions of planktonic C in fish, dendritic wetlands were intermediate, and lagoon wetlands had highest proportions. A regression between the loading index and planktonic C in fish was an improvement over the residence time regression (F = 11.7, R2 = 0.59, p = 0.009). We conclude that coastal wetland food webs are strongly affected by hydrology and further by nutrient enrichment. This work has implications for the development of food web-based ecological indicators of nutrient enrichment and the use of hydrology as a classification factor in the prediction of nutrient effects on food webs.
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Vol. 26 • No. 4