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

Abstract

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 (μ0) and replete (μn) nutrient concentrations. The rates were measured on 4 phytoplankton fractions: bulk, > 5 μm, < 5 μm, and cyanobacteria. Many similarities existed among phytoplankton fractions: grazing rates were positively correlated with both μ0 and μn, the relationship between μ0 and m was nearly 1:1, and μn always exceeded m. The 1:1 relationship between μ0 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, μ0 < μn for all phytoplankton fractions, indicating persistent nutrient limitation. Because μn 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. © Inter-Research 2005.