The development of the phytoplankton winter bloom and the accumulation of particulate suspended matter (PSM) inside sediment collectors were assessed in the inner zone of the Bahía Blanca Estuary. The phytoplankton bloom (chlorophyll up to 25 μg l−1 and abundance up to 8 × 106 cells l−1) was related with high levels of dissolved inorganic nutrients and underwater light availability (Im up to 355 μE m−2 s−1) and was dominated by relatively small diatoms, e.g. Chaetoceros sp.1 (3–8 μm). Conversely, large planktonic diatoms, mostly Thalassiosira spp. 20–60 μm, were found in the accumulated material inside the collectors, together with benthic microalgae and high concentrations of chlorophyll, phaeopigments, particulate organic matter (POM between 18 and 32% of total PSM) and C:N ratios >12. The composition of the settled material indicated vertical exportation of phytoplankton to the benthos, external loads of detritus and bottom resuspension. The present study highlights the close benthic‐pelagic interactions in shallow coastal environments characterized by high productivity.
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