Relationship between Dissolved Oxygen and the Vertical and Longitudinal Distribution of Zooplankton off the Namibian coast

Abstract

Zooplankton play an important role in the marine food web and are abundant on the Namibian coast, which is part of the highly productive Benguela upwelling system. In the Benguela system zooplankton populations are dominated by copepods and euphausiids. The abundance and distribution of zooplankton are affected by various environmental factors such as temperature, light intensity and dissolved oxygen (DO). This study investigated the relationship between DO and the diversity of zooplankton at different depths in the water column on the continental shelf off the Namibian coast. There was a positive correlation between DO and diversity of zooplankton offshore (Pearson’s r =0.83), while there exists a very weak positive correlation between DO and zooplankton diversity onshore (r= 0.196). Diversity of zooplankton does not differ significantly with depth at both onshore and offshore stations (Shannon’s Index H0 < 1). However, Divesity of zooplankton between offshore (H0=2.8 to 3.6) and onshore (H0=0.8 to 0.9) stations differs significanty (p = 0:0271;d f = 4) in the top 90m, but is not significantly different below 90m (p = 0:406;d f = 4). Nevertheless, the onshore stations had higher species richness compared to the offshore stations. Dissolved oxygen does not have a direct effect on the diversity of zooplankton, but an indirect effect and it is the interplay and interaction between several biological and environmental factors that affects zooplankton assemblage composition.