Benthic fluxes of oxygen, carbon and nutrients in the Marano and Grado Lagoon (northern Adriatic Sea, Italy)

Benthic metabolism and carbon and nutrient cycling at the sediment-water interface were studied seasonally in the Marano (sites MB and MC) and Grado Lagoon (sites ART and BAR), northern Adriatic Italy, using porewater vertical profiles and daily fluxes of O2, DIC, DOC, NO3-, NH4+, PO43- and SiO44- measured in situ deployed transparent and dark benthic chambers. Diffusive and benthic fluxes of solutes were evaluated on a seasonal basis. Sites MC and ART were characterized by higher Corg. contents due to input of riverine organic matter and mariculture, respectively. The Corg., Ntot., Ptot., Porg. and Sibiog. contents decreased along the sediment cores while porewater concentrations of DIC, DOC, NO3-, NH4+, PO43- and SiO44- increased along the sediment cores at all study sites due to the degradation of labile sedimentary matter. Higher concentrations of all porewater solutes and higher diffusive fluxes were observed in warmer periods. Benthic fluxes of O2, DIC, NO3-, NH4+, PO43- and SiO44- showed intensive seasonal variations. Based on O2 and DIC metabolism, the lagoon sediments were highly heterotrophic except at BAR being in trophic balance or weakly heterotrophic. NO3- and SiO44- exhibited influxes due to intense microphytobenthic assimilation, mostly by diatoms, and denitrification while extremely low PO43- fluxes suggest P as a limiting factor. The great difference observed between the diffusive and the in situ benthic fluxes suggests the importance of bioturbation and that the pertinent processes occur at the sediment-water interface. Tentative annual budgets of carbon and nutrients in surface sediments of studied sites indicate that their cycling, compared to burial flux, is more 39 intensive at the sediment-water interface. These basic benthic biogeochemical processes can be important to better understand the trace metal cycling, especially Hg mobilization and sequestration, in the lagoon environment.