Patterns of nutrient limitation of algal productivity from headwaters to estuary

Studies of algal growth responses to nutrient availability at the catchment scale are sparse. Here we study algal nutrient responses in headwater streams, rivers, lakes, and an estuary in a temperate coastal catchment. Nutrient enhancement bioassays were used to determine phytoplankton and periphyton growth limitation by nitrogen (N) and/or phosphorus (P) availability. In addition, we measured N and P concentrations and ratios in water. In the headwaters, algae were most often limited by N and N + P while algae further downstream became progressively less nutrient limited. Unlike other sites in the catchment, phytoplankton in a lowland headwater lake was almost always N limited, indicating that local characteristics shifted nutrient availability and limitation status towards P saturation. Nutrient limitation state could be correctly inferred from dissolved inorganic nitrogen:P ratios at only 4 of the 8 sites, highlighting the limited efficacy of nutrient ratios alone for inferring nutrient limitation status. Our results show that reduction of N and/or P concentrations to below growth-limiting thresholds could reduce algal productivity and biomass in the lower catchment, but not consistently, because nutrient limitation status varied greatly at sites within the catchment. Our study adds to the growing literature indicating that drivers of eutrophication can be spatially and temporally variable, even within a single catchment.