Dynamics of Near-Bottom Currents in Cold-Water Coral and Sponge Areas at Valdivia Bank and Ewing Seamount, Southeast Atlantic

Abstract

This study investigates near-bottom currents and physical processes from simulations with the hydrodynamic model ROMS-AGRIF at two seamounts of the northeast Walvis Ridge to obtain valuable insights about drivers of observed occurrences of benthic suspension feeders (cnidarians and sponges) in this data-poor area. The spatial resolution in each model area was increased across two levels of nested grids from 1,500 m to 500 m resolution with 32 stretched terrain-following (s-) layers in the vertical with high resolution close to the bottom. The parent grids receive initial and boundary conditions from the basin-scale model INALT20 and from solutions of the OTIS inverse tidal model. The model topography is based on GEBCO data with local refinements from multi-beam data collected during different surveys in 2008, 2009, and 2010. Increasing model resolution is an important advancement for precisely evaluating the intrinsic dynamics within challenging rough terrain. We examined how near-bottom currents vary over space and time and investigated potential links between observed Cnidarian and Porifera occurrences and ranges of physical variables and processes. We identified a close link between physical processes and species distributions and suggested that physical processes such as kinetic energy dissipation and internal wave dynamics may be considered in future research as proxies of food supply to benthic suspension feeders. Such mechanistic variables may also be used to supplement more traditional descriptors such as water mass and terrain properties in species distribution models, thus enhancing our ability to predict the occurrence of benthic communities characterized by cnidarians and sponges.