Mesozooplankton Abundance, Composition and Response to Global Climate Change in the Volta Estuarine System

Abstract

Projections suggest that global climate change is warming and changing the salinity of brackish waters (e.g., lagoons, estuaries), with impacts on nursery and feeding habitats for fish and other organisms. This study evaluated the response of mesozooplankton - responsible for energy transfer to higher trophic levels - to the combination of these climate change factors. It was conducted using samples from the Volta estuary. A field study on transects indicated that, in terms of species composition, mesozooplankton of the area were dominated by copepods (68%) followed by decapods (25%), cladocerans (4.5%) and rotifers (0.5%); these were related to chlorophyll-a concentration, temperature and dissolved oxygen of the estuary. Combined impacts of warming and changes in salinity were assessed on two cosmopolitan mesozooplankton - Temora Stylifera and Paracalanus parvus - using microcosm experiments involving different levels of salinity (22, 21, 25, 29 and 30 ppt) and warming (+0, +2 and +4 °C). Combination of these factors could explain ≈74% of the variations in egg production rate (EPR) by Temora; EPR of the copepod decreased (70%) with each degree of warming. In contrast, only feacal pellet production (FPP) by Paracalanus could be related to the combination of the two climate change factors; FPP decreased by ≈56% when the copepod was exposed to increasing temperature and salinity. It can therefore be said that different species of estuarine mesozooplankton respond differently to the combined impacts of surface warming and salinity changes expected under global climate change