AME 13:275-284 (1997)  -  doi:10.3354/ame013275

Comparative effects of benthic microalgae and phytoplankton on dissolved silica fluxes

Sigmon DE, Cahoon LB

Microalgal effects on dissolved silica fluxes in an estuarine sandflat were investigated over an annual cycle. Benthic microalgal and phytoplankton chlorophyll a were measured at 17 times in Masonboro Sound, NC, USA, between June 1994 and May 1995. Using light and dark benthic chambers, dissolved silica (DSi) concentrations were measured in situ and used to calculate flux rates. Mean annual biomass was 64.6 and 10.0 mg chl a m-2 for benthic microalgae and phytoplankton, respectively. Benthic microalgae and phytoplankton (both containing diatoms), and cultured benthic diatoms were used to calculate Si:chl a weight ratios. Ratios of Si:chl a showed that benthic microalgae contain more silica per unit chlorophyll than phytoplankton, with mean Si:chl a ratios of 14.3, 10.9, and 2.8:1, for benthic microalgae, cultured benthic diatoms, and phytoplankton, respectively. DSi flux rates were always higher in the dark, while some negative flux rates (into the sediment) were seen in the light. DSi flux rates averaged 81.0 μmol m-2 h-1 in the dark and 37.0 μmol m-2 h-1 in the light. Lab and field experiments showed that benthic microalgae can take up water column DSi, while typical phytoplankton concentrations exert little control of water column DSi. Temperature was found to be the major regulator of DSi fluxes, with flux rates decreasing with decreasing temperature. Benthic microalgae were significant regulators of light chamber DSi flux at temperatures <20°C. Due to benthic microalgal uptake and low water column DSi concentrations observed under lower temperatures, nutrient limitation experiments were performed to determine if phytoplankton were silicate-limited. Of the 3 major nutrients, nitrogen, phosphorus, and silicon, only silicon was limiting for phytoplankton growth in every experiment. Benthic microalgae are important regulators of DSi flux, and can regulate water column DSi concentrations for at least half of the year, possibly limiting phytoplankton growth and species composition in shallow, coastal environments.

Diatoms · Benthic microalgae · Phytoplankton · Dissolved silica · Sediments · Flux rates

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