Climate-driven changes in energy and mass inputs alter N:P stoichiometry differently in deep and shallow sites in a large lake

The balance of nitrogen (N) and phosphorus (P) is a key factor controlling the phytoplankton community in lakes and the likelihood of cyanobacteria dominance. Concentrations of N and P in lakes may be affected differently by nutrient delivery from watersheds and by changes in rates of internal nutrient cycling driven by physical conditions such as temperature and stratification, and both nutrients and energy inputs are likely to change with climate warming. Responses of lake nutrients to changing climate may be different in deep stratified basins and in shallow basins, where lake sediments are a site for both nutrient release and nutrient removal. In this study, we use 23 years of monitoring data to compare long-term trends in total N (TN), total P (TP) and TN:TP at 15 sites in Lake Champlain to external nutrient inputs as well as long-term meterological trends. We find that TN:TP has declined sharply lake wide, particularly in the past decade. In deep sites, these declines appear to have been driven by the ratio of dissolved N:P in tributary inputs and by decreases in hypolimnetic dissolved oxygen. In shallow sites, declines in N:P appear to be primarily driven by long-term increases in temperature and decreases in wind speeds. These patterns suggest that while shallow and deep lake segments are likely to respond to different external drivers, both are likely to experience declines in N:P and increases in cyanobacteria dominance if historical trends in environmental change continue.