CHEMICAL, ISOTOPIC AND ENZYMATIC MONITORING OF FREE AND ENCLOSED SEAWATER - IMPLICATIONS FOR PRIMARY PRODUCTION ESTIMATES IN INCUBATION BOTTLES

Enclosing seawater samples in incubation bottles had 3 main effects on the phytoplankton: first, a large (5-fold in 12 h) increase of phytoplankton biomass, due to the release of grazing pressure by herbivores, Second, an increase (2-fold in 6 h) in ribulose biphosphate carboxylase specific activity (per unit chlorophyll a), which could be due to a smaller dependence on regenerated production (less heterotrophy on excreted compounds). This was also supported by a tight relationship between phosphoenol pyruvate carboxykinase activity and dissolved ammonium. Finally, the large changes in the carbon isotopic composition of the incubated particulate matter, which were highly dependent on the dissolved inorganic nitrogen concentration, were due mainly to physiological processes such as biochemical fractionation rather than physical processes such as CO2 limitation. For environments with high phytoplankton growth rate and high grazing pressure, a duration of incubation greater than 3 h may significantly change the physiology of the organisms and lead to different estimates of carbon assimilation by autotrophs than bulk water estimates.