Oithona nana: population dynamics at the EPEA coastal station (38º28´S 57º41´W) and at the Rio de la Plata estuarine front

CEPEDA, G. C.; HERNANDÉZ, D.; VIÑAS, M. D.

Simposio; 5th International Zooplankton Production Symposium; 2011

The key role of Oithona spp. in coastal marine ecosystems is being increasingly recognised worldwide. In Argentine coastal waters, Oithona nana is one of the dominant copepod species. Nevertheless, studies on the ecology of this species are still incipient. We investigated the annual population dynamics (copepodites CI-CV and adults) and seasonal variations in prosome length (PL) and reproductive parameters of this species at the EPEA (Estación Permanente de Estudios Ambientales) coastal station (38º28´S 57º41´W). The same ecological parameters were studied in two sections of the Rio de la Plata estuarine front (RdPF) during spring. O. nana occurred throughout the year at the EPEA station, varying between 36 ind. m-3 (0.07 mg C m-3) in May and 5,800 ind. m-3 (7.58 mg C m-3) in January. PL of each stage was shortest in summer-autumn compared to spring-winter. Clutch size was constant, but egg diameter varied seasonally. At RdPF, O. nana abundances increased seawards from 761 ind. m-3 (0.98 mg C m-3) at the inner brackish water stations to ~7,000 ind. m-3 (9 mg C m-3) at the midestuarine stations. At the offshore marine stations, O. nana was replaced by O. helgolandica (sensu Ramirez, 1969). PL of each stage varied spatially without a discernible pattern. No variation was detected in clutch sizeor in mean egg size at RdPF. During spring, O. nana was two (females, CIV, CV) or even three (CI-CIII) times higher at RdPF than at EPEA, whereas abundance of males was similar. The results will be discussed in relation to the environmental variables, and the influence of the RdPF influence will be highlighted.