Biomass and composition of the phytoplankton in the Río de la Plata: large-scale distribution and relationship with environmental variables during a spring cruise

The distributions of phytoplankton taxa and biomass were recorded to assess their association to environmental variables in the Río de la Plata, a shallow and highly turbid estuary. Forty-seven CTD stations covering the whole estuary were sampled from 5 to 16 November 2001. At 31 stations, dissolved oxygen concentration, pH, and light intensity at 50 cm depth intervals were recorded in situ; also, samples were taken for measurement of dissolved nutrient levels (nitrate, ammonium, phosphate, and silicate), suspended particulate matter and particulate organic matter loads, chlorophyll (a, b, and c) concentration, and phytoplankton identification. Stations ordination (multi-dimensional scaling, MDS, analysis) on environmental data suggested two main groups (upper-middle and lower estuary) but stressed the gradual change of conditions along the ecosystem. MDS on pigment data suggested a tendency for higher biomass levels in the lower estuary; MDS based on species indicated two main groups corresponding to lower and upper-middle estuarine stations. Spearman rank correlations of environmental and pigment similarity matrices suggested that salinity, the ratio of mixed to photic depth, and nitrates were the variables best explaining pigments distribution; in turn, the variables best explaining species distribution were salinity and seston. Results suggest an overriding importance of salinity-light gradients in modulating biomass levels and species distribution in the Río de la Plata. This result is consistent with processes known to modulate phytoplankton distribution and production in other highly turbid estuaries.