Modelling biogeochemical processes in sediments from the northwestern Adriatic Sea: response to enhanced POC fluxes

This work presents the result of a study carried out in the north-western Adriatic Sea, by combining two different types of biogeochemical models with field sampling efforts. A longline mussel farm was taken as a local source of perturbation to the natural POC downward flux. This flux was first quantified by means of a pelagic model of POC deposition coupled to sediment traps data, and its effects on sediment bioirrigation capacity and OM degradation pathways were investigated by constraining an early diagenesis model, linked to new data in sediment porewaters. The measurements were performed at stations located inside and outside the area affected by mussel farm deposition. Model-predicted POC fluxes showed marked spatial and temporal variability, which were mostly associated with the dynamics of the farming cycle. Sediment traps data at the two sampled stations (in and out of the mussel farm) showed average POC background flux of 20.0–24.2 mmol C m −2  d −1 . The difference of OC fluxes between the two stations was in agreement with model results, ranging between 3.3 and 14.2 mmol C m −2  d −1 , and primarily associated with mussel physiological conditions. Although restricted, these changes in POC fluxes induced visible effects on sediment biogeochemistry. Observed oxygen microprofiles presented a 50 % decrease in oxygen penetration depth (from 2.3 to 1.4 mm), accompanied by an increase in the O 2 influx at the station below the mussel farm (19–31 versus 10–12 mmol O 2  m −2  d −1 ) characterized by higher POC flux. DIC and NH 4 + concentrations had similar behavior, with a more evident effect of bioirrigation underneath the farm. This was confirmed through constraining the early diagenesis model, which calibration leads to an estimation of enhanced and shallower bioirrigation underneath the farm: bioirrigation rates of 40 y −1 and irrigation depth of 15 cm were estimated inside the shellfish deposition footprint versus 20 y −1 and 20 cm outside. These findings were confirmed by independent data on macrofauna composition collected at the study site. Early diagenesis model results indicated a larger organic matter mineralization below the mussel farm (11.1 versus 18.7 mmol m −2  d −1 ), characterized by similar proportions between oxic and anoxic degradation pathways at the two stations, with an increase in the absolute values of oxygen consumed by OM degradation and reduced substances re-oxidation underneath the mussel farm.