Dynamics and sources of organic carbon in suspended particulate matter and sediments in Pialassa Baiona lagoon (NW Adriatic Sea, Italy)

Abstract Organic carbon (OC), total nitrogen (TN) and stable carbon isotopic signature (δ 13 C) of suspended particulate matter and of surface sediments were investigated to assess temporal dynamics and relative contribution of autochthonous and allochthonous OC sources in the coastal eutrophic Pialassa Baiona lagoon. Water sampling for particulate organic carbon (POC), total nitrogen (PTN), nutrients and chlorophyll a (Chl a ) concentrations, and bacterial abundance was carried out over a year. Seasonal changes of δ 13 C POC signature and C/N ratio discriminated two main areas within the lagoon: the partially preserved northern-central area and the southernmost impacted area. Except for the southernmost impacted area, δ 13 C POC (mean value: 22.11‰) showed the highest depleted values in winter and the most enriched in summer following phytoplankton seasonal development as also suggested by changes in C/N ratios, and by the significant correlations found between Chl a and POC, and δ 13 C POC . Moreover, the two areas showed different timing and isotopic signature of phytoplankton blooms (e.g. Chl a 23.22 and 29.27 μg L −1 , and δ 13 C POC  −19.15 and −30.70‰, June and March, in the northern-central and southern area, respectively). The summer bloom in the northern-central area was also associated with high bacterial abundances (up to 7.30 × 10 9  cells L −1 ), suggesting the establishment of a strong structured microbial food loop and organic matter recycling. Surface sediments from the southern impacted area showed significant higher values in OC and TN contents (3.05 and 1.44%), and significant depleted δ 13 C signal (−23.03‰) when compared to the control area (1.09 and 0.16%, −19.40‰); whereas no differences were found in C/N ratios (8.1 and 8.2, in northern-central and southern impacted area, respectively). Elemental and isotopic composition data showed a strong coupling between POC and surface sediments. The relative contribution of three different sources (marine, estuarine, terrestrial) to POC and surface sediments were estimated using a mixing model, which predicted a predominant fraction of marine phytoplankton in POC during spring-summer and in surface sediments from the northern-central area. Conversely, dominant allochthonous sources were predicted for POC in winter months and in impacted area sediments. δ 13 C values of surface sediment reflected an isotopic overprint of refractory terrestrial-derived (allochthonous) organic carbon agreeing with urban/industrial wastewaters origin.