Nutrient limitation, organic matter cycling, and plankton dynamics during an Aureococcus anophagefferens bloom

Abstract

We evaluated plankton dynamics and the role of nutrients (organic and inorganic) during an intense (>1.5 × 10 6 cells ml -1 ) and sustained (>4 mo) brown tide bloom of the pelagophyte Aureococcus anophagefferens during the summer of 2000 in Great South Bay (GSB), New York, USA. During the study, light and dissolved inorganic nitrogen (DIN) levels were low (mean 1% light depth = 2 m, mean DIN = 0.7 µM), while dissolved organic carbon and nitrogen levels were high (mean = 600 and 45 µM, respectively). Co-occurring with the A. anophagefferens bloom was a consortium of other small phytoplankton: 60 ± 19% of chlorophyll present during the bloom was 10 6 cells ml -1 following the peak of the brown tide. Although the growth of non-brown-tide phytoplankton, such as picocyanobacteria, was consistently enhanced by nitrate and urea additions (during 100% of experiments conducted, n = 18), the net population growth of A. anophagefferens was not stimulated by N-enrichment in 60% of the experiments. During bloom initiation, decreases in DOC, DON, and DOP concentrations in GSB were similar to concurrent calculated increases in cellular C, N, and P of the burgeoning A. anophagefferens population, suggesting direct or indirect utilization of compounds within these pools by brown tide and associated microbes. In contrast, levels of DOM rose sharply when A. anophagefferens densities decreased and picocyanobacteria became the dominant phototrophs. These results illustrate a dichotomy in the algal communities of estuaries which host brown tides: often nitrogen-replete A. anophagefferens populations which are associated with DOM degradation and non-brown-tide phytoplankton populations such as picocyanobacteria which are consistently nitrogen-limited and associated with water column accumulation of DOM.