Hydrography and phytoplankton of the Ría de Vigo before and during a red tide of Gymnodinium catenatum Graham

Abstract

At the end of summer 1986, hydrographic data and phytoplankton samples were collected on three occasions in the Ría de Vigo. The phytoplankton composition was studied in relation to the hydrography using principal component analysis and canonical correlation analysis. Upwelling and the response of the plankton community emerge as the main source of variation in the analyses Within this framework, nutrient regeneration was observed, partly attributable to ciliates and small flagellates. The second source of variation of the phytoplankton was the presence in the later samples of a Gymnodinium catenatum red tide. Two other phytoplankton populations were clearly differentiated One oceanic, dominated by Erythropsis sp, Ceratium horridum and Stauroneis membranacea, which practically disappeared when the red Ode was established. The other, dominated by Solenicola setigera, dinoflagellate cysts and elongated diatoms, was a permanent population located below the populations which responded to upwelling. The penetration of warm oceanic water into the ría during the second day of observations altered the distributions of all the planktonic populations, isolating them in a downwelling zone. The energy of the oceanic intrusion could have been sufficient to remove sediments from the interior of the ría and to stimulate the formation of the red tide, by confining a water body in the ría which lost little by mixing. It is likely that the resuspension of G.catenatum cysts provided the necessary inoculum.