Spatial and temporal environmental heterogeneity induced by internal tides influences faunal patterns on vertical walls within a submarine canyon

Abstract

Vertical walls of submarine canyons represent features of high conservation value that can provide natural areas of protection for vulnerable marine ecosystems under increasing anthropogenic pressure from deep-sea trawling. Wall assemblages are spatially heterogeneous, attributed to the high environmental heterogeneity over short spatial scales that is a typical feature of canyons. Effective management and conservation of these assemblages requires a deeper understanding of the processes that affect faunal distribution patterns. Canyons are recognised as sites of intensified hydrodynamic regimes, with focused internal tides enhancing near-bed currents, turbulent mixing and nepheloid layer production, which influence faunal distribution patterns. Faunal patterns also respond to broad-scale hydrodynamics and gradients in water mass properties (e.g. temperature, salinity, dissolved oxygen concentration). Oscillating internal tidal currents can advect such gradients, both vertically and horizontally along a canyon's walls. Here we take an interdisciplinary approach using biological, hydrodynamic and bathymetry-derived datasets to undertake a high-resolution analysis of a subset of wall assemblages within Whittard Canyon, North-East Atlantic. We investigate if, and to what extent, patterns in diversity and epibenthic assemblages on deep-sea canyon walls can be explained by spatial and temporal variability induced by internal tides. Vertical displacement of water mass properties by the internal tide was calculated from autonomous ocean glider and shipboard CTD observations. Spatial patterns in faunal assemblage structure were determined by cluster analysis and non-metric Multi-Dimensional Scaling plots. Canonical Redundancy Analysis and Generalised Linear Models were then used to explore relationships between faunal diversity and assemblage structure and a variety of environmental variables. Our results support the hypothesis that internal tides influence spatial heterogeneity in wall faunal diversity and assemblages by generating both spatial and temporal gradients in hydrodynamic properties and consequently likely food supply.

Monitoring Strategies of Suspended Matter after Natural and Deep-Sea Mining Disturbances

“…Similar patterns are thus expected in Whittard Canyon since a similar zonation in organic matter deposition was observed. Canyon walls on the other hand have shown to be hotspots of biodiversity and biomass likely related to the continuous resuspension and delivery of (fresh) organic matter with each tidal cycle (Huvenne et al, 2011;Pearman et al, 2023). Cold-water corals that are known to mainly feed on fresh phytodetritus are mainly observed in the shallow parts of Whittard Canyon (Morris et al, 2013).…”

Patterns in the deep sea

“…We show here that cold-water corals globally occur more often on those continental slope parts that are (super)critical to the M2 tide or supercritical to the K1 tide, than what would be expected based on the percentage of (super)critical topography on all transects (figure 6.9 and table 6.1). Several casestudies on continental slopes (Frederiksen et al 1992, Hanz et al 2019 and in submarine canyons (Pearman et al 2020(Pearman et al , 2023 relate cold-water corals to (super)critical topography. Such (super)critical reflection of internal tides can be a more local phenomenon, and, indeed, with higher resolution bathymetry the relationship between cold-water corals and (super)critical topography is even more pronounced (figure 6.9 and table 6.1).…”

“…Cold-water coral reefs are deep-sea ecosystems that have a particularly high organic-matter processing rate and have indeed been associated with internal (tidal) waves (e.g., Hanz et al, 2019;Roberts et al, 2021;Mohn et al, 2014;Davies et al, 2009;Wang et al, 2019;Juva et al, 2020;van Haren et al, 2014;Pearman et al, 2023). Cold-water coral reefs and mounds are built up of dead coral framework, coral rubble, and sediment, often with thriving cold-water coral reefs on the mound tops (Roberts et al 2009).…”