Environmental controls on the conversion of nutrients to chlorophyll in lakes.

Anthropogenic inputs of nitrogen and phosphorus to lakes have increased worldwide, causing phytoplankton chlorophyll concentrations to increase at many sites, with negative implications for biodiversity and human usage of lake resources. However, the conversion of nutrients to chlorophyll varies among lakes, hindering effective management actions to improve water quality. Here, using a rich global dataset, we explore how the relationship between chlorophyll-a (Chla) and nitrogen and phosphorus and inferred nutrient limitation is modified by climate, catchment, hydrology and lake characteristics. Phosphorus was the dominant control in oligotrophic/mesotrophic lakes, both nitrogen and phosphorus co-limitations were dominant in (hyper)eutrophic lakes, apart from hypereutrophic shallow lakes, where nitrogen was the main limiting factor. A generalized additive model of Chla vs nutrients identified a sigmoidal-type relationship with clear breakpoints between Chla and nutrients in all depth-dependent lake categories, except for nitrogen in shallow lakes. The model revealed that Secchi depth, as the predominant factor explaining the residuals, followed by the lake thermal region, elevation, and maximum depth. Lake shoreline slope, hydraulic retention time, mean depth, shoreline length, and watershed area were also statistically significant drivers for deep lakes. Surface area was only significant in shallow lakes, as it directly affects surface heating and surface contact with the wind, resulting in non-significant impact of thermal region in shallow lakes. These findings provide new insights into the response of global lake eutrophication and its main drivers, which could assist lake managers and policy-makers in mitigating widespread lake eutrophication.