Nitrogen limitation of algal biofilms in coastal wetlands of Lakes Michigan and Huron
Coastal wetlands of the Laurentian Great Lakes are critical habitats for many plants and animals of ecological and economic importance and are particularly susceptible to nutrient runoff from the landscape given their proximal location to inflows. We used nutrient diffusing substrata (NDS) to determine whether N, P, or both nutrients limited algal biomass accrual in 54 wetlands of Lakes Michigan and Huron. N was the most common limiting nutrient. Forty-three percent of NDS assays exhibited N limitation, 18% exhibited N+P colimitation, 3% exhibited P limitation, and 36% exhibited neither N nor P limitation. The magnitude of nutrient limitation was negatively correlated with water-column nutrients (dissolved and total N and P) and surrounding agricultural and developed land cover, results suggesting that anthropogenic nutrient loading partially relieved nutrient limitation in many locations. We also analyzed benthic algal assemblages to assess whether N2-fixing taxa were associated with N-limited wetlands. N2-fixing algae were common in our study wetlands, and their abundance was related to nutrient conditions. For example, the percentage of N2-fixing diatoms was significantly higher in N-limited wetlands, positively correlated with the magnitude of N limitation, and negatively correlated with dissolved N concentrations. Our results demonstrate that anthropogenic N loading, which receives considerably less management attention than P in the Great Lakes, can affect coastal wetland algal communities. Benthic algae is an important energy source for much of the food web in Great Lakes coastal wetlands, so effects associated with N loading may have broad implications.