Phytoplankton morphological traits in a nutrient-enriched, turbulent Mediterranean microtidal lagoon

High environmental variability is a feature of transitional water ecosystems, which also affects temporal and spatial patterns of phytoplankton. The Lagoon of Venice is characterised by a wide range of environmental conditions, high phytoplankton species diversity and abundance variability. Here we focus on morphological traits and specifically, for the first time in transitional waters, on phytoplankton geometric shape. Our hypothesis is that despite high taxonomic diversity, the prevalent features of the lagoon (shallow, permanently nutrient-enriched, turbulent and light-attenuated) should select a small number of dominant morphological types and adaptive strategies. We tested the hypothesis on a 13-year (1998-2010) phytoplankton data series and related abiotic factors. Seven shapes were found to be dominant and widespread in space and time in the lagoon, with two adaptive strategies: high surface area-to-volume ratio (S/V) together with low greatest axial linear dimension (GALD), and low S/V together with high GALD. Certain morphological traits, which allow a high S/V (from 0.8 to >1), appear quite constant: (i) small size (GALD < 15 ?m), in particular in round-shaped organisms, and (ii) high GALD together with shape attenuation, within the same species or across a range of species. We observed that the lagoon regime selects for the coexistence of competing species with similar morphological traits.