Interactions between nutrients, phytoplankton growth, and microzooplankton grazing in a Gulf of Mexico estuary

Dilution grazing experiments were conducted on 9 dates over a 16 mo period in Santa Rosa Sound (Florida, USA) measuring microzooplankton grazing (m) and phytoplankton gross-growth rates under in situ (mu(0)) and replete (mu(n)) nutrient concentrations. The rates were measured on 4 phytoplankton fractions: bulk, >5 pm, <5 pm, and cyanobacteria. Many similarities existed among phytoplankton fractions: grazing rates were positively correlated with both mu(0) and mu(n), the relationship between mu(0) and m was nearly 1: 1, and mu(n) always exceeded m. The 1: 1 relationship between mu(0) and m implied that microzooplankton grazing accounted for essentially all in situ phytoplankton growth, allowing no net accumulation under ambient nutrient concentrations. Despite this strong grazing pressure, mu(0) < mu(n) for all phytoplankton fractions, indicating persistent nutrient limitation. Because mu(n) always exceeded m, additional nutrient influx to the sound would generate a disparity between microzooplankton-grazing and phytoplankton-growth rates, resulting in increased biomass in all phytoplankton fractions. However, grazing would remain a major loss term for phytoplankton such that quantitative prediction of the biomass increase would have to incorporate grazing rates. This study therefore provides a useful example of simultaneous 'top-down' and 'bottom-up' control of phytoplankton biomass, We additionally observed that increased nutrient availability led to greater dominance by larger eukaryotic phytoplankton, due to differences in gross-growth rates between the phytoplankton fractions rather than differential grazing. Grazing rates on and gross-growth rates of cyanobacteria, but not the other phytoplankton fractions, were strongly correlated to temperature.