A unifying theory for global benthic biomass distribution: the relative importance of water column and topographic drivers
Most benthic fauna in the deep sea relies on the flux of organic carbon synthetized in the surface ocean as a source of food. Particulate organic carbon, derived from primary and secondary production, sinks through the water column and gets 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 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. Here, 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 (hydrodynamic scenario) can be reversed when the slopes are steep (gravitational scenario). Furthermore, a small effect of the interaction between current direction and seafloor morphology is detected, suggesting an asymmetric distribution of sediments around topographic features. An attempt is made to build a global model for benthic biomass distribution in relation to seafloor morphology. This model improves by 120 times the resolution of a similar existing model. The promising results highlight that the effect of seafloor morphology on benthic biomass can be detected at global scale. Nevertheless, the small number of datapoints and the limited range of features 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 5% under climate change scenarios; therefore, accurate estimates of carbon stocks and fluxes through the benthic community, and their spatial variability, are needed to improve models of climate change impact models as they become more accurate and, therefore, more sensible to spatial variability at fine scale.