JultikaUniversity of Oulu repository

Abstract

Phytoplankton are the base of marine food webs, and zooplankton an important link between phytoplankton and fish. Zooplankton are highly sensitive to environmental changes and thus can act as indicators for the changes. There are four Eastern Boundary Upwelling Systems in the world, including the Benguela Current Upwelling System (BUS) in the South-Eastern Atlantic. They are highly productive marine ecosystems, contributing a great proportion to the world’s ocean productivity and up to 20% of global fish catch. The mesozooplankton abundance along a latitudinal 23°S transect in the northern Benguela (nBUS) and between 30–31°S in the sBUS was determined from samples collected between February and March 2019, and compared with respective environmental values in the area. An ongoing upwelling event was observed close to the shore in the sBUS, based on low sea surface temperature and high chlorophyll a concentration. Thus, mesozooplankton was highest onshore in the sBUS, ranging from 1 490 x 103 m⁻² to 47 x 10³ m⁻² offshore. In the nBUS, the mesozooplankton abundance was ranging from 313 x 10³ m⁻² onshore to 17 x 10³ offshore. Mesozooplankton abundance was lower in the nBUS than in the sBUS, but the difference was not statistically significant. Oxygen minimum zones were observed in the nBUS, but they did not seem to impact the mesozooplankton abundance. OMZs however impacted the community structure in the nBUS. The observed species composition was typical for the BUS; crustaceans dominated the mesozooplankton community with their 80% abundance, and 96% of crustaceans were copepods. Species diversity was decreasing with distance to the shore in the nBUS, but increasing in the sBUS. Shannon diversity indices (H’) and NMDS analysis based on abundances revealed significant differences in the mesozooplankton communities in the nBUS and sBUS. In the nBUS, fluorescence and salinity were the strongest contributors to the diffenrences, while in the sBUS they were fluorescence and oxygen concentration. This pattern was most likely driven by the contrasting seasons in the subsystems; upwelling in the sBUS and quiescence in the nBUS. Oxygen concentration could be one of the major factors shaping the communities in the different subsystems but is only impacting with very low concentrations. The present study provides information about the current mesozooplankton community structure in the BUS. Long-term monitoring of the BUS zooplankton community structure, climate change effects, as well as the physical conditions, such as temperature, oxygen concentration, and upwelling patterns is important for future management and protection of this highly productive and exploited ecosystem.