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Marine Ecology Progress Series

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MEPS 212:39-52 (2001)  -  doi:10.3354/meps212039

Benthic-pelagic exchange of microalgae at a tidal flat. 2. Taxonomic analysis

Cathy H. Lucas*, Clare Banham, Patrick M. Holligan

School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom

ABSTRACT: Benthic-pelagic exchange of diatoms over a tidal cycle has been examined using a combination of field measurements and annular flume experiments. The relative contribution of benthic and planktonic species to water-column abundance and biomass has been estimated using taxonomic counts, and cell volume to carbon conversions. In June, peaks in chlorophyll a (chl a) biomass occurred either side of the high-water slack period, which corresponded with increased abundance of benthic diatoms, as well as the large centric diatom Coscinodiscus spp. In numerical terms, benthic diatoms accounted for, on average, 15 to 42% of total diatom abundance over the flood-ebb period, while Coscinodiscus spp. accounted for 12 to 17% in June and 3% in late August. Because of the large size of planktonic forms (e.g., Coscinodiscus spp., Rhizosolenia spp., Biddulphia spp.) compared with the small benthic Nitzschia and Navicula species, the contribution of benthic diatoms was significantly lower in terms of biomass. The average contribution of benthic diatoms to total diatom carbon was <1% in June, but up to 25% in late August. In contrast, the relative contribution of Coscinodiscus spp. was ~90% in June and 73 to 80% in late August. The flume experiments confirmed that, in addition to increased benthic abundance in response to stepwise increases in current velocity, the large centric diatom Coscinodiscus spp. was also resuspended from sediments at high current velocities. At low current velocities, these cells rapidly sank to the seabed, the rate probably enhanced by deposition of sand grains out of the water column. Ecological implications related to water-column biomass and production, macro-infauna diet, and composition of benthic chl a are discussed.

KEY WORDS: Resuspension · Sinking · Benthic diatoms · Phytoplankton · Coscinodiscus spp. · Annular flume

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