Watershed versus within-lake drivers of nitrogen: phosphorus dynamics in shallow lakes

Research on lake eutrophication often identifies variables affecting amounts of phosphorus (P) and nitrogen (N) in lakes, but understanding factors influencing N:P ratios is important given its influence on species composition and toxin production by cyanobacteria. We sampled 80 shallow lakes in Minnesota (USA) for three years to assess effects of watershed size, proportion of watershed as both row crop and natural area, fish biomass, and lake alternative state (turbid versus clear) on total N: total P (TN:TP), ammonium, total dissolved phosphorus (TDP), and seston stoichiometry. We also examined N:P stoichiometry in 20 additional lakes that shifted states during the study. Lastly, we assessed importance of denitrification by measuring denitrification rates in sediment cores from a subset of 34 lakes, and by measuring seston δ15N in four additional experimental lakes before and after they were experimentally manipulated from turbid to clear states. Results showed alternative state had t...