Nitrogen response of natural phytoplankton communities: a new indicator based on photosynthetic efficiency Fv/Fm

The availability of the primary limiting nutrient plays a major role in driving the pro-ductivity, biomass and community composition of coastal phytoplankton, but there is an ongoing challenge to define the relationship between nutrient enrichment and phytoplankton response. This study explored a new indicator (N95) for quantifying nutrient response in natural phytoplank-ton communities. In this study, N95 specifically represented the required nitrogen addition (nor-malised to chlorophyll a) for the sampled phytoplankton population to reach 95% of nitrogen-replete photosynthetic efficiency. To calculate N95, an asymptotic model was applied to measures of phytoplankton photosynthetic efficiency (Fv/Fm) taken after 24 h incubations under a range of nitrogen enrichment treatments. Applying the method in an Australian subtropical bay, Moreton Bay, nitrogen response was lowest nearshore and high, yet variable, offshore. This pattern reflected the ambient nutrient gradient of declining concentrations with distance offshore. A Moreton Bay-specific threshold in phytoplankton nutrient response was identified at the ambient dissolved inorganic nitrogen concentration of 2 µmol l−1, above which the response to nitrogen enrichment did not increase for 82% of samples. Advantages to measuring N95 were that incuba-tions were simple to set up, incubation period was short, and Fv/Fm measures were simple to make. This approach advances options for using Fv/Fm measures to assess phytoplankton nutrient response and provides a new and sensitive biological indicator applicable to long-term water quality monitoring programs, relevant to parameterising ecological models and supporting deci-sion making on nutrient management.Griffith Sciences, Griffith School of EnvironmentFull Tex