Pergamon 0273-1223(95)00680-X War Sci Vol. 32. No 4. pp 55-58. 1995. Copynght @ 1995 IAwQ Printed ID Great Bnlam. All nghts reserved. 0273-1223195 $9'50 ... 0'00 ON THE NET GROWTH OF PHYTOPLANKTON IN TWO DUTCH ESTUARIES J. Kromkamp and J. Peene Netherlands Institute of Ecology. Centre for Estuarine and Coastal Ecology. Vierstraat 28. NL-4401 EA Yerseke, The Netherlands ABSTRACT Langdon's model has been used in the calculation of net primary production of phytoplankton in the (turbid, eutrophic) Westerschelde and (relatively transparent) Oosterscbelde eSluaries. The results for the Westerschelde are shown to be much closer 10 the observed values than is achieved using other methods (fixed respiration rate) of calculation. KEYWORDS Primary production; phytoplankton; estuaries; respiration; C-balance. Estuaries fonn a land-water interface. Due to relatively high nutrient concentrations primary production by phytoplankton can be high in comparison to nearby coastal areas. Nevertheless. this potentially high productivity is not always reached. because of turbId conditions in many estuaries. whIch Imposes a light limitation. Primary production generally increase towards the mouth of the estuary (see Help et al .. 1995. for a recent review on primary production in estuanes). Only a few studies report on variation in annual productivity. Cloem (1989) concluded that the onset of phytoplankton blooms in San Francisco Bay was initiated by density stratification due to increased river runoff. causing a more transparent photic zone. This might indicate that phytoplankton in turbid estuaries might be growing near their light compensation intensity. Cole et al. (1992) even concluded that phytoplankton could not maintain a positive C-balance in the turbid Hudson River estuary. Whether net primary production (Pn) by phytoplankton will occur is mainly dependent on the balance between gross photosynthesis (Pg) and respiration (R). which will determine the critical depth. nonnally assumed to be 5-6 times the photic depth. Unfortunately. respiration of phytoplankton cannot be measured with the 14('-technique. and estimations of net production by 24h incubations suffer from several serious artifacts (Williams. I 993a.b). Therefore. a commonly used method to estimate respiratory losses is to assume that respiration is a fixed percentage of the maximum rate of photosynthesis. A value of 10% is often used. although it can vary between 5-40% (Langdon. 1993). When this approach was taken to calculate the C-balance of phytoplankton in the turbid Westerschelde estuary. net annual phytoplankton primary production was negative along the entire estuary (Kromkamp and Peene. 1995). This was in contrast to observations. which showed clear phytoplankton biomass dynamics. whIch could not be explained by import of populations from the river or from resuspended microphytobenthos from the intertidal flats.. However. when they used a model described by Langdon (1993) to calculate respiratory losses for station Vlissingen. the most marine station in the estuary. net primary production could be calculated. According to this model. the rate of respiration (R) can be expressed :is: 55