Abstract
The seasonal variation of diatoms in the inshore waters off Paraná State, Southern Brazil was investigated to analyse their temporal dynamics and to detect the main environmental constraints of the planktonic community. Biomass peaks occurred from May to August and from December to March. Among the microplanktonic diatoms, Cerataulina pelagica, Chaetoceros spp., Dactyliosolen fragilissimus, Guinardia striata, Lauderia annulata, Leptocylindrus spp., Pseudo-nitzschia cf. delicatissima, P. australis, Rhizosolenia spp., Skeletonema costatum and Thalassionema nitzschioides were dominant species. Nanoplanktonic diatoms were dominated by Naviculaceae, Nitzschia spp., Thalassiosira spp. and Chaetoceros cf. tenuissimus. Concentrations of most of the species decreased during and just after the blooms of Phaeocystis pouchetii in September and of Coscinodiscus wailesii in April. Based on cluster analysis and interpretations of the environmental parameters monitored, six diatom associations were discerned. Four main environmental factors were attributed as the determinants for the grouping: (a) the stronger influence of the tropical oligotrophic waters of the Brazil Current in spring/summer, (b) the alternation between dry and rainy (nutrient-richer) seasons, (c) the influence of subantarctic waters mixed with coastal ones in fall/winter, bringing cold species and promoting the growth of autochthonous species, and (d) the blooms of nanoplanktonic (Phaeocystis) and microplanktonic (C. wailesii) species.
Highlights
Phytoplankton studies of South Brazilian waters have documented its composition, distribution, biomass and primary productivity in relation to hydrographic conditions
The environmental factors determining the changes in the structure of diatom associations in coastal areas off South Brazil appear to be seasonal fluctuations in the hydrographic regime and land drainage, which affect temperature, salinity and nutrient concentrations (Brandini, 1990)
Off Rio Grande State (3842°S), the tropical waters meet the colder, nutrientrich waters of the Malvinas Current flowing northwards over the shelf and continental slope of Argentina and Uruguay, which are greatly affected by freshwater discharges from the La Plata river and the Patos Lagoon (Boltovskoy, 1981; Hubold 1980, 1980a)
Summary
Phytoplankton studies of South Brazilian waters have documented its composition, distribution, biomass and primary productivity in relation to hydrographic conditions (see review of Brandini et al, 1997 and references therein; Fernandes & Brandini, 1999). As for other investigations carried out in different marine ecosystems around the world, these works have found that diatoms (Bacillariophyta) are responsible for high cell densities and most of the primary productivity. They dominate the phytoplankton community in midshelf and coastal regions of southeastern Brazil, gradually diminishing oceanwards, where the relative contribution of dinoflagellates and nanoplankton increases (Brandini & Fernandes, 1996; Brandini et al, 1997). The influence of the Malvinas Current is stronger in the winter, when the penetration of a tongue of subantarctic waters is regularly detected along the south Brazilian continental shelf (Campos et al, 1996)
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