Possible transport pathway of diazotrophic Trichodesmium by Agulhas Leakage from the Indian into the Atlantic Ocean.

Diazotrophic cyanobacteria such as Trichodesmium play a crucial role in the nitrogen budget of the oceans due to their capability to bind atmospheric nitrogen. Little is known about their interoceanic transport pathways and their distribution in upwelling regions. Trichodesmium has been detected using a Video Plankton Recorder (VPR) mounted on a remotely operated towed vehicle (TRIAXUS) in the southern and northern Benguela Upwelling System (BUS) in austral autumn, Feb/Mar 2019. The TRIAXUS, equipped with a CTD as well as fluorescence and nitrogen sensors, was towed at a speed of 8 kn on two onshore-offshore transects undulating between 5 and 200 m over distances of 249 km and 372 km, respectively. Trichodesmium was not detected near the coast in areas of freshly upwelled waters but was found in higher abundances offshore on both transects, mainly in subsurface water layers down to 80 m depth with elevated salinities. These salinity lenses can be related to northward moving eddies that most probably have been detached from the warm and salty Agulhas Current. Testing for interaction and species-habitat associations of Trichodesmium colonies with salinity yielded significant results, indicating that Trichodesmium may be transported with Agulhas Rings from the Indian Ocean into the Atlantic Ocean.

Chaetognatha of the Namibian upwelling region: taxonomy, distribution and trophic position.

In October 2010, the vertical distribution, biodiversity and maturity stages of Chaetognatha species were investigated at four stations located off Walvis Bay, Namibia. Seventeen species were detected and classified as pelagic, shallow-mesopelagic, deep-mesopelagic and bathypelagic species based upon the weighted mean depth derived from their average vertical distribution. High abundances of Chaetognatha were found in the upper 100 m at all stations of the Walvis Bay transect with a maximum value of 20837 ind. 1000 m(-3) at the outer shelf station near the surface. The community was dominated by species of the Serratodentata group. Furthermore, the distribution of Chaetognatha did not seem to be influenced by low oxygen concentrations. Stable isotope ratios of carbon and nitrogen in Chaetognatha were determined for seven different areas located off northern Namibia. The values of δ(15)N ranged from 6.05 ‰ to 11.39 ‰, while the δ(13)C values varied between -23.89 ‰ and -17.03 ‰. The highest values for δ(15)N were observed at the Walvis Bay shelf break station. The lowest δ(13)C values were found at the Rocky Point offshore station, which was statistically different from all other areas. Stable isotopes of carbon and nitrogen were determined for four taxa (Sagitta minima, Planctonis group, Sagitta enflata, Sagitta decipiens). In this case, the δ(15)N values ranged from 6.17 ‰ to 10.38 ‰, whereas the δ(13)C values varied from -22.70 ‰ to -21.56 ‰. The lowest δ(15)N values were found for S. minima. The C- and N-content revealed maximum C-values for S. decipiens and maximum N-values for the Planctonis group. The C:N ratio of Chaetognatha ranged between 5.25 and 6.20. Overall, Chaetognatha are a diverse group in the pelagic food web of the Benguela Upwelling System and act as competitors of fish larvae and jelly fish by preying on copepods.