Distribution modeling of seagrasses in brackish waters of Grado-Marano lagoon (Northern Adriatic Sea)

Seagrasses play an important role in coastal lagoons both as primary producers and ecosystem engineers, thus sustaining biodiversity and ecosystem services. In recent decades, their populations have shown a rapid decrease, mainly due to their vulnerability to environmental degradation. Their ecology was widely investigated in the marine domain whereas the knowledge of their distribution patterns in lagoon waters is still not exhaustive. This study aimed at improving the knowledge of seagrass ecology in such ecosystem. Three seagrass species (i.e. Zostera marina, Zostera noltii and Cymodocea nodosa) occurring in Grado-Marano lagoon (Northern Adriatic Sea -Italy) were studied by examining: (i) the distribution of each taxon, (ii) the main water, and (iii) geomorphological variables, gathered in 466 sample points during field surveys and from literature, respectively. Logistic-Generalized Linear Models (GLMs) were used to develop species distribution models (SDM) of seagrass meadows and single species. The seagrass presence was mainly explained by two clear ecological gradients: (i) sea-inner shoreline and (ii) channel-tidal flats. In particular, seagrasses thrive in areas mostly subjected to marine influence, in both terms of proximity to lagoon inlets and main channels, avoiding areas near major rivers mouths. Species distribution models highlighted the crucial role of water salinity and distance from fresh water sources (positively and negatively related, respectively). Zostera marina and Zostera noltii showed similar comparable effects for most of the ecological predictors, with the exception of water depth (ecological vicariance). Cymodocea nodosa showed a wider ecological variability, with lower goodness of model selection.