CONTRIBUTION OF RESUSPENDED MICROPHYTOBENTHOS TO TOTAL PHYTOPLANKTON IN THE EMS ESTUARY AND ITS POSSIBLE ROLE FOR GRAZERS

Data are presented on the amounts of chlorophyll a from microphytobenthos (mainly benthic diatoms) and phytoplankton in lower, middle and upper reaches (Dollard) of the Ems estuary (The Netherlands/Federal Republic of Germany). The data are used to estimate the contribution of suspended microphytobenthos to the total phytoplankton biomass in the main channels of the different reaches. This calculation is based on a relation between 'effective wind speed' and the resuspended fraction of the total microphytobenthos. The total amount of chlorophyll a from vital algae in the water column and in the top 0.5 cm of the sediments in the lower reaches of the estuary (18-10(3) kg) is twice the amount in the Dollard (8.10(3) kg). For the entire estuary, the amount of chlorophyll a derived from microphytobenthos present in the top 0.5 cm of the sediments and in the water column (15.10(3) kg) is significantly greater than the amount from real phytoplankton (10.10(3) kg). In the lower reaches of the Ems estuary suspended microphytobenthos makes up approximately 22% of the total phytoplankton; for the Dollard, it is approximately 60%. Over 30% of the total chlorophyll a in the water column of the entire estuary consists of suspended microphytobenthos. Therefore, it may be an important food source for grazing calanoid copepods in the main channels during spring when relatively little food is available and the contribution of suspended microphytobenthos equals the amount of phytoplankton sensu stricto. On an annual basis, the resuspended microphytobenthos in the water above the tidal flats makes up half the food available to filter feeders living on these tidal flats.