PRIMARY PLANKTON PRODUCTION, RESPIRATION AND NITRIFICATION IN A SHALLOW TEMPERATE ESTUARY DURING SUMMER

Patterns of longitudinal and temporal variation in rates of primary pr oduction, respiration and nitrification were investigated in the Urdai bai estuary during August 1994. Rates of primary production, as measur ed with the C-14 technique, varied between 0.002 and 2.828 mg C l(-1) d(-1). C-14 and oxygen based rates of primary production showed a good correlation, but photosynthetic quotients (mean PQ of 2.2) were highe r than expected from considerations of the N source used by the algae. Chlorophyll a biomass showed an inverse correlation with salinity and tidal flushing is hypothesized to be a major controlling factor of it s spatial variability. Temporal variations in chlorophyll a concentrat ion were associated primarily to changes in the lunar cycle and follow ing rain events. Rates of community respiration varied between 0.154 a nd 6.857 mg O-2 l(-1) d(-1) and were significantly and positively corr elated with both bacterial abundance and chlorophyll a concentration. The relationship between respiration and primary production suggested that respiration rates based on substrates not associated with the aut ochthonous pelagic primary production increased greatly from the outer to the inner estuary and that the consumption of autotrophic producti on by heterotrophs in the water column was higher in the less producti ve outer estuary than in highly productive intermediate and inner area s. Except at the innermost station, temporal variations in respiration rates followed variations in temperature. The ratio of gross primary production to respiration (GP/R) showed marked spatial and temporal va riations, ranging from 0.24 to 17.94. Rates of nitrification, measured as allylthiourea sensitive rates of CO2 fixation, varied from undetec table levels to 9.3 mu g C l(-1) d(-1). In general, highest rates of n itrification were observed at the inner estuary under conditions of el evated ammonia concentration, high bacterial abundance and high turbid ity. Rates of nitrification were frequently associated with the >3 mu m size fraction. Oxygen consumed in,nitrification was estimated to acc ount on average for 4.7 +/- 8.3% of total oxygen consumption.