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Aquatic Microbial Ecology

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AME 38:147-156 (2005)  -  doi:10.3354/ame038147

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

Andrew R. Juhl1,2,*, Michael C. Murrell1

1Gulf Ecology Division, United States Environmental Protection Agency, Office of Research and Development,National Health and Environmental Health Research Laboratory,1 Sabine Island Drive, Gulf Breeze, Florida 32561, USA
2Present address: Lamont-Doherty Earth Observatory, Marine Biology Room 2A, 61 Route 9W, Palisades, New York 10964, USA

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 situ0) 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.

KEY WORDS: Phytoplankton growth · Grazing · Nutrients · Dilution experiments · Pensacola Bay

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