Primary plankton production, respiration and nitrification in a shallow temperate estuary during summer

Abstract

Patterns of longitudinal and temporal variation in rates of primary production, respiration and nitrification were investigated in the Urdaibai estuary during August 1994. Rates of primary production, as measured with the 14C technique, varied between 0.002 and 2.828 mg C l −1 d −1. 14C and oxygen based rates of primary production showed a good correlation, but photosynthetic quotients (mean PQ of 2.2) were higher 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 its spatial variability. Temporal variations in chlorophyll a concentration were associated primarily to changes in the lunar cycle and following rain events. Rates of community respiration varied between 0.154 and 6.857 mg O 2 l −1 d −1 and were significantly and positively correlated 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 autochthonous pelagic primary production increased greatly from the outer to the inner estuary and that the consumption of autotrophic production by heterotrophs in the water column was higher in the less productive outer estuary than in highly productive intermediate and inner areas. 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 variations, ranging from 0.24 to 17.94. Rates of nitrification, measured as allylthiourea sensitive rates of CO 2 fixation, varied from undetectable levels to 9.3 μg C l −1 d −1.In general, highest rates of nitrification were observed at the inner estuary under conditions of elevated ammonia concentration, high bacterial abundance and high turbidity. Rates of nitrification were frequently associated with the > 3 μm size fraction. Oxygen consumed in nitrification was estimated to account on average for 4.7 ± 8.3% of total oxygen consumption.