EFFECT OF SCALLOP SHELLS AND SEDIMENT GRAIN-SIZE ON PHYTOPLANKTON FLUX TO THE BED

A flume study was made of bed skin friction and phytoplankton (Thalass iosira weissflogii deposition about a sea scallop (Placopecten magella nicus) mimic on a coarse (mean grain dia. = 1200 mu m), medium (615 mu m) and fine (170 mu m) quarry sand. Bed skin friction immediately ups tream, and at one shell diameter downstream of the mimic was 1.2-2.4 t imes higher than ambient values (8.1 x 10(-2) Pa). Directly downstream of the mimic there was a region of near-zero skin friction. Bed diato m density was correlated with changes in skin friction; after 21 h, ce ll densities were 36-87% greater in the regions of high skin friction upstream and downstream of scallop than in control experiments. The pa ttern of bed diatom density about the scallop was similar in the coars e and medium sands, but deposition to the fine bed was not affected by the mimic. Diatom density was significantly correlated with grain dia meter; deposition in the coarse bed was 2.3 times higher than in the m edium sand, and 7.4 times higher than in the fine sand. A field experi ment confirmed that the coarse sand was a greater sink of phytoplankto n pigment than the fine sand. Previous studies and scaling arguments s uggest that the differences in bed diatom density were controlled by t he magnitude of interfacial solute fluxes. Regions of high skin fricti on about the mimic increased the porewater exchange, resulting in grea ter concentrations of diatoms retained within the bed. Similarly, the more permeable coarse sand would have a greater rate of porewater exch ange than the fine sand, explaining the higher bed diatom density. Dif ferences in the predicted volume of interstitial void space as a funct ion of grain size closely matched the observed differences in bed diat om density. Results suggest that alteration of boundary layer flows by centimetre scale topography such as scallops, increases the flux of p articulate organic matter toward the bed, but whether it is retained w ithin the bed, and thus made available to the benthos is dependent on the sediment granularity. (C) 1998 Elsevier Science Ltd. All rights re served.