Biogeochemical responses to late-winter storms in the Sargasso Sea. IV. Rapid succession of major phytoplankton groups

In this paper, we present multi-parameter data on phytoplankton community composition, and its response to storm events in the Sargasso Sea in late February and early March of 2 years (2004 and 2005). Observed physical conditions spanned a continuum from pulsed destratification/stratification to continuous mixing, with a corresponding range of phytoplankton growth responses. The pulsed destratification/stratification condition resulted in a rapid (1–2 d) doubling of euphotic zone chlorophyll (Chl-a) along with a rapid succession, days timescale, from diatoms to haptophytes and then to cyanobacteria. Deep (>300 m) continuous mixing led to a slow (8–9 d) doubling of autotrophic biomass with no observed succession in the phytoplankton community. These different temporal responses appear to be due to differences between nutrient-limited and light-limited phytoplankton growth, although differences in grazing rates or selective grazing cannot be ruled out. Unexpectedly, we found that flow cytometrically enumerated picoeukaryotes were not accounted for in HPLC-pigment derived phytoplankton classifications and did not covary with any of the pigments quantified. Yet, the picoeukaryotes were positively related to increases in total Chl-a and increased carbon export, suggesting an important but as yet unknown role in the Sargasso Sea carbon cycle.