Wind effect as forcing factor controlling distribution and diversity of copepods in a shallow temperate estuary (Solis Grande, Uruguay)

Abstract

Spatial and temporal variations of planktonic copepods were investigated in relation to environmental conditions in the Solís Grande estuary (34°22´S, 55°33´W), Uruguay. Over a period of fifteen days, samples were taken daily at three stations along the main axis of the estuary, and the species composition and abundance were determined. The Solís Grande is a shallow estuary (2.0 m) with no vertical discontinuities. Comparisons of surface versus bottom hydrographic values indicated no vertical stratification of temperature and salinity during the studied period. The species-environmental relationships explain 63.6% of the system´s variability considering the two first axes from Redundance Analysis (RA). Copepods showed strong differences in distribution and abundance between stations and successive days. In the region the winds quickly rotate from sectors S-SE (frontal period) to E-NE (post-frontal period). Changes in abundance were significantly related to the inflow and outflow of water produced by the rotation of winds. At smaller space and time scales, the differences highlighted by the RA were the result of wind-forced hydrodynamics after the frontal period. In this situation hydrographic features were dominant (factor one of RA) and wind effects were of secondary importance (factor two of RA). Six species were identified and the estuary was clearly dominated by Acartia tonsa. The results showed two negatively correlated groups: one was integrated by Oithona nana, O. simplex, Paracalanus parvus and P. crassirostris while the second group was integrated by A. tonsa and Euterpina acutifrons. Environmental variability may be responsible for the low diversity.