Comparative effects of benthic microalgae and phytoplankton on dissolved silica fluxes

Abstract

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-' for benthic microalgae and phytoplankton, respectively. Benthic microalgae and phytoplankton (both containing diatoms), alid 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 Sl.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 pm01 m-' h-' in the dark and 37.0 pm01 m-' h-' 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 <2O0C. Due to benthic microalgal uptake and low water column DSi concentrations observed under lower temperatures, nutnent limitation expenments 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 concentratlons for at least half of the year, possibly limiting phytoplankton growth and species composition in shallow, coastal environments.

Nitrogen fluxes, denitrification and the role of microphytobenthos in microtidal shallow-water sediments:an annual study

“…This 'filter' function is linked to at least 2 algal processes, algal assimilation of nutrients and photosynthetic oxygenation of the sediment surface, and has since then been shown in several studies (e.g. Krom 1991, Rizzo et al 1992, Wiltshire 1992, Risgaard-Petersen et al 1994, Reay et al 1995, van Luijn et al 1995, Sigmon & Cahoon 1997.…”

“…The 5 times higher IN winter concentrations at Rortbngen resulted in IN/IP ratios of 60 to 90, suggesting P limitation due to an additional N load, presumably from local sewage outlets. It must be kept in mind, however, that microphytobenthic communities assimilate nutrients from both the overlying water and the sediment pore water , Sigmon & Cahoon 1997.…”

Effect of continuous nutrient enrichment on microalgae colonizing hard substrates

“…However, the decrease of Si:N supply ratios may be less influential for diatoms inhabiting sediments which represents an undepletable pool of PSi. This pool may prevent Si-limitation (Sundbäck & Snoeijs, 1991), if the temperature-dependent dissolution rates meet the demand of the biofilm (Sigmon & Cahoon, 1997) or if bacterial mediated PSi-dissolution is fast enough (Bidle & Azam, 1999).…”

No limit? The multiphasic uptake of silicic acid by benthic diatoms

“…They colonize intertidal and subtidal areas in all latitudes and can contribute significantly to primary production in coastal zones (MacIntyre et al 1996;Cahoon 1999). By forming dense microbial mats at the sediment-water interface, benthic diatoms also alter the properties of sediments (Ziervogel and Forster 2006), and play a key role in controlling nutrient fluxes across the sediment-water interface (Sigmon and Cahoon 1997) with important implications for the functioning of the pelagic ecosystem. For example, due to their direct uptake of dissolved silica (DSi) at the sediment-water interface, benthic diatoms can greatly influence DSi released to the water column (Sigmon and Cahoon 1997;Srithongouthai et al 2003) and play an important role in the dominance of specific pelagic phytoplankton groups (Conley et al 1993).…”

“…By forming dense microbial mats at the sediment-water interface, benthic diatoms also alter the properties of sediments (Ziervogel and Forster 2006), and play a key role in controlling nutrient fluxes across the sediment-water interface (Sigmon and Cahoon 1997) with important implications for the functioning of the pelagic ecosystem. For example, due to their direct uptake of dissolved silica (DSi) at the sediment-water interface, benthic diatoms can greatly influence DSi released to the water column (Sigmon and Cahoon 1997;Srithongouthai et al 2003) and play an important role in the dominance of specific pelagic phytoplankton groups (Conley et al 1993). Indeed, when DSi becomes limiting, a shift from diatoms to nonsiliceous algae can occur, which may increase the likelihood of harmful algal blooms and have dramatic consequences for the transfer of carbon to higher trophic levels.…”