Micro- and Mesozooplankton Distribution Related to an Upwelling Filament in the Northern Benguela Current Region

Upwelling filaments are formations of cold upwelled water which develop near the coast and extend up to hundreds of kilometers offshore. They are important for the dynamics of upwelling systems as they transport nutrients and plankton to the open ocean. On regional scales, the transport of water masses by upwelling filaments occasionally can be higher and more important than the Ekman transport. Such filaments are able to transport a large fraction of coastal primary production even during low upwelling intensity. However, the dimension and nature of transported organic matter depends largely on the size structure of the involved plankton community. In the course of the GENUS project (Geochemistry and Ecology of the Namibian Upwelling System), we investigated the micro- and mesozooplankton community associated with upwelling filaments. Small-scale differences in the distribution of micro- and mesozooplankton species were detected and related to different water masses. Depending on the developmental stage of the filament, zooplankton abundance can be higher or lower than in the surrounding offshore waters. Along the filament, the microzooplankton composition changed from ciliates-dominated in the in the centre of the filament to dinoflagellate-dominated further offshore. Several mechanisms like the exhaustion of available nutrients and the subsequent die off and sinking of the community or increased predation pressure within the filament are discussed to enlighten the fate of the transported organic material.