Rixen, Tim ORCID: https://orcid.org/0000-0001-8376-891X, Borowski, T., Duncan, S, Heinatz, K., Hirschmann, S., Horton, M., Hüge, Fabian, Janßen, S., Jordan, T., Kaufmann, M., Kremer, K, Labis, E., Martin, B., Mayer, B., Meiritz, L., Paulus, E., Pinter, S., Plewka, J., Reule, N., Rommel, A., Schneider, T., Siddiqui, Claire, Springer, B., Stanbro, K., Stegeman, H., Wallschuss, S., Welsch, A., Wenzel, J., Witting, K. and Zankl, S. (2021) Trophic Transfer Efficiency in the Benguela Current, Cruise No. SO285, August 20th - November 2nd 2021, Emden (Germany) - Emden (Germany) (Englisch). . SONNE-Berichte, SO285 . Gutachterpanel Forschungsschiffe, Bonn, 128 pp. DOI https://doi.org/10.48433/cr_so285.

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Abstract

Cruise SO285 is the second field campaign of the BMBF-funded joint project TRAFFIC (Trophic TRAnsfer eFFICiency in the Benguela Current) with the overall aim to explore climate change and human impacts on ecosystem structure and function as well as on ecosystem services such as fisheries and CO2 sequestration in the Benguela Upwelling System (BUS). In contrast to the first TRAFFIC cruise with RV Meteor (M153) during the austral summer/autumn in 2019, cruise SO285 took place during the austral winter/spring of the year 2021 in order to capture ecosystem responses to seasonally varying physical conditions. Since the strong Lüderitz Upwelling Cell divides the BUS into a northern (NBUS) and a southern subsystem (SBUS), we conducted research between 20°–32°S and 10–18°E and focussed on two regions, one off Namibia
(NBUS: 20°–23°S and 10°E–coastline) and a second one off South Africa (SBUS: 28°–32°S, 13°E–coastline). Due to the COVID-19 pandemic, the design and conduct of the cruises SO285 and M153 differed
partly from each other but during both cruises almost the same gears have been used. This includes underway measurements of greenhouse gases and associated parameters, shipboard measurements
of the EK60 echo sounder to obtain information on vertical distribution of plankton and nekton organisms, the CTD rosette sampler for water sampling, six different nets for plankton and fish sampling as well as sediment traps to measure the export of organic matter from the euphotic zone. Additionally, we recovered two oceanographic observation (PIRATA) buoys during the transit into the work area and deployed six autonomously operating CTD profilers (five ARGO and one
BIO-ARGO floats) within the work area. The preliminary results were surprising as they showed seasonal variations, which were expected
and unexpected. For instance, sea water temperatures revealed a pronounced seasonal cycle as expected. At the sea surface as well as within the upwelling source waters, sea water temperatures
were lower during the winter/spring cruise SO285 than during the summer/autumn cruise M153. A resulting weaker stratification in winter seem to have also raised oxygen levels within the upwelling source waters by increasing the intrusion of oxygen-enriched surface water via an enhanced vertical mixing. Furthermore, similar to our findings during M153 and to other major upwelling regions, the phytoplankton and mesozooplankton composition showed pronounced cross shelf gradients but the mesozooplankton composition differed also between the SBUS and NBUS. However, in contrast to our expectation of an enhanced productivity in winter/spring, the NBUS chlorophyll a concentrations in line with net samples surprisingly indicated a similar low productivity as during M153. Additionally, on the NBUS shelf, masses of the jellyfish Chrysaora fulgida were found, while mesopelagic fishes were absent in our net hauls. In the SBUS, the near-coastal occurrence of dense and massive diatom blooms implies in line with satellite-derived chlorophyll a concentrations a higher productivity compared to the NBUS. Net
catches at the sea surface with a neuston catamaran as well as catches with a mid-water trawl down to a water depth of 500 m revealed, in turn, a higher abundance of salps on the SBUS than on the NBUS shelf. Enhanced abundance of salps was associated with a lack of sardine larvae, while mesopelagic fish Maurolicus walvisensis was still found in high numbers in the SBUS. These observations support our TRAFFIC hypothesis suggesting that the NBUS and SBUS differ in the
proportion of “dead end” species such as jellyfish and salps, which are assumed to outcompete fishes (jellyfish versus fish in the NBUS and salps versus sardines in the SBUS). However, the data density is still too low to prove trends due to the high spatial and temporal variability of these systems. Nevertheless, we made these observations and are now keen to study possible causes like global warming and impacts of such changes in ecosystem composition on the marine carbon
cycle. Therefore, we will evaluate our underway measurements in combination with high resolution nutrient profiles and results obtained from our drifting and moored sediment trap arrays jointly with satellite data.

Document Type:	Report (Cruise Report)
Programme Area:	PA2
Research affiliation:	Biogeochemistry and Geology > Carbon and Nutrient Cycling Infrastructure > Chemistry Laboratory
DOI:	https://doi.org/10.48433/cr_so285
ISSN:	2510-764X
Date Deposited:	04 May 2022 13:45
Last Modified:	25 Jun 2024 10:56
URI:	http://cris.leibniz-zmt.de/id/eprint/4917

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