MEPS 552:81-92 (2016)  -  DOI: https://doi.org/10.3354/meps11729

ABSTRACT: The availability of the primary limiting nutrient plays a major role in driving the productivity, 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 phytoplankton communities. In this study, N₉₅ specifically represented the required nitrogen addition (normalised 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 (F_v/F_m) 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 incubations were simple to set up, incubation period was short, and F_v/F_m measures were simple to make. This approach advances options for using F_v/F_m 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 decision making on nutrient management.

KEY WORDS: Microalgae · Microbial · Eutrophication · Bioassay · PHYTO-PAM · Fluorescence · Maximum quantum yield of PSII