Assessment of the nutrient status of phytoplankton: a comparison between conventional bioassays and nutrient-induced fluorescence transients (NIFTs)

Determining nutrient limitation in natural phytoplankton populations is problematic, given the theoretical and practical limitations of popularly used methods such as nutrient enrichment bioassays, which involve adding various nutrient mixtures to samples in containers and then measuring growth over an incubation period of several days. Laboratory studies suggest that a rapid fluorometric technique based on the concept of nutrient-induced fluorescence transients (NIFTs) may provide a direct measure of the nutrient status of natural phytoplankton populations. were measured on laboratory cultures of Chlorella emersonii and Oscillatoria sp., and consistent NIFT responses were recorded in nutrient depleted cultures. NIFTs and bioassays were compared using water samples from the Yarra River and Port Phillip Bay, Victoria, and from the Derwent Estuary in Tasmania. Bioassays indicated potential (Liebig) limitation by either phosphorus or nitrogen or both at every site sampled, while NIFT measurements taken on the day of collection indicated no nutrient limitation at any site at that time. After a number of days of controlled incubation, positive NIFT responses were observed in several of the water samples, although not always in response to the nutrient that the bioassay indicated was limiting. The data presented suggest that conventional bioassays do not measure true, in situ nutrient limitation, and instead only provide information about potential limitation (Liebig limitation) in the absence of other limiting factors.