PARTICULATE AND DISSOLVED PHYTOPLANKTON PRODUCTION OF THE PATOS LAGOON ESTUARY, SOUTHERN BRAZIL - COMPARISON OF METHODS AND INFLUENCING FACTORS

Uptake rates of C-14 (filtration and the acidification-bubbling method-ABM) were measured weekly in a shallow region of the Patos Lagoon estuary (32-degrees-07'S, 52-degrees-06'W) between March 1989 and March 1990. Phytoplankton production varied seasonally, the lowest values occurring in the austral winter (June-August 1989) and the highest rates during spring and summer (March 1989; September 1989-March 1990). Particulate carbon production varied between 0.65 and 70.6 mg C m-3 h-1 and was mostly associated with organisms <20 mum (mean = 73.4%). Dissolved organic carbon (DOC) released by phytoplankton varied between 0.1 and 89.3 Mg C M-3 h-1, representing approximately 22% of total (particulate + dissolved) carbon uptake. Production rates estimated by the ABM ranged from 2.2 to 159.4 Mg C M-3 h-1 and were equivalent to the sum of particulate and dissolved carbon production, simultaneously determined by the filtration technique. In general, primary production rates related to incident irradiance and water temperature, whilst high seston loads and low nitrate concentrations appeared to limit photosynthetic activity. High rates of absolute DOC excretion were positively related to the particulate carbon production, although increased excretion percentages were found under high levels of both salinity and mean irradiance in the water column.