The Budget of Macrobenthic Reworked Organic Carbon: A Modeling Case Study of the North Sea

Abstract

The importance of macrobenthos in benthic‐pelagic coupling and early diagenesis of organic carbon has long been recognized but has not been quantified at a regional scale. By using the southern North Sea as an exemplary area we present a modeling attempt to quantify the budget of total organic carbon (TOC) reworked by macrobenthos in seafloor surface sediments. Vertical profiles in sediments collected in the field indicate a significant but nonlinear correlation between TOC and macrobenthic biomass. A mechanistic model is used to resolve the bidirectional interaction between TOC and macrobenthos. A novelty of this model is that bioturbation is resolved dynamically depending on variations in local food resource and macrobenthic biomass. The model is coupled to 3‐D hydrodynamic‐biogeochemical simulations to hindcast the mutual dependence between sedimentary TOC and macrobenthos from 1948 to 2015. Agreement with field data reveals a satisfactory model performance. Our simulations show that the preservation of TOC in the North Sea sediments is determined not only by pelagic conditions (hydrodynamic regime and primary production) but also by the vertical distribution of TOC, bioturbation intensity, and the vertical positioning of macrobenthos. Macrobenthos annually ingest 20–35% and in addition vertically diffuse 11–22% of the total budget of TOC in the uppermost 30‐cm sediments in the southern North Sea. This result indicates a central role of benthic animals in modulating the organic carbon cycling at the sediment‐water interface of continental margins.

What global biogeochemical consequences will marine animal–sediment interactions have during climate change?

“…However, few included dynamic benthic ecological elements, specifically coupling biogeochemical fluxes and microbes (Yakushev et al, 2017), or feedbacks between OC supply and macrofaunal biomass (Ehrnsten et al, 2019) and their responses to hypoxia and nutrient loading (Soetaert and Middelburg, 2009). Furthermore, ecosystem models only recently linked macrofaunal dynamics to changes in biological transport and their effects on biogeochemical fluxes (Daewel and Schrum, 2013;Butenschön et al, 2016;Zhang and Wirtz, 2017), inducing strong temporal dynamics resulting from high sensitivity to environmental drivers (Timmermann et al, 2012;Ehrnsten et al, 2019;Zhang et al, 2019). The major limitation of such coastal ecosystem models rests in their highly simplified representation of the vertical sedimentary structure, which cannot resolve strong biogeochemical and biological spatial gradients with sediment depth.…”

“…The major limitation of such coastal ecosystem models rests in their highly simplified representation of the vertical sedimentary structure, which cannot resolve strong biogeochemical and biological spatial gradients with sediment depth. Recent explicit representations of organism-sediment interactions as a function of depth in sediment cores (Zhang et al, 2019) nonetheless link dynamics to local macrobenthic biomass and food resources, both of which may vary temporally and spatially.…”

“…The vertically resolved coupling of OC degradation, macrobenthos dynamics, and biologically induced transport recently developed at the scale of coastal ecosystem models could provide an important building block for a unified modeling framework (Zhang et al, 2019). These ecosystem models do not resolve the sharp biogeochemical gradients necessary to predict the impact of changing macrobenthos and associated transport regimes on biogeochemical cycles.…”

Identifying Trawl Marks in North Sea Sediments

“…Mérillet et al [16] observed that bioturbation could lead to a lower persistence of TM. In general, bioturbation is higher in muddy than in sandy sediments [63], therefore, bioturbation is likely not a significant controlling factor for the preservation of TM in the three study sites, where sediments are mainly made by sand with a very low mud content. However, in their model, Zhang et al [63] described a pattern of increased bioturbation activity at Dogger Bank during summer.…”

“…In general, bioturbation is higher in muddy than in sandy sediments [63], therefore, bioturbation is likely not a significant controlling factor for the preservation of TM in the three study sites, where sediments are mainly made by sand with a very low mud content. However, in their model, Zhang et al [63] described a pattern of increased bioturbation activity at Dogger Bank during summer. As a consequence, the TM at the DB site should be less stable during summer months, which is in contrast with the highest TBB mark densities recorded in that period in our datasets.…”