Modelling biogeochemical processes in sediments from the north-western Adriatic Sea: response to enhanced particulate organic carbon fluxes

Daniele Brigolin,Bruno Bombled,Silvia Colla,Christophe Rabouille,Roberto Pastres,Salvatrice Vizzini,Fabio Pranovi

doi:10.5194/bg-15-1347-2018

Abstract

Abstract. 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 particulate organic carbon (POC) downward flux. This flux was first quantified by means of a pelagic model of POC deposition coupled to sediment trap data, and its effects on sediment bioirrigation capacity and organic matter (OM) degradation pathways were investigated constraining an early diagenesis model by using original data collected in sediment porewater. 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 was mostly associated with the dynamics of the farming cycle. Sediment trap data at the two sampled stations (inside and outside of the mussel farm) showed average POC background flux of 20.0–24.2 mmol C m−2 d−1. The difference of organic carbon (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 was 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 O2 influx at the station below the mussel farm (19–31 versus 10–12 mmol O2 m−2 d−1) characterised by higher POC flux. Dissolved inorganic carbon (DIC) and NH4+ concentrations showed similar behaviour, with a more evident effect of bioirrigation underneath the farm. This was confirmed through constraining the early diagenesis model, of which calibration leads to an estimation of enhanced and shallower bioirrigation underneath the farm: bioirrigation rates of 40 yr−1 and irrigation depth of 15 cm were estimated inside the shellfish deposition footprint versus 20 yr−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 mineralisation below the mussel farm (11.1 versus 18.7 mmol m−2 d−1), characterised by similar proportions between oxic and anoxic degradation rates 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.

Highlights

Disturbance gradients in benthic habitats of marine soft sediment can cause shifts in species behaviours and interactions, affecting the biodiversity–ecosystem function relationship (Snelgrove et al, 2014; Villnäs et al, 2013)
Mathematical models of early diagenesis – reaction transport models that allow the simulation of organic matter degradation and kinetic and equilibrium reactions, as well as biologically mediated processes, such as bioturbation and bioirrigation (e.g. Arndt et al, 2013) – can represent useful tools to study sediment biogeochemistry and animal–sediment interactions under specific conditions of organic matter deposition and to generalise results from in situ observation and/or lab experiments (Volkenborn et al, 2012)
Offshore mussel farming is extensively practised in the northern Adriatic Sea, and farmed mussels account for approximately two-thirds of the national Italian production (MiPAAF, 2014)

Summary

Introduction

Disturbance gradients in benthic habitats of marine soft sediment can cause shifts in species behaviours and interactions, affecting the biodiversity–ecosystem function relationship (Snelgrove et al, 2014; Villnäs et al, 2013). D. Brigolin et al.: Modelling biogeochemical processes in sediments web functioning (Rooney et al, 2008). Arndt et al, 2013) – can represent useful tools to study sediment biogeochemistry and animal–sediment interactions under specific conditions of organic matter deposition and to generalise results from in situ observation and/or lab experiments (Volkenborn et al, 2012) Their use for studying the effects of disturbance gradients in coastal sediments was very limited in the past (Paraska et al, 2014)

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