Phytoplankton and zooplankton seasonal dynamics in a subtropical estuary: importance of cyanobacteria

A seasonal study of phytoplankton and zooplankton was conducted from 1999 to 2001 in Pensacola Bay, Florida, USA, to further the understanding of pelagic food webs in sub-tropical estuaries. Monthly measurements included size-fractionated chlorophyll (whole water, <5 μm, <20 μm), net- and picophytoplankton composition analyzed using microscopy, flow cytometry, and HPLC pigment analysis. Additionally, zooplankton abundance and dry weight were determined from net tows. The results show a phytoplankton community dominated by the small size fraction (<5 μm), especially during the warm periods. The <5 μm chlorophyll fraction was strongly correlated with cyanobacterial abundance and zeaxanthin. Cyanobacteria (cf. Synechococcus) abundance peaked during summer in the upper estuary, typically exceeding 3 × 109 L−1, and was strongly correlated with temperature. Cyanobacteria abundance at the freshwater end of the Bay (in the Escambia River) was very low, suggesting that cyanobacteria were not delivered via freshwater. Two pigmentation types of cyanobacteria were observed. Phycoerythrin-containing cells (PE-rich) were more abundant at the marine end, while phycocyanin-containing cells (PC-rich) were more abundant in the upper estuary. The larger algae (>5–10 μm) were predominantly composed of diatoms, followed by chlorophytes, cryptophytes and dinoflagellates. The three most abundant genera of diatoms were Thalassiosira, Pennales and Cyclotella. Zooplankton biomass averaged 12.2 μg C L−1, with peak biomass occurring during May (∼30 μg C L−1). Zooplankton abundance averaged 16.7 ind. L−1, peaking at 30 ind. L−1 during May. During the summer, the zooplankton community shifted from the ubiquitous Acartia tonsa towards Oithona sp. The increase in Oithona coincided with increases in picophytoplankton and may reflect the changing food resources available to zooplankton. Thus, the trophic implications of cyanobacterial dominance in sub-tropical estuaries need to be more fully assessed.