Nutrient availability and phytoplankton nutrient limitation across a gradient of atmospheric nitrogen deposition

Abstract

Atmospheric nitrogen (N) deposition to lakes and watersheds has been increasing steadily due to various anthropogenic activities. Because such anthropogenic N is widely distributed, even lakes relatively removed from direct human disturbance are potentially impacted. However, the effects of increased atmospheric N deposition on lakes are not well documented. We examined phytoplankton biomass, the absolute and relative abundance of limiting nutrients (N and phosphorus [P]), and phytoplankton nutrient limitation in alpine lakes of the Rocky Mountains of Colorado (USA) receiving elevated (>6 kg N·ha−1·yr−1) or low (<2 kg N·ha−1·yr−1) levels of atmospheric N deposition. High‐deposition lakes had higher NO 3‐N and total N concentrations and higher total N : total P ratios. Concentrations of chlorophyll and seston carbon (C) were 2–2.5 times higher in high‐deposition relative to low‐deposition lakes, while high‐deposition lakes also had higher seston C:N and C:P (but not N:P) ratios. Short‐term enrichment bioassays indicated a qualitative shift in the nature of phytoplankton nutrient limitation due to N deposition, as high‐deposition lakes had an increased frequency of primary P limitation and a decreased frequency and magnitude of response to N and to combined N and P enrichment. Thus elevated atmospheric N deposition appears to have shifted nutrient supply from a relatively balanced but predominantly N‐deficient regime to a more consistently P‐limited regime in Colorado alpine lakes. This adds to accumulating evidence that sustained N deposition may have important effects on lake phytoplankton communities and plankton‐based food webs by shifting the quantitative and qualitative nature of nutrient limitation.