The central importance of macrobenthos in benthic-pelagic coupling in coastal shelf seas

Benthic oxygen fluxes measured in the south-eastern North Sea indicate a prominent annual cycle characterized by a low level between mid-autumn (October) and early spring (March), a slow increase since mid-spring (April) till late summer (late August/early September), and a subsequent accelerated decrease in early autumn (September). A significant positive correlation between the benthic oxygen flux, total organic carbon (TOC) and macrobenthic biomass in surface sediments suggests their potential mutual dependence. To understand their interactions quantitatively, 3-D benthic-pelagic coupled modelling was used to reconstruct the benthic status. Simulation results based on a satisfactory agreement with field data reveal that the benthic oxygen flux is determined by not only pelagic drivers (hydrodynamics, temperature and primary production) but also internal dynamics associated with the interaction between organic carbon and benthic fauna, and bedform morphodynamics. The slow increase of benthic flux since mid-spring till late summer is a compound effect of several processes with dominant contribution by accumulation of labile OC and growth of macrobenthos in surface sediments. Bioturbation intensity peaks in late summer, resulting in highest oxygen flux into sediments and promoting remineralization of subsurface OC and release of nutrients. Shutdown of pelagic primary production in combination with enhanced wind-waves in early autumn cause a systematic shift of the benthic carbon pool from deposition to erosion within a few weeks, accounting for the accelerated decrease of benthic oxygen flux. Our results indicate a central role of macrobenthos in modulating the rate of both solute and solid fluxes across the sediment-water interface.