Zhang, W., Neumann, A., Daewel, U., Wirtz, K., van Beusekom, J.E.E., Eisele, A., Ma, M., & Schrum, C. (2021): Quantifying importance of macrobenthos for benthic-pelagic coupling in a temperate coastal shelf sea. Journal of Geophysical Research: Oceans, 126, e2020JC016995, doi:10.1029/2020JC016995
Benthic oxygen fluxes consist mostly of advective and diffusive terms. Both terms in the German Bight exhibit a prominent annual cycle but with opposite variation patterns. To understand the driving mechanisms quantitatively, a novel 3-D benthic-pelagic coupled model resolving interactions among macrobenthos, bioturbation, oxygen consumption and carbon early diagenesis was applied to reconstruct the benthic states. Simulation results show a satisfactory agreement with field data and reveal that the benthic oxygen flux is determined by not only pelagic drivers but also by internal dynamics associated with the interaction between organic carbon and macrobenthos, and bedform morphodynamics. Variation of advective flux, characterized by summer-low and winter-high, is mainly driven by hydrodynamics and bedform morphodynamics, while variation of diffusive flux, featured by summer-high and winter-low, is a compound effect of pelagic and benthic drivers with a dominant control by macrobenthos through bioturbation. The role of bioturbation in benthic oxygen consumption is twofold: (i) on the one hand, it alters the particulate organic carbon (POC) distribution in surface sediments, thereby changing the availability of POC to oxygen consumption; (ii) on the other hand, it mixes oxygen down into sediments, thereby facilitating oxygen consumption. Our results indicate that the first role prevails in sandy seafloor characterized by energetic hydrodynamics, while the second role becomes increasingly important along with a weakening of bottom currents. We found that bioturbation contributes up to 874 % and 55 8 % of the total benthic oxygen fluxes in muddy seabed and at a regional scale (the German Bight), respectively.
Tian, D., Zhou, F., Zhang, W., Zhang, H., Ma, X., & Guo, X. (2021): Effects of dissolved oxygen and nutrients from the Kuroshio on hypoxia off the Changjiang River estuary. J. Ocean. Limnol., doi:10.1007/s00343-021-0440-3
The intrusion of the Kuroshio into the East China Sea (ECS) affects the development of hypoxia off the Changjiang (Yangtze) River estuary; however, quantitative analysis of its impacts is lacking. In this study, the Regional Ocean Modeling Systems (ROMS) model coupled with the Carbon, Silicate and Nitrogen Ecosystem (CoSiNE) model was used to investigate the relative importance of dissolved oxygen (DO) and different nutrients (silicate, nitrate, and phosphate) in the Kuroshio on hypoxia in the ECS. Results show that changes in DO concentrations in the Kuroshio modify the distribution and intensity of hypoxia through direct onshore transport by hydrodynamic processes. An increase in Kuroshio DO concentration by 25% or 50% would result in a decrease of the maximum hypoxia extent (MHE) in the ECS by 76% or 86%, respectively, while a 25% decrease in Kuroshio DO would increase the MHE by up to 219%. The contribution of DO in the Taiwan Strait is almost negligible. In contrast to Kuroshio DO, nutrients affect hypoxia in the ECS through onshore transport by hydrodynamic and biochemical processes. Changes in phosphate and nitrate concentrations by 25% in the Kuroshio would change the MHE by up to 30% and 18%, respectively, accompanied by apparent changes in surface chlorophyll-a concentrations. The effect of silicate on hypoxia is negligible because a 25% change in silicate concentrations in the Kuroshio would result in less than 1% change in the MHE. Our results reveal a hierarchical rank of importance for environmental variables in the Kuroshio (i.e., DO > phosphate > nitrate > silicate) in modifying the development of hypoxia in the ECS.