The gelatinous component of the mesozooplankton community in the northern Benguela Upwelling System Rolf Koppelmann, Karolina Bohata, Sarina Jung, Pawel Kassatov, Björn Kullmann, Bettina Martin, Christian Möllmann and Marvin Reymers Institute for Hydrobiology and Fisheries Science, Center for Earth System Research and Sustainability, University of Hamburg, Germany
Gelatinous and semi-gelatinous organisms B E Semi-gelatinous zooplankton organisms consisting of Thecosomata (Pteropoda), Chaetognatha and Thaliacea and gelatinous organisms consisting of Cnidaria and Ctenophora are important members of the zooplankton community of the northern Benguela Upwelling System (BUS). The trophic positions of these animals range from herbivore and omnivore (Pteropoda and Thaliacea) to strictly carnivore (Cnidaria, Ctenophora and Chaetognatha). Thaliacea
Problem Cnidaria F Ctenophora There is some evidence that the number of large cnidarians like Chyrsaora spp. and C Aequorea spp. has increased in the northern BUS since the 70th after the collapse of small-pelagic fish stocks (Clupeidae). In recent literature such increases in gelatinous organisms is referred to as jellyfication. However, little is known so far about the G development of smaller Cnidaria and other gelatinous and semi-gelatinous organisms D in the northern BUS. In the framework of GENUS (Geochemistry and Ecology of the Namibian Upwelling System), we investigated the zooplankton community off the
Namibian coast and focussed on this faunal component. Chaetognatha Siphonophora Pteropoda Pteropoda Carnivorous species Herbi-/Omnivorous species A,B,E: from Larink and Westheide (2011); C,D,F,G: from Lalli and Parsons (1993)
Material and Methods Several stations were sampled off Walvis Bay, Namibia, between 2008 4000 3000 Siphonophora parts and 2011 using a multiple closing net (MOCNESS) with 330 µm mesh 2000 15 10 1000 aperture. The distribution and taxonomic composition of gelatinous 5 0 0 and semi-gelatinous organisms was analyzed for different depths from 300 200 Cnidaria and Ctenophora onshore to offshore. 10 100 5 0 0 3 Stable carbon and nitrogen isotope values of the groups were - 16000 14000 determined. Since trophic level calculations over different years are 12000 10000 1000 m 8000 Chaetognatha baseline-sensitive, values of the omni-/herbivorous group Thaliacea 6000 15 4000 10 were used as a baseline and set to zero. 2000 5 Sampling sites off Namibia 0 0
6000 taxa # of 5000
concentration 4000 3000 Thaliacea Results and Conclusion 15 2000 10 Mean 1000 5 40 taxa of Cnidaria (22 Medusae and 18 Siphonophora), 1 taxon of Ctenophora, 20 taxa of Thecosomata 0 0
(Pteropoda), 17 taxa of Chaetognatha and 7 taxa of Thaliacea (3 Doliolida and 4 Salpida) were sampled. 1500 1000 Generally, the lowest diversity of all groups was detected on the shelf with an increasing trend towards the 15 500 Pteropoda 10 open ocean. The highest abundance of the carnivorous groups Coelenterata (Cnidaria and Ctenophora) and 5 0 Chaetognatha was encountered on the shelf, but Cnidaria were also abundant offshore. Pteropoda were most 0 common at the slope stations whereas high numbers of Thaliacea were sampled at the shelf break during a salp bloom in December 2009.
Stable isotope analysis confirmed the trophic level of the groups. Pteropods and Thaliacea compete with micro- and mesozooplankton for food, whereas Cnidaria and Chaetognatha are strong competitors of fish larvae. Oceanic Slope Shelf Shelf break Future research will focus on the long-term temporal development of the gelatinous part of the zooplankton by retrospective analyses and future monitoring, as well as on the role of these faunal components in the Abundance and number of taxa of gelatinous and semigelatinous zooplankton trophic cascade and in elemental cycling.
14 8 Chaetognatha Siphonophora Cnidaria Pteropoda Ctenophora Thaliacea 6 12 TL4Carniv.
4 10 N 15
2 TL3Carniv. δ 8
N baseline corrected baseline N 0 15
δ 6 Mar/Apr 2008 TL2Herbiv. -2 Dec 2009 Sep 2010 Feb 2011 4 -4 -26 -24 -22 -20 -18 -16 -14 -12 -26 -24 -22 -20 -18 -16 -14 -12 13 TL1Phyto δ C δ 13C
Stable isotope data of different groups during Baseline-adjusted stable isotope data, setting Pelagic food web based on an Ecopath with Ecosim-Model. The trophic level of different season Thaliacea to zero the components is based on GENUS field data. The size of the squares represent the biomass. From Auel et al. (unpubl. results)