Phyto- and protozooplankton assemblages and hydrographic variability during an early winter survey in the Southern Brazilian Continental Shelf

Abstract

Abstract The distribution of phytoplankton and protozooplankton assemblages in relation to hydrographic variability across the southern Brazilian continental shelf were studied based on data collected during early winter (June/2012), complemented with MODIS-Aqua satellite imagery. The in situ data set was comprised by water column structure properties, dissolved inorganic nutrients (ammonium, nitrite, nitrate, phosphate and silicate), phytoplankton and protozooplankton composition and biomass [chlorophyll a (Chl a) concentration and carbon content]. Phytoplankton assemblages were assessed by both microscopy and HPLC-CHEMTAX approaches. A canonical correspondence analysis associating physical, chemical and phytoplankton composition data at surface revealed a tight coupling between the phytoplankton community and hydrographic conditions, with strong environmental gradients across three different domains: the pelagic, outer shelf Tropical Water (TW); the mid shelf domain under influence of Subtropical Shelf Water (STSW); and the inner shelf domain, mainly under influence of riverine outflow of the La Plata River Plume Water (PPW). Results showed that diatoms dominated the phytoplankton assemblages throughout the study region, but other groups were also important and varied according to the different water masses at surface. The low salinity and nutrient-rich PPW stimulated a high phytoplankton biomass and diversity of diatoms and dinoflagellates within the inner shelf region, with enhanced Chl a levels (>1.3 mg m−3) and a great abundance of large and medium-sized diatoms (some of these appeared exclusively in PPW), as well as ciliates, dinoflagellates, raphidophyceans and cryptophytes. Conversely, smaller diatoms (e.g. Rhizosolenia clevei), picoplanktonic species (prochlorophytes and cyanobacteria) and a noticeable contribution of dinoflagellates and other flagellates, associated with lower Chl a levels (