Sediment microbial enzyme activity as an indicator of nutrient limitation in Great Lakes coastal wetlands B. H. HILL,* C. M. ELONEN,* T. M. JICHA,* A. M. COTTER,* A. S. TREBITZ* AND N. P. DANZ † *US Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Laboratory, Mid-Continent Ecology Division, Duluth, MN, U.S.A. † Center for Water and the Environment, Natural Resources Research Institute, University of Minnesota, Duluth, MN, U.S.A. SUMMARY 1. We compared the extracellular enzyme activity (EEA) of sediment microbial assemblages with sediment and water chemistry, gradients in agricultural nutrient loading (derived from principal component analyses), atmospheric deposition and hydrological turnover time in coastal wetlands of the Laurentian Great Lakes. 2. There were distinct increases in nutrient concentrations in the water and in atmospheric N deposition along the gradient from Lake Superior to Lake Ontario, but few differences between lakes in sediment carbon (C), nitrogen (N) or phosphorus (P). Wetland water and sediment chemistry were correlated with the agricultural stress gradient, hydrological turnover time and atmospheric deposition. 3. The N : P ratio of wetland waters and sediments indicated that these coastal wetlands were N-limited. Nutrient stoichiometry was correlated with the agricultural stress gradient, hydrological turnover time and atmospheric deposition. 4. Extracellular enzyme activity was correlated with wetland sediment and water chemistry and stoichiometry, atmospheric N deposition, the agricultural stress gradient and the hydrological turnover time. The ratios of glycosidases to peptidases and phosphatases yielded estimates of nutrient limitation that agreed with those based solely on nutrient chemistry. 5. This study, the first to link microbial enzyme activities to regional-scale anthropogenic stressors, suggests that quantities and ratios of microbial enzymes are directly related to the concentrations and ratios of limiting nutrients, and may be sensitive indicators of nutrient dynamics in wetland ecosystems, but further work is needed to elucidate these relationships. Keywords: Laurentian Great Lakes, microbial enzymes, nutrients, stoichiometry, wetlands Introduction Historically, concerns about nutrient enrichment in Laurentian Great Lakes ecosystems focused on phos- phorus and its role in stimulating algal production (with concurrent degradation of water clarity, oxygen content and biotic condition). With the initiation of phosphorus controls of point-source dischargers in the 1970s, water quality in the open waters of the Laurentian Great Lakes has been improving, and most lakes now meet water quality standards for nutrients (Schelske, 1991; Nicholls et al., 2001). Nevertheless, nutrient loading to coastal wetlands is a widespread problem in the lower lakes (Lakes Erie, Ontario and the southern half of Lake Michigan) and a more localised problem in the upper lakes (Lakes Superior, Huron and the northern half of Lake Michigan). In addition to nutrient loadings via surface and sub-surface runoff, atmospheric deposition of NH þ 4 and NO  3 can contribute significantly to annual Correspondence: B. H. Hill, US Environmental Protection Agency, Mid-Continent Ecology Division, 6201 Congdon Blvd., Duluth, MN 55804, USA. E-mail: hill.brian@epa.gov Freshwater Biology (2006) 51, 1670–1683 doi:10.1111/j.1365-2427.2006.01606.x 1670 Ó 2006 The Authors, Journal compilation Ó 2006 Blackwell Publishing Ltd