Spatio-temporal variation of zooplankton biomass in an estuarine system of the Western Caribbean during two annual cycles

The bay of Chetumal, in the Western Caribbean, is regarded as an uniformly hypohaline and oligotrophic system. In order to define if the zooplankton biomass shows a homogeneous behavior in this coastal system we analyzed the spatio-temporal variation of this parameter (wet weight: mg m(-3)) during two years (1996, 1997). During 1996, the average annual biomass (11.56 mg m(-3)) was significantly lower than that recorded during 1997 (16.18 mg m(-3)). Slightly lower biomass values were observed in northerlies during both years, followed by dry and wet periods, thus suggesting seasonality in the dynamics of this factor in the zooplankton community. Monthly variations were also considerable and suggest that important changes could also be occurring at scales smaller than the seasonal or interannual. Between 45 and 51% of the biomass in 1996 and 1997 was distributed at the outermost and more saline stations of the bay regardless of the season. The high primary production usually related to the inner zone of the bay was not reflected in the zooplankton biomass; this was explained by local distributional restrictions of the resident herbivorous zooplankton and possibly to hypoxia related to local eutrophication. The interannual differences, with higher biomass, temperature, and salinity found in 1997 and also the weak seasonality of that year could be related to the regional effect of El Nino 1997-98. The zooplankton biomass in the bay was relatively low year-round regardless of the season; this was attributed to its low internal hydrographic energy, scarce aquatic vegetation, and to its isolation from the marine influence.