Environmental controls upon picophytoplankton growth and biomass in a eutrophic estuary

We assessed the role of top-down versus bottom-up factors in regulating picophytoplankton (PicoP) growth and overall phytoplankton size structure in a eutrophic estuary. PicoP biomass reached an annual maximum in summer/fall and was positively correlated with temperature. Ephemeral blooms (chlorophyll a > 20 µg l–1) of PicoP were observed in the upper and middle regions of the estuary despite inorganic nitrogen concentrations <1 µmol l–1. Nutrient-amended PicoP growth rates were similar to in situ growth rates in the upper estuary, and PicoP biomass was negatively correlated with river-derived inorganic nitrogen concentrations, indicating that regenerated nutrients are a major source of nitrogen supporting PicoP growth. Microzooplankton grazing rates routinely exceeded PicoP growth rates during summer; therefore, grazing must have become uncoupled from PicoP growth on timescales shorter than the interval between grazing experiments (i.e. 2 to 4 wk) for PicoP to have bloomed. Field data point to the possibility of trophic cascades involving copepods, protistan grazers, and phytoplankton as a mechanism for this growth–grazing uncoupling. These and other recent findings indicate that bottom-up factors alone cannot explain the PicoP blooms observed in some estuarine systems and emphasize the need for grazing control studies to better understand the regulation of primary production.