Quantitative assessment of near-seafloor flow dynamics and physical drivers for material and larval transport

At Walvis Ridge, two nested high-resolution implementations of the hydrodynamic model ROMS-AGRIF (Regional Ocean Modelling System with grid refinement) were employed, aiming to identify and quantify the physical drivers of the distribution of benthic cold-water coral communities at spatial scales of several metres vertically to several hundreds of metres horizontally.

In contrast, modelling of plume dynamics at the Lucky Strike vent field focussed on the vertical motion from injection of hydrothermal tracers at the vent to the neutral level, where the hydrothermal material is then dispersed horizontally. The vertical motion is due to the turbulent buoyant plume. To study this plume, large eddy simulations (LES) with grid sizes down to 1 cm were analysed to resolve the outlet of a deep-sea vent. 

Two main conclusions are drawn from this study. The first is that increasing resolution horizontally and vertically close to the seafloor is an important advancement for precisely evaluating the intrinsic dynamics within challenging, rough terrain such as the Walvis Ridge seamounts. We found that the dissipation of kinetic energy in near-bottom ocean currents and internal wave dynamics may serve as effective proxies of food supply to cold-water corals and sponge communities. Our findings suggest that the predictive capacity of species distribution models for benthic communities may be improved by considering physical processes in addition to more traditional descriptors like water mass and terrain properties, thus enhancing our ability to predict the occurrence of benthic species communities in general.

The second key finding is that the simplified classical 1D model, the so-called MTT model, fails at predicting important features of the plumes at Lucky Strike, such as its height. Results show that the vertical speed increases in the metres immediately above the vent but to correctly represent this acceleration, it is crucial to include a full nonlinear equation of state. Results also reveal that the Lucky Strike plumes have a very large mixing efficiency in the first 10 metres of plume ascent, well above the oceanic background rate, which emphasises the extraordinary regime of hydrothermal plumes compared to the rest of the ocean processes.