Observations on suspended particulate matter (SPM) and microalgae in the Dollard Estuary, The Netherlands: Importance of late winter ice cover of theintertidal flats

Three years of combined physical and biological measurements in the Dollard tidal basin showed that in the early spring of 1996, suspended particulate matter (SPM) concentrations were low compared to observations in spring 19 95 and 1997. Since in 1996 the duration of ice formation on the tidal flats in winter was much longer than in 1995 or 1997 (until late February), it i s proposed that ice cover reduced resuspension of bed material by enhancing consolidation of the sediment and thus increasing the critical shear stres s for erosion (tau (crit)), and also by preventing the occurrence of high w aves. Both phenomena led to low suspended particulate matter (SPM) concentr ations. After the ice cover vanished, the clear water phase was followed by a microalgal bloom both in the water column and on the sediment. During th e algal bloom, SPM concentrations remained low. Floc size measurements indi cated that low SPM concentrations during the benthic and pelagic algal bloo ms were not caused by increased flocculation. On the sediment bed however b enthic microalgae produced large amounts of extracellular carbohydrates, wh ich led to an increase in bed strength (tau (crit)) and a reduction of resu spension, and thus kept SPM concentrations low during the algal bloom. (C) 2001 Academic Press.