Latitudinal variation in nutrient stoichiometry and chlorophyll-nutrient relationships in lakes: A global study

Abstract

We present analysis of variations in relationships between nitrogen (N), phosphorus (P) and chlorophyll-a (chl-a) in lakes along a gradient of latitude inclusive of tropical, temperate and polar regions. Total nitrogen (TN), total phosphorus (TP), chl-a, latitude and depth data were collated for 1316 lakes situated between 70 degrees S and 83 N. Latitudinal variation was then analysed for three empirical measures of phytoplankton nutrient limitation and/or nutrient assimilation. Lastly, chl-a near-maxima conditional on TN and TP abundance were empirically defined for this global dataset using quantile regression. Mean TN:TP increases with distance from the equator. This relationship is independent of variation in either lake depth or trophic state, reflecting latitudinal variation in nutrient cycling processes and/or nutrient sources. There is a negative linear relationship between latitude and chl-a:TN which similarly suggests that N is less abundant relative to phytoplankton growth requirements at lower latitudes. Relative to temperate lakes, the statistical capability of TN and TP to predict chl-a is poor for both tropical and polar lakes, reflecting latitudinal variation in lake ecosystem functioning and the subsequent potential unsuitability of applying relationships derived for temperate lakes elsewhere. Chl-a near-maxima correspond to chl-a:TN and chl-a:TP yields of 0.046:1 and 0.87:1 respectively, although some observations greatly exceed near-maxima, suggesting possible physiologically plastic phytoplankton responses in these exceptional cases. Deficiencies in understanding the mechanisms that drive variation in macro-nutrient stoichiometry and phytoplankton biomass-nutrient relationships across large spatial scales necessitates further landscape-scale research on this topic, particularly in the tropics.