Microplankton assemblages in three Bias Baixas (Vigo, Arosa and Muros, Spain) with a subsurface chlorophyll maximum: their relationships to hydrography

Abstract

Dunng summer 1984, when coastal upwelling had diminished, a subsurface chlorophyll maximum (SCM) was found in 3 Galician rías which are charactenzed by strong thermal stratification and a nutricline at 10 to 30 m depth. The SCM. formed directly above the nutricline and was composed mainly of diatoms, although the dinoflagellates Ceratium horridum and Ceratium fusus and the ciliate Mesodinium rubrum were also present. The contribution of M. rubrum was especially important in the center of the Ría de Muros. It was possible to recognize 3 phytoplankton assemblages by means of principal component analysis. Assemblage 1, composed of heterotrophic dinoflagellates and ciliates, was found above the SCM, indicating a possible vertical segregation in the feeding mode when an SCM is formed in the rías. Assemblage 2, consisting of flat dinoflagellates (Dinophysis spp.), elongated diatoms (Nitzschia delicatissima, Rhizosolenia alata gracillina, Nitzschia seriata-like) and the dinoflagellates C. horridum and C. fusus, was present in more oceanic stations, suggesting some kind of interchange between rias and ocean. A third assemblage (diatoms) showed a marked neritic distribution, and was found below the SCM in waters with higher nutrient concentrations. Similarly, canonical correlation analysis showed that all the phytoplankton were characteristic for a post-bloom period, when important trophic relationships began establishing themselves in the system. However, Assemblage 1 was more heterotrophic in character than Assemblage 2. Within this general background there were differences among rias related to hydrography. In Arosa, with a hydrographic structure suggesting more mixing than in the other 2 rías, a cluster composed of diatoms was more abundant and frequent. Muros, with the strongest fluvial input, occupied an intermediate position between Arosa and Vigo.