Impact of two decades of shellfish farming on phytoplankton biomass, abundance and productivity of a coastal lagoon influenced by upwelling

Resumo

Long-term impact of the Pacific oyster Crassostrea gigas farms on the phytoplankton biomass as chlorophyll a concentration (chl-a), abundance and productivity (PP) of the western arm of Bahía San Quintín (Falsa Bay), a coastal lagoon influenced by the California Current coastal upwelling, was evaluated after two decades of commercial cultures. High-resolution time series (on an hourly basis) of these variables were generated in 1999 and 2000, for locations with and without oyster cultures in Falsa Bay. Representative averages of these data were compared with data reported for 1979, when the commercial cultures were just starting. Also, data of 1979 and recent data were used separately to explore possible short-term (hourly) effects of oysters on the phytoplankton variables. The available data are scarce and do not support the hypothesis of diminished phytoplankton biomass, abundance and PP due to the oyster cultures. In general, there were no statistically significant differences between the averages of chl-a (2.5 mg m-3 ), microphytoplankton abundance (from 42·103 to 194·103 cells L-1, depending upon the location) and PP (12 mgC m-3 h-1 ) when comparing 1979, and 1999 - 2000 data. This was possibly due to the relatively low residence time of Falsa Bay waters (~8 days) and relatively low oyster density (a total of ~900 tons, with an estimated maximum carrying capacity from ~2,000 to 6,000 tons for this arm of the bay).