Mutual Dependence Between Sedimentary Organic Carbon and Infaunal Macrobenthos Resolved by Mechanistic Modeling

Abstract

The mutual dependence between sedimentary total organic carbon (TOC) and infaunal macrobenthos is here quantified by a mechanistic model. The model describes (i) the vertical distribution of infaunal macrobenthic biomass resulting from a trade‐off between nutritional benefit (quantity and quality of TOC) and the costs of burial (respiration) and mortality, and (ii) the variable vertical distribution of TOC being in turn shaped by bioturbation of local macrobenthos. In contrast to conventional approaches, our model emphasizes variations of bioturbation both spatially and temporally depending on local food resources and macrobenthic biomass. Our implementation of the dynamic interaction between TOC and infaunal macrobenthos is able to capture a temporal benthic response to both depositional and erosional environments and provides improved estimates of the material exchange flux at the sediment‐water interface. Applications to literature data for the North Sea demonstrate the robustness and accuracy of the model and its potential as an analysis tool for the status of TOC and macrobenthos in marine sediments. Results indicate that the vertical distribution of infaunal biomass is shaped by both the quantity and the quality of OC, while the community structure is determined only by the quality of OC. Bioturbation intensity may differ by 1 order of magnitude over different seasons owing to variations in the OC input, resulting in a significant modulation on the distribution of OC. Our relatively simple implementation may further improve models of early diagenesis and marine food web dynamics by mechanistically connecting the vertical distribution of both TOC and macrobenthic biomass.

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

“…This is because the freshly deposited OC remains at the seafloor surface and degrades with its first‐order degradation rate at z = 0 cm. The sedimentation rate (i.e., w in equation ) is in general very small (≤1.6 × 10 −3 · cm · day ‐1 , see Zhang & Wirtz, ) in the southern North Sea due to energetic hydrodynamic condition, except at some local sites (e.g., off estuaries) where deposition is sporadically enhanced due to storms or river floods. Without a mixing into deeper sediments by bioturbation the labile and semilabile OC are depleted within 18 and 180 days after deposition, respectively, according to our model setting.…”

“…A mechanistic model, namely, TOC‐MAcrobenthos Interaction Model (TOCMAIM), has been developed to quantify the two‐way interactions between infaunal macrobenthos and TOC in marine sediments (Zhang & Wirtz, ). The theoretical basis of TOCMAIM is built on the hypothesis that (1) the community structure of macrobenthos is mainly dependent on the quality of OC, which further controls bioturbation; (2) bioturbation in turn affects the vertical diffusion of OC; and (3) the vertical positioning of macrobenthos reflects a trade‐off between benefits (i.e., quantity and quality of food) and costs (i.e., respiration and mortality).…”

“…The last term on the right‐hand side represents a net loss due to degradation and macrobenthic uptake (digestion). R i is the depth‐dependent first‐order degradation rate constant of pool i and is calculated by R i ( z ) = e −0.3 z R i (0), where R 1 (0) = 5.5 × 10 −2 · day ‐1 , R 2 (0) = 5.5 × 10 −3 · day ‐1 , and R 3 (0) = 5.5 × 10 −5 · day ‐1 are derived by Zhang and Wirtz (). B ( z , t ) is the time‐dependent biomass density of macrobenthos (g C/m 3 , in ash‐free dry weight) at depth z , ρ s is the sediment density (g/m 3 ), ε i (≤1) is the portion of ingested OC that is not digested by macrobenthos, and γ is the macrobenthic uptake coefficient representing the ratio of the amount of sediment particles processed by macrobenthos to the biomass of macrobenthos per unit time (day).…”

“…In particular, it is suitable to model natural seafloor sediments, which benthic animals have continuously reworked for considerably long (Meysman et al, ). Bioturbation diffusivity D b ( z , t ) in our model is assumed to scale with the local macrobenthic biomass B ( z , t ) through a power law and inversely proportional to the value of local food resource: where β , b are empirical parameters linking body‐size and abundance to biomass (Zhang & Wirtz, ). The above formulation indicates that bioturbation is dependent upon (1) the body‐size and abundance of infaunal macrobenthos (through scaling with biomass) located at z and (2) the intensity of vertical movements of macrobenthos to derive enough nutrition for metabolism and growth.…”

“…TOCMAIM has been calibrated by station data from the North Sea assuming an equilibrium distribution of macrobenthic biomass, that is, the rate of gain G equals to the rate of loss L (Zhang & Wirtz, ). The concept of equilibrium distribution allows us to constrain the unknown model parameters to a certain range.…”