Impact of climate change on long-term zooplankton biomass in the upwelling region of the Gulf of Guinea

We investigated long-term changes in coastal zooplankton in the upwelling region in the Gulf of Guinea, 1969 - 1992, in relation to climatic and biotic factors. We considered the role of hydrographic and climatic factors, i.e. sea surface temperature (SST), salinity, sea level pressure, windfield, and Southern Oscillation Index (SOI), in the long-term variation of zooplankton in a multiple regression analysis, along with the abundance of Sardinella. Annual variation in zooplankton biomass was cyclical, with the annual peak occurring during the major upwelling season, July-September. Over the 24-year period, there was a downward trend in zooplankton biomass (equivalent to 6.33 ml per 1000 m(3) per year). The decomposed trend in SST during the major upwelling revealed gradual warming of surface waters. This trend was believed to be the main influence on the abundance of the large copepod Calanoides carinatus (sensitive to temperatures above 23 degrees C), which appears in the coastal waters only during the major upwelling season. The warming trend associated with global climate change could affect zooplankton community structure, especially during the major upwelling season. Global warming coupled with top-down (predation) control by Sardinella might be responsible for the long-term decline in zooplankton biomass in the upwelling region of the Gulf of Guinea.