Time variation in phytoplankton assemblages in a subtropical lagoon system after the 1982-1983 "El Niño" event (1984 to 1986)

An analysis of seasonal and geographical distribution and abundance of the total and separate fractions of phytoplankton (nanno- and microphytoplankton) in the Magdalena-Almejas lagoon system was done after the 1982-1983 "El Niño" event. In spite of its being in a subtropical region, the annual variation of phytoplankton abundance in the area was similar to the annual cycle of production of coastal lagoons in temperate regions. There were two peaks of phytoplankton abundance, in spring and in autumn. The upwelling and tidal currents enriching the waters of Bahia Magdalena were responsible for the high concentrations of phytoplankton in the bay. Microphytoplankton was the most important fraction throughout the study period. Nannophytoplankton was somewhat abundant. Using principal component analysis, seasonal variation and frequency were the two factors determining the structure of species assemblages. Lowest values of diversity and dominance were related to circulation patterns and to the phytoplankton blooms that occurred throughout the year in Bahía Magdalena-Almejas. High values of diversity and low dominance were estimated at those areas under the influence of oceanic waters. The 1982-1983 El Niño caused a drastic drop in phytoplankton abundance during 1984. The recuperation process was slow, starting in 1985 and completed by 1986. Recorded increases in phytoplankton abundance surpassed all previous records. "El Niño" caused changes in the structure of the microphytoplankton assemblages. Species richness and specific diversity diminished because of the occurrence of few species.