Preliminary notes on C-N ppols in sediments from the Sacca di Goro coastal lagoon (Po delta, northern Italy)

A very hot debate is currently focused on submerged substrates that can be classified either as sediments or subaqueous soils. In this paper, the controversy is contextualized on a specific case study, i.e. the Sacca di Goro coastal lagoon facing the North Adriatic Sea. The submerged substrate has been sampled along a a E-W transect and subsequently analyzed to investigate the C-N elemental and isotopic compositions. The recorded concentration of organic carbon (OC) and nitrogen is 0.26-0.50 and 0.02-0.06 wt%, respectively, and the spatial distribution of these parameters highlights a general decrease of concentration from W to E. The carbon isotopic composition of the organic matter (δ13COC) indicates the incorporation within the substrate of significant amount of seaweed and seagrass. Seaweed prevails in the west (δ13COC down to -19‰) due to a nutrient-rich freshwater inflow (Po di Volano), whereas seagrass is preponderant eastward (δ13COC up to -10‰) in a sector of the lagoon fed by riverine waters characterized by lower nutrient load (Po di Goro). The existence of these biological components plausibly implies a lush benthic vegetation, properly rooted on the submerged floor. For this reason, in our view the substrate of the investigated lagoon can be regarded as a subaqueous soil.