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C/Chl-a ratios were determined for benthic diatom populations at 6 stations in the Ems estuary. For mixed field populations previous data are not available. Measurements were canied out over a period of nearly 3 years. The data were statistically analysed. Statistically significant differences in rat10 values were indicated within and between years. No indications were found for a statistically significant station effect upon the ratio values. The ratio values over the 3 year period investigated ranged from 10.2 up to 153.9 with yearly averages and standard deviations of 40.3 f 13.8; 41.2 f 20.4 and 61.4 -+ 22.0, respectively. It was concluded that chlorophyll a measurements in itself are insufficient to describe fluctuations in benthic diatom standing stock. Mechanisms possibly regulating the CIChl-a ratios are discussed.

The temporal dynamics of benthic chlorophyll a (chl a ) in the Ems estuary (The Netherlands, NW Europe) were studied over ca 3 yr at 6 stations. The mean annual concentrations of chl a ranged from 28.6 to 247.0 mg m-' in the top 0.5 cm of sediment. The relative vertical distribution of chl a in the sediment revealed a profile which was slrnilar for 5 of the 6 stations This distribution of chl a, representing vital cells, was reconstructed from typical vertical distnbuhons of chl a including the denvatives with a n absorption peak at the same wavelength. The graphs were converted and used to calculate the total biomass of the microphytobenthos expressed as organic carbon. The lowest value of the mean annual biomass of inicrophytobenthos per station was 3.1 g C m-', and the highest was 16.7 g C m-' Average values over the total period investigated ranged from 3.2 to 11.0 g C m-2. With one exception where a value of 0.6 was calculated, the ratio between the biomass from the 0.5 to 2.0 cm sedlrnent layer and that from the 0 to 2 cm 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 microphytobenthos chl a and multiply~ng t h~s value by the corresponding C/chl a ratio results in the carbon biomass being grossly (40 to 60%) underestimated. The use of a n incorrect C/chl a ratio (which vaned between 40 and 61 dunng the study period) in this calculation can increase thls deviation by a further 5 0 % KEY WORDS blicrophytobenthos biomass . Chlorophyll a . Carbon . Carbon to chlorophyll a ratlo (C/chl a) . Ems estuary .

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