Combining in-situ and satellite derived proxies of temperature and chlorophyll with microplankton analyses to determine differences at the base of the food web in the northern and southern Benguela Upwelling Systems

Intensive fishery pressure in the high productive Benguela Upwelling System (BUS) caused a decline in fish stocks of small pelagic schooling fishes in the second half of the last century. While the fish stocks in the southern BUS slowly recovered, the ecosystem of the northern part did not return to its original status. The project TRAFFIC (Trophic Transfer efficiency in the Benguela Current) investigates differences in structure between both food webs and the effects on the efficiency of energy transfer rates. Primary producers and microzooplankton are at the base of the food chain and the composition of phytoplankton and microzooplankton determines the quality and quantity of food for higher trophic levels. Abiotic parameters and chlorophyll a were measured by satellite and in-situ during an expedition to the northern and southern BUS in February/March 2019. Microplankton organisms were sampled with water bottles (CTD rosette) and different nets (20 µm Apstein, 55 µm Multinet). A remotely operated towed vehicle (TRIAXUS), a gear equipped with several sensors (temperature, salinity, oxygen, nitrate, light, pigments, video plankton recorder, LOPC) was towed behind the ship undulating between the surface and 200 m depth. The gear was deployed on several transects across the shelf and along the coast and delivered a broad spectrum of data over a distance of 586 km in the northern and 944 km in the southern BUS. Differences in composition, abundance and photoactive fitness/performance of the phytoplankton community were analyzed in both regions. Stable nitrogen and carbon isotopes of the different size components were measured to determine the isotopic values at the base of the food web. We will present similarities and dissimilarities of these parameters between both sub-systems and test whether the differences in fishery yields between the northern and southern BUS correspond to those at the base of the food web.