Morphological traits variability reflects light limitation of phytoplankton production in a highly productive subtropical estuary (Río de la Plata, South America)

Trait-based approaches are increasingly used to understand the responses of phytoplankton communities to changing environmental conditions. This paper analyses phytoplankton morphological traits (i.e. volume, surface to volume ratio) in a highly productive subtropical estuary (Río de la Plata, South America). The goal was to investigate whether morphological variability, weighted by biovolume, supports the hypothesis that light is a key limiting factor for primary producers in this turbid and nutrient-rich estuary. A transect from nearly freshwater to marine stations was sampled with increasing mixing depth, decreasing turbidity and dissolved inorganic nitrogen concentration. Primary production (PP) and total biovolume were highest on the marine side of the turbidity front. In this area, morphologies of different high-order taxa converged towards smaller individual size and higher surface to volume ratio (S/V). Phytoplankton weighted average S/V increased with turbidity along the gradient, while individual size did so with salinity. High S/V and low morphological variability supported higher PP, appearing as responses to cope with light limitation in this environment. Multiple linear regressions indicated that morphological variability alone explained 72 % of PP, reinforcing the utility of trait-based approaches when targeting relations between phytoplankton communities and ecosystem functioning.