Plankton Community and Environmental Conditions During a Mid Shelf Waters Intrusion and Upwelling at the EPEA Station (Argentina)

NEGRI, R.M.; AKSELMAN, R.; CARIGNAN, M.O.; CUCCHI COLLEONI, A.D.; DÍAZ, M.V.; DIOVISALVI, N; HOZBOR, C.; LEONARDUZZI, E.; LUTZ, V.; MOLINARI, G.; PÁJARO, M.; SILVA, R. I.; SEGURA, V.; VIÑAS, M.

Foz do Iguaçu

Congreso; The Meeting of The Americas (AGU); 2010

During December 2008 sea surface temperature (SST) and chlorophyll a (Chla) concentrations were elevated with respect to those registered for the same time of the year since 2000 at the Coastal EPEA station (38º 28’ S – 57º 41’ W); part of the Antares-ChloroGIN networks. In this work planktonic community and environmental variables measured during this special event are analyzed in relationship to data collected at the time-series for the months of November-December-January. Variables considered were (field): temperature, salinity, nutrients, Chla, bio-optical properties, bacterioplankton, phytoplankton, protozooplankton, zooplankton, and icthyoplankton; as well as wind fields and satellite information on SST and Chla. Field SST showed an anomaly of 4 ºC below the average for the period, while mid-depth (16 to 24 m) temperature was 6 ºC below the average (considering a historical field data-base 1986-2009). Salinity, homogenous in the water column, was one of the lowest of the series (33.4). Nitrate concentration at the surface was similar to the average for the period, while that at depth was the highest for the series. Surface Chla concentration was within the higher, being that at 12 m the highest registered (14.6 mg m-3). Phytoplankton was responsible for a strong light absorption, being the depth of the 1 % surface irradiance at 12 m. Diatom concentrations were unusually high (> 106 cells L-1) mainly Chaetoceros spp. and Leptocylindrus spp., the presence of Thalassiosira spp. and Emiliania huxleyi, species not common of this coastal zone, was marked. Coccal forms of picophytoplankton reached high concentrations (22x103 cells L-1). Among the copepods it was notorious the absence of Oithona nana, a usually abundant coastal species, together with the presence of typical shelf species Drepanopus forcipatus, O. atlantica, O. helgolandica y Ctenocalanus vanus. Equally rare was the absence of eggs of Engraulis anchoita in the icthyoplankton, while concentrations of larvae were the usual (1.23 larvae m-3). At the end of November satellite images of SST showed a noticeable cooling of a large sector of the shelf reaching the coast. At that time, and at beginning of December, persistent N-E winds were registered, causing upwelling events. These processes were detectable in the SST images, and would be responsible for the elevated Chla concentrations seeing in the satellite images, which were coincident with in situ data. Our observations at EPEA would indicate two events, one the entrance of Mid Shelf Waters –colder, richer in nutrients, with different planktonic species-, and the other high phytoplankton concentrations related to upwelling. Even though these upwellings could be explained by the persistent N-E winds, linked to the south shift of the South Atlantic Anticyclonic Gyre, the mechanism provoking the advection of Mid Shelf Waters towards the coast is still unknown.