An overview of ecological status, vulnerability and future perspectives of European large shallow, semi-enclosed coastal systems, lagoons and transitional waters. Estuarine, Coastal and Shelf Science,

From 1976 through 1978 primary production of microphytobenthos was measured at 6 stations on intertidal flats in the Ems-Dollard estuary using the 14C method. The purpose of the measurements was to estimate the annual primary production at different sites in the estuary and to investigate the factors that influence the rates of primary production. Therefore benthic chlorophyll a and a set of environmental factors were measured. Only primary production correlated sigruficantly with chlorophyll a concentration in the superficial (0.5 cm) sediment layer; other factors (temperature. in situ irradiance) did not correlate with primary production, primary production rate or assimilation number. Annual primary production ranged from ca. 50 g C m-' to 250 g C m-2 and was closely related to elevation of the tidal flat station. However, highest values were also recorded at the station closest to a waste water discharge point in the inner part of the estuary. Annual primary production can be roughly estimated from the mean annual content of chlorophyll a in the sediment. Use of different calculation methods results in annual primary production values that do not differ greatly from each other. Also productivity rates did not differ much over most of the estuary, except at the innermost station which showed a high production rate in combination with high microalgal biomass; this could not be explained by the high elevation of the station alone. A hypothesis is offered to explain the limited primary production of microphytobenthic vegetations.

A linear transect with 12 sampling points was sampled in a tidal estuarine environment during 1973 for chlorophyll a and pheophytin, and for species composition of epipelic diatoms. A gradual pattern in sediment from coarse to fine was found from the most seaward station towards the most landward station of the transect. Differences in station level gave emersion times of 37 to 67 % of the tidal cycle. In the course of the year, fluctuations in chlorophyll a content (mg chlorophyll a m-* in the top 2 cm of the sediment) appeared to be distinctly parallel at all stations. However, mean yearly values differed widely from station to station, ranging between 20 and 100mg chlorophyll a m-2. Chlorophyll a concentration was positively related to the amount of particles < 16 pm in the sediment; this proportion was considered to relate negatively to the degree of exposure to currents and wave action. It is likely that low daily winter irradiances limited diatom growth. Heavy rain and strong wave action during storms suddenly reduced the chlorophyll a content considerably. Species composition of epipelic benthic diatoms could partly b e related to the sediment gradient and time of year. It seems that high biomass values are formed almost exclusively by single species which are indifferent in their spatial distribution.

PAM (pulse-amplitude-modulated) fluorescence measurements of motile microphytobenthic algae were carried out in June 1996 at Sylt, Germany. Compansons between "C-based and fluorescence-based production rates were made. A very high correlation between 14C-and fluorescence-based production rates was found for maximal production rates (P, , values). I4C-based maximal production rates var~ed during the study penod between 0.65 and 1.7 mg C mg chl a-' h-', comparable to variations of P,,, measured with the fluorescence-based method. For other photosynthetic parameters [a (maximum light utilization coefficient). Ek (light saturation index), E,,, (light intensity at which P, , , , , is reached)], differences between the 2 methods were much larger. Highest carbon quantum yields ( @, , , ) (m01 C m01 quanta-' absorbed) were obtained at low irradiances. Considering the whole range of investigated carbon quantum yields, we found that initially these values decreased at low to moderate irradiances without a concomitant decline of the actual photochemical efficiency (F,' -F)/F,,' (Fand F,'. m~nimal and maximai iiuoresce~lct' signals in :he !igh!) Therefcre, e high !ine~rit)r between the actual photochemical efficiency and the carbon quantum yield could only be observed up to values of 0.018 m01 C m01 quanta-' This is different to higher plants, for which linearity can be observed up to carbon quantum yields of 0.042 m01 C m01 quanta-' It was shown that, for the calculation of the overall production rates based on the fluorescence method, it is necessary to carefully measure the mean specific absorption coefficient (a') of the algae. Unless this is achieved. PAM measurements cannot be used to calculate absolute production rates.

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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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