Benthic-pelagic exchange of microalgae at a tidal flat. 1. Pigment analysis

Annular flume experiments and high-performance liquid chromatography (HPLC) were used to quantitatively and qualitatively measure benthic-pelagic exchange of microphytobenthos from natural sediments. Clear spatial and temporal differences in. microphytobenthos resuspension in response to stepwise increases in current velocity were observed. Resuspension of chlorophyll a (chl a) from sandy sediments containing low levels of microphytobenthos biomass (<11.5 mg chl a m(-2)) occurred gradually and continuously over a range of current velocities from 10 to 40 cm s(-1). In June, well-developed diatom mats at siltier sites (>56 mg chl a m(-2)) displayed strong resistance to erosion at currents <20 to 25 cm s(-1), above which there was a very rapid increase in the amount of chi a in suspension following the stripping of the algal mat from the sediment surface. In September, when the diatom bloom was over, these sediments were less resistant to erosion and resuspension of microalgae occurred at current velocities above 15 to 20 cm(-1). Site 1, situated at the edge of the flat, had a dense algal mat but low sediment stability. Microalgae were readily resuspended because the extracellular polymeric substances (EPS) produced by the migratory diatoms were unable to consolidate during the short emersion period. As a rule more chi a was resuspended from sandy sediments at current velocities 15 and 20 cm(-1), but above this current velocity chi a resuspension was greater from silty sediments. Although sandy sites have low biomass in the surface layer, the greater depth of sediment erosion during bedload transport exposes more chl a to the surface. The percentage of sedimentary chl a lost at selected current velocities was estimated, and the implications for carbon supply to the pelagic and benthic systems discussed. Much of the biomass resuspended may be deposited locally, particularly in sandier regions. During the flume experiments it was observed that settling of fine sediment and microalgae was extremely rapid, because it was being biodeposited by suspension-feeding activity, and 'stripped' out of the water column by rapidly sinking suspended particulate matter (SPM). Qualitative changes in suspended material were measured as % chl a (chl a/phaeopigments x 100%) and accessory pigment content. As current velocity increased the relative proportion of phaeopigments increased, which has important implications for benthic suspension feeders feeding in the benthic boundary layer. The findings from the flume experiments have been compared with in situ measurements of current velocity, SPM and chl a.