Zooplankton community composition, distribution and trophic structure in the San Jorge Gulf during austral summer

ELOÍSA MARIANA GIMÉNEZ

Rimouski

Workshop; Workshop on the PROMESse project: results and opportunities; 2017

ISMER, UQAR, MINCyT, Gob. de Chubut

The objective of this study was to investigate zooplankton community structure, to determine its relation with environmental parameters and to examine its food webs in the San Jorge Gulf (SJG, 45°-47°S, Southwestern Atlantic Ocean). Sampling was conducted in February 2014, comprising 14 stations covering the entire SJG with a net of 243µm mesh size. Temperature, chlorophyll a and stratification of water column allowed identifying two zones comprising the north-center and the south of SJG, characterized by unstratified and colder water column. Those two water masses encompass two distinctive zooplankton assemblages. The north-central zone was strongly influenced by the small copepod Ctenocalanus vanus, copepodites stages of C.vanus, Clausocalanus brevipes and Paracalanus parvus, appendicularians of genus Oikopleura and the cyclopoid Oithona helgolandica. The southern zone was characterized by the small copepod P.parvus. copepodites stages of C.vanus, C. brevipes and P.parvus, the cosmopolite copepod Acartia tonsa and Drepanopus forcipatus. Zooplankton community was composed of 30 taxa, being copepods the main component representing 83% of total abundances. Mean zooplankton abundance and biomass throughout the SJG ranged from 668 to 5006 ind m-3 and 6 to 49 mg DW m-3, respectively. Community structure was most strongly related to surface temperature and stratification of the water column. We used stable isotope analyses (13C/12C and 15N/14N) to evaluate the trophic structure of zooplankton community in three zones of the SJG and to compare trophic roles of major taxa throughout the SJG. Surface and bottom particulate organic matter (POM) was enriched in 13C from north to south. More negative values in the north may be explain by terrigenous inputs introduced in the gulf by the strong westerly winds Large variations were found in 13C values of zooplankton taxa within each zone and among zones. The spread of values (-16.8 ± 1.5 to -30.6 ± 3.4) reflects a wide diversity of food sources among the taxa analysed. A strong tendency to enrichment in 13C from north to south was observed in the majority of the FW components. Higher 13C values in the southern zone FW might be due to the grazing on diatoms, as first carbon source. Biplots of 13C vs. 15N were used to provide a good visualization of zooplankton FW structure in the SJG. A strong distinction between the different taxa and their trophic positions was shown. Narrow trophic space observed in southern FW could indicate similar feeding strategies within the taxa. Chaetognathes occupy the highest trophic position in the north and in the center, while in the south its position was replaced by the ubiquitous copepod A. tonsa. Appendicularians showed the lowest trophic levels (TL) Two and three TL were found in the north and in the center, respectively. An increase in TL values was observed from north to south for copepods, euphausiids, appendicularians and chaetognathes. This occurs due to a decrease in POM 15N values observed from north to south. Thus, TL should be considered as relative values for the FW of SJG rather than absolute values. Further studies should integrate complementary approaches (i.e: fatty acids analyses), microzooplankton and algae data to add substantial information in trophic structure of zooplankton FW in the SJG.