DYNAMICS OF MICROPHYTOBENTHOS BIOMASS IN THE EMS ESTUARY

The temporal dynamics of benthic chlorophyll a (chl a) in the Ems estu ary (The Netherlands, NW Europe) were studied over ca 3 yr at 6 statio ns. The mean annual concentrations of chl a ranged from 28.6 to 247.0 mg m(-2) in the top 0.5 cm of sediment. The relative vertical distribu tion of chl a in the sediment revealed a profile which was similar for 5 of the 6 stations. This distribution of chl a, representing vital c ells, was reconstructed from typical vertical distributions of chl a i ncluding the derivatives with an absorption peak at the same wavelengt h. The graphs were converted and used to calculate the total biomass o f the microphytobenthos expressed as organic carbon. The lowest value of the mean annual biomass of microphytobenthos per station was 3.1 g C m(-2), and the highest was 16.7 g C m(-2). Average values over the t otal period investigated ranged from 3.2 to 11.0 g C m(-2). With one e xception where a value of 0.6 was calculated, the ratio between the bi omass from the 0.5 to 2.0 cm sediment layer and that from the 0 to 2 c m sediment layer was fairly constant (0.40 to 0.47). This illustrates that sampling only the top 0.5 cm of the sediment to determine the mic rophytobenthos chl a and multiplying this value by the corresponding C /chl a ratio results in the carbon biomass being grossly (40 to 60%) u nderestimated. The use of an incorrect C/chl a ratio (which varied bet ween 40 and 61 during the study period) in this calculation can increa se this deviation by a further 50 %.