The ‘El Niño’ effect on the phytoplankton of a north-western Baja California coastal lagoon

Abstract

San Quintin bay is a coastal lagoon having strong tidal water exchange with the adjacent ocean. It has been reported that, during spring and summer, upwelling events are the main cause of variability for most water properties in the lagoon. Usually, diatoms and dinoflagellates dominate phytoplankton assemblages in the bay, during summer. During the summer of 1984, upwelling was masked by the warming of surface and near-surface waters as a product of El Niño influence, and nanoplankton dominated the phytoplankton assemblages of the lagoon. Diatom and dinoflagellate abundances were between one and two orders of magnitude lower, and nanoplankton was an order of magnitude higher, than those reported for the summers of 1977 and 1979, years with no El Niño events. In 1984, chlorophyll a concentrations in the lagoon were 20–25% of the values reported for 1977 and 1979. Twenty-day time series show that assimilation numbers of phytoplankton had a range of 3·7–10·0 mgC mg Chl a −1 h −1. These values are lower than the assimilation ratios reported for 1977 and 1979. These relatively low assimilation numbers were possibly due to the presence of abundant cyanobacteria, as was reported from other coastal areas of the California Current for summer 1983.