Unifying theories for global benthic biomass distribution: the relative importance of water column and topographic drivers

Abstract

Most benthic fauna in the deep sea relies on the flux of organic carbon synthesised in the surface ocean as a source of food. Particulate organic carbon, derived from primary and secondary production, sinks through the water column and some of this is remineralised; therefore, vertical food input to the benthos decreases as depth increases. This results in a reduction of biomass along the depth gradient, which can be observed across the full benthic size spectrum. Nevertheless, unexpectedly large benthic standing stocks have been observed, generally associated with topographic features such as seamounts and trenches. At these sites, biomass can be higher than what would be expected under normal conditions of vertical flux and attenuation of organic carbon. Lateral fluxes of organic particles are believed to sustain the growth of excess biomass in these areas, which could not be sustained by vertical fluxes alone. In this thesis, I show how the synergy of hydrodynamic and gravitational processes can explain the distribution of benthic communities around three types of topographic features in the deep sea. In fact, a positive effect of increasing slope and relative elevation on biomass (a more hydrodynamic scenario) can be reduced or reversed when the slopes are steeper (a more gravitational scenario). Furthermore, a small effect of the interaction between current direction and seafloor morphology is detected, suggesting an asymmetric distribution of particular organic matter settling around topographic features. Using knowledge from these processes, a global model for benthic biomass distribution is built that more fully considers seafloor morphology in comparison to earlier global seafloor biomass models. This model also improves the resolution of an existing seafloor biomass model by 120 times. The promising results highlight that the effect of seafloor morphology on benthic biomass can be detected at global scale. Nevertheless, the small number of data points and the related limited ranges in the variables covered by the dataset greatly reduce the model’s predictive power. While this thesis focuses on a relatively remote portion of the planet’s biome, its scope is much wider. Global benthic biomass has been forecasted to decrease by up to 5% under climate change scenarios with some areas declining nearly 50%. Therefore, accurate estimates of carbon stocks and fluxes through the benthic community, and their spatial variability, are needed to improve models of human impact.

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