POSSIBILITY OF NET PHYTOPLANKTON PRIMARY PRODUCTION IN THE TURBID SCHELDE ESTUARY (SW NETHERLANDS)

Phytoplankton primary production was studied in the turbid estuary of the river Schelde (The Netherlands). Measured rates of gross primary production were comparable to other estuaries. Respiration rates were calculated as being a fixed percentage of the maximum rate of photosynthesis (P-max(B)). We calculated that net primary production would be possible only when the rate of respiration was less than 1.5% of P-max(B). This respiration rate seemed too low to be realistic, but was necessary to explain the observed growth of phytoplankton. Near the maximum turbidity zone, no net annual primary production could be calculated. Maximum rates of photosynthesis were similar to those reported in the literature. Phytoplankton had relatively high rates of photosynthetic affinity [alpha(B), 0.032 to 0.043 mg C mg(-1) chl h(-1) {mu mol m(-2) s(-1))(-1}], although they were within the range reported in the literature. When lower values for alpha?B were used, estimates of net primary production decreased significantly. We propose that it is probably better to use the 0.1 % light level as the base of the photic depth than the 1 % light level when considering the critical depth. We also calculated respiratory losses with a simple 2-compartment model assuming that respiration was determined by maintenance processes and by the growth rate. Using this model with published parameter values, the calculated respiratory losses in the summer were comparable to the results in which a respiration rate equal to 1.5 % of P?B?(max) was assumed. Hence, this physiologically more sound approach allowed calculation of net primary production whereas the more rigid approach assuming that respiration is a fraction of P-max(B) did not.