Investigations of the variability of tidal mixing fronts and their importance for shelf-sea ecosystems across multiple trophic levels

Abstract

Tidal mixing fronts establish during the summer months over shelf-seas, and separate tidally-mixed from stratified water masses. They play an important part in shelf-sea bio-physical processes, including volume transport and facilitation of primary productivity. Frontal hydrodynamics provide the physical necessities for prey aggregations to develop, holding the potential for biodiversity hotspots. However, there is limited knowledge on long-term variability of tidal mixing fronts and its effect on associated ecosystems, due to a lack of adequate datasets. Such information would greatly benefit spatial conservation efforts and improve our understanding of ecosystem dynamics on the continental shelf.

Satellite-derived frontal maps and extensive biological datasets (from 1990-2010) are employed here to investigate spatio-temporal variability of tidal mixing fronts and their significance for shelf-sea biology from zooplankton tomegavertebrates in the Celtic Sea. In addition, this study assesses the suitability and limitations of satellite-derived frontal metrics for quantitative analyses and employs innovative technology (submarine gliders) to fill data gaps in species-environment interactions.

This research provides guidance on the use of frontal metrics in quantitative analysis, such as the need to account for data variability over the years and the careful consideration of the employed frontal metric. This thesis furthermore, represents the first description of long-term temporal variability of tidal mixing fronts on the European shelf and highlights a potential sensitivity to climate change due to positive correlations with rising temperatures. Consequences could include extension of the frontal season and intensification of the frontal density gradient with knock-on effects on associated biota.

The density gradient of tidal mixing fronts was shown to act as a direct distribution boundary for plankton between different shelf-sea domains. Climate-change-driven shifts in the seasonality of these fronts may have a direct impact on dispersal of passive floating organisms, habitat connectivity and adult populations of species with planktonic larvae, including commercially important fish and the benthos. Apart from a barrier function, fronts were also found to be important foraging areas for specialist megavertebrates, which were strongly associated with persistent frontal areas, whereas generalist feeders were not. Tidal mixing fronts represent suitable conservation areas for sensitive species in shelf-seas. The underlying mechanisms leading to bio-aggregations at these sites require more research. High-resolution data, simultaneously collected across multiple trophic levels can be obtained by autonomous robotic fleets in the near future.

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