P. Van Rijswijk

Nutrients, light and primary production by phytoplankton and microphytobenthos in the eutrophic, turbid Westerschelde estuary (The Netherlands)

Abiotic factors and primary production by phytoplankton and microphytobenthos was studied in the turbid Westeschelde estuary. Because of the high turbidity and high nutrient concentrations primary production by phytoplankton is light-limited. In the inner and central parts of the estuary maximum rates of primary production were therefore measured during the summer, whereas in the more marine part spring and autumn bloom were observed. Organic loading is high, causing near anaerobic conditions upstream in the river Schelde. Because of this there were no important phytoplankton grazers in this part of the estuary and hence the grazing pressure on phytoplankton was minimal. As this reduced losses, biomass is maximal in the river Schelde, despite the very low growth rates.On a number of occasions, primary production by benthic micro-algae on intertidal flats was studied. Comparison of their rates of primary production to phytoplankton production in the same period led to the conclusion that the contribution to total primary production by benthic algae was small. The main reason for this is that the photosynthetic activity declines rapidly after the flats emerged from the water. It is argued that CO2-limitation could only be partially responsible for the noticed decrease in activity.

Enzymatically hydrolyzable amino acids in North Sea sediments and their possible implication for sediment nutritional values

The nutritional value of peptidic material in e ve North Sea sediments that differ in organic matter quality has been investigated with an enzymatic approach measuring the digestibility (rate constant k of proteolysis by protease) and the proportion of bioavailable fraction of amino acids (%EHAA-i: THAA). The bioavailable fraction in sediments varied between 14‐ 50% EHAA-i:THAA and was generally lower than in potential source organisms such as algae (40‐ 43%), plankton ( , 80%) and bacteria (57‐ 72%).The rate of proteolysis ofthe amino acids varied among stations ( k 5 0.3‐3.1h 2 1 ) with a systematic decrease when going from labile to more refractory organic matter. The concentration of both the total and the enzymatically hydrolyzable amino acid pool increased toward the north where e ne-grained sediments accumulate whereas the relative contribution of the labile fraction (%EHAA-i:THAA) decreased. Differences were found between the amino acid composition of the EHAAand the THAApool in sedimentary organic matter, but these differences were minor in source organisms. Reasons for the selective liberation behavior of protease-k toward peptidic material in sediments include (A) specie c cleavage pattern of protease-k preferring certain bounds (B) selection against amino acids protected by structural matrixes (holds for glycine and methionine) (C) protection by adsorptive binding to sediment particles (basic amino acids arginine, histidine, methionine). It is argued that arginine and histidine play an important role in deposit feeder nutrition since they are dee cient in the sedimentary food source and therefore may be limiting growth. Their strong adsorption affinity, low susceptibility toward cleavage and occurrence in structural organic matter make them poorly available to deposit feeders. The relative content of basic amino acids also co-varies systematically with increasing nutritional quality.

Zooplankton biomass in the Oosterschelde (SW Netherlands) before, during and after the construction of a storm-surge barrier

The hydrodynamic consequences of large coastal engineering (barrier-construction) works in the Oosterschelde were: prolonged residence times of the water, increased sinking of particulate material, and higher water transparencies. This strongly influenced the phytoplankton (Bakker et al., 1990; 1994) and phytoplankton biomass increased in the shallow Eastern compartment of the Oosterschelde (Bakker & Vink, 1994) while phytoplankton concentration of the seston rose.

Plankton size distributions and trophic relations before and after the construction of the storm-surge barrier in the Oosterschelde estuary

Phytoplankton and Zooplankton biomass distributions were calculated on a carbon basis for the inland part of the Oosterschelde, in the period before (1983), during (1984, 1986) and after (1987, 1988) the construction of the storm- surge barrier. In all years studied, both phytoplankton and Zooplankton distributions are very irregular, and little consistent patterns emerge. The data were used to test the model of Sheldon et al (1977). The observed standing stock ratios of Zooplankton to phytoplankton agree with the model predictions in 1983, and are slightly higher during the period 1984–1987. In 1988, the model predictions are very different from the observed values, because of important changes in the Zooplankton species abundance occurring in this year.