Nutrient Limitation of Upper Great Lakes Periphyton across Large Spatial and Environmental Gradients

Abstract

Periphyton plays an important role in the food webs of large lakes, but their over-proliferation can be a major water quality concern in many systems. Managing periphyton blooms requires an understanding of what nutrients limit algal growth and how nutrient limitation interacts with other parameters in the highly dynamic and spatially heterogeneous littoral zone. To assess the nutrient limitation status and its correlates for periphyton in the Upper Great Lakes, I deployed nutrient diffusing substrata (NDS) at 28 sites in Lake Superior, northern Lake Michigan, and Green Bay that spanned broad gradients in shoreline land use and water column conditions. To understand the drivers of periphyton isotopic values and elemental composition, δ13C, δ15N, and C:N of periphyton growing on natural substrata were also measured at each site. Results from NDS experiments showed that periphyton biomass, measured as chl-a, was primarily nitrogen (N) and secondarily phosphorus (P) limited in Lake Superior and Northern Lake Michigan. Green Bay showed primary N limitation. Periphyton biomass measured as ash-free-dry weight showed simultaneous colimitation of nitrogen and phosphorus in Lake Superior, but was not limited by nutrients in northern Lake Michigan or Green Bay. Periphyton in more eutrophic sites had higher biomass, were more enriched in δ15N, displayed lower C:N, and were overall less nutrient limited. Periphyton C:N was positively correlated with water column NO3- and negatively correlated with TKN (measured as TN-NO3-), suggesting that periphyton preferentially uptake more bioavailable iii forms of N. There were substantial ranges in periphyton δ15N (-3.00 to 7.14 per mil) and δ13C (-25.5 to -12.0 per mil). Researchers interested in using stable isotopes to quantify food web relationships in the Great Lakes should keep in mind the large spatial variability that exists in periphyton δ15N and δ13C. My results suggest that the limitation status of nearshore autotrophs may differ than that of offshore autotrophs (that are phosphorus limited in the Upper Great Lakes), and that anthropogenic nutrient loading has relieved nutrient limitation of periphyton in more impacted areas of the Upper Great Lakes. Future management should take into consideration the dichotomy of limitation status that may occur between nearshore and offshore habitats by incorporating strategies to monitor inputs of both nitrogen and phosphorus into the Great Lakes.