Variations in phytoplankton assemblages during 1988-1989 in a subtropical lagoon sytem on the west cost of mexico

Abstract

The total phytoplankton abundance of the Magdalena-Almejas lagoon system from 1988 to 1989 was found to be similar in surface and subsurface water samples. The pattern of sea surface temperature in the system, typical of a temperate lagoon, shows a well-defined warm season from July to October and a cool one from November to May. The maximum average values for total phytoplankton abundance and microplankton diversity (H') were recorded during the cool season, whereas the warm periods are characterized by low mean values. Nanoplankton (cells 2-20 μm) may contribute most to the total phytoplankton abundance throughout the year and dominate at the two sampling levels. Microplankton (cells > 20 μm) is abundant mainly during the cool season and reaches its largest abundances around May, when temperature increases. The study area is located in a subtropical transition zone which is influenced by tropical and temperate waters, which determines the presence of tropical and temperate species during the corresponding period. The 1988- 1989 period may be considered a normal year for the lagoon system, but the phytoplankton dynamics described in terms of our observations can not be considered typical of the system, where interannual variations in the structure of phytoplankton assemblages are determined by a complex interaction of hydrological factors. This variation is further affected by El Niño/La Niña events, which should be considered common factors determining the changes observed in the phytoplankton assemblages in this area, not anoma lies.