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 p roduction were comparable to other estuaries. Respiration rates were c alculated as being a fixed percentage of the maximum rate of photosynt hesis (P-max(B)). We calculated that net primary production would be p ossible 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 nece ssary to explain the observed growth of phytoplankton. Near the maximu m turbidity zone, no net annual primary production could be calculated . Maximum rates of photosynthesis were similar to those reported in th e literature. Phytoplankton had relatively high rates of photosyntheti c 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 li terature. When lower values for alpha?B were used, estimates of net pr imary production decreased significantly. We propose that it is probab ly 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 assum ing that respiration was determined by maintenance processes and by th e growth rate. Using this model with published parameter values, the c alculated respiratory losses in the summer were comparable to the resu lts in which a respiration rate equal to 1.5 % of P?B?(max) was assume d. Hence, this physiologically more sound approach allowed calculation of net primary production whereas the more rigid approach assuming th at respiration is a fraction of P-max(B) did not.