Dmitry Aleynik

Offshore marine renewable energy devices as stepping stones across biogeographical boundaries

Summary Offshore renewable energy provides an increasing component of our electricity supply. We have limited understanding of the potential environmental impacts of these developments, particularly in the move to larger scales. Surfaces provided by devices offer novel habitat to marine organisms, which may allow species to spread to new areas. We used coupled biological and hydrodynamic models to investigate the spread of intertidal marine organisms with pelagic larvae (such as barnacles or gastropods) in the region around south-western Scotland. We assessed the impact of novel habitat on dispersal and its role in allowing transgression of physical barriers. Model renewable energy device sites provided habitat for pelagic larval particles that would otherwise have been lost offshore. They also provided a source of larvae for existing coastal sites. Many offshore devices fulfilled source and destination (or intermediate connection) roles, creating new dispersal pathways, and allowing previously impossible northward dispersal from the Northern Irish coast to Scotland. Synthesis and applications. New habitat close to biogeographical barriers has implications for existing species’ distributions and genetic population structure. It also affects the spread of non-native species and ‘climate migrants’. Monitoring these sites for the presence of such species will be important in determining the future ecology of coastal habitat and in maintaining economic aquaculture and marina operations. Future model studies should focus on particular species of importance, taking account of their biology and current distribution.

Focusing of baroclinic tidal energy in a canyon

Strong three-dimensional focusing of internal tidal energy in the Petite Sole Canyon in the Celtic Sea is analyzed using observational data and numerical modeling. In a deep layer (500–800 m) in the center of the canyon, shear variance was elevated by an order of magnitude. Corresponding large vertical oscillations of deep isotherms and a local maximum of horizontal velocity were replicated numerically using the MITgcm. The elevated internal tidal activity in the deep part of the canyon is explained in terms of the downward propagation and focusing of multiple internal tidal beams generated at the shelf break. The near-circular shape of the canyon head and steep bottom topography throughout the canyon (steeper than the tidal beam) create favorable conditions for the lens-like focusing of tidal energy in the canyons center. Observations and modeling show that the energy focusing greatly intensifies local diapycnal mixing that leads to local formation of a baroclinic eddy.

A high resolution hydrodynamic model system suitable for novel harmful algal bloom modelling in areas of complex coastline and topography

Fjordic coastlines provide sheltered locations for finfish and shellfish aquaculture, and are often subject to harmful algal blooms (HABs) some of which develop offshore and are then advected to impact nearshore aquaculture. Numerical models are a potentially important tool for providing early warning of such HAB events. However, the complex topography of fjordic shelf regions is a significant challenge to modelling. This is frequently compounded by complex bathymetry and local weather patterns. Existing structured grid models do not provide the resolution needed to represent these coastlines in their wider shelf context. In a number of locations advectively transported blooms of the ichthyotoxic dinoflagellate Karenia mikimotoi are of particular concern for the finfish industry. Here were present a novel hydrodynamic model of the coastal waters to the west of Scotland that is based on unstructured finite volume methodology, providing a sufficiently high resolution hydrodynamical structure to realistically simulate the transport of particles (such as K. mikimotoi cells) within nearshore waters where aquaculture sites are sited. Model-observation comparisons reveal close correspondence of tidal elevations for major semidiurnal and diurnal tidal constituents. The thermohaline structure of the model and its current fields are also in good agreement with a number of existing observational datasets. Simulations of the transport of Lagrangian drifting buoys, along with the incorporation of an individual-based biological model, based on a bloom of K. mikimotoi, demonstrate that unstructured grid models have considerable potential for HAB prediction in Scotland and in complex topographical regions elsewhere.

Glider observations of enhanced deep water upwelling at a shelf break canyon: A mechanism for cross-slope carbon and nutrient exchange

Using underwater gliders we have identified canyon driven upwelling across the Celtic Sea shelf-break, in the vicinity of Whittard Canyon. The presence of this upwelling appears to be tied to the direction and strength of the local slope current, which is in itself highly variable. During typical summer time equatorward flow, an unbalanced pressure gradient force and the resulting disruption of geostrophic flow can lead to upwelling along the main axis of two small shelf break canyons. As the slope current reverts to poleward flow, the upwelling stops and the remnants of the upwelled features are mixed into the local shelf water or advected away from the region. The upwelled features are identified by the presence of sub-pycnocline high salinity water on the shelf, and are upwelled from a depth of 300 m on the slope, thus providing a mechanism for the transport of nutrients across the shelf break onto the shelf.

Impact of remotely generated eddies on plume dispersion at abyssal mining sites in the Pacific

Proposed harvesting of polymetallic nodules in the Central Tropical Pacific will generate plumes of suspended sediment which are anticipated to be ecologically harmful. While the deep sea is low in energy, it also can be highly turbulent, since the vertical density gradient which suppresses turbulence is weak. The ability to predict the impact of deep plumes is limited by scarcity of in-situ observations. Our observations show that the low-energy environment more than four kilometres below the surface ultimately becomes an order of magnitude more energetic for periods of weeks in response to the passage of mesoscale eddies. The source of these eddies is remote in time and space, here identified as the Central American Gap Winds. Abyssal current variability is controlled by comparable contributions from tides, surface winds and passing eddies. During eddy-induced elevated flow periods mining-related plumes, potentially supplemented by natural sediment resuspension, are expected to spread and disperse more widely and rapidly. Predictions are given of the timing, location and scales of impact.