Björn Elsäßer

Understanding macroalgal dispersal in a complex hydrodynamic environment: a combined population genetic and physical modelling approach

Gene flow in macroalgal populations can be strongly influenced by spore or gamete dispersal. This, in turn, is influenced by a convolution of the effects of current flow and specific plant reproductive strategies. Although several studies have demonstrated genetic variability in macroalgal populations over a wide range of spatial scales, the associated current data have generally been poorly resolved spatially and temporally. In this study, we used a combination of population genetic analyses and high-resolution hydrodynamic modelling to investigate potential connectivity between populations of the kelp Laminaria digitata in the Strangford Narrows, a narrow channel characterized by strong currents linking the large semi-enclosed sea lough, Strangford Lough, to the Irish Sea. Levels of genetic structuring based on six microsatellite markers were very low, indicating high levels of gene flow and a pattern of isolation-by-distance, where populations are more likely to exchange migrants with geographically proximal populations, but with occasional long-distance dispersal. This was confirmed by the particle tracking model, which showed that, while the majority of spores settle near the release site, there is potential for dispersal over several kilometres. This combined population genetic and modelling approach suggests that the complex hydrodynamic environment at the entrance to Strangford Lough can facilitate dispersal on a scale exceeding that proposed for L. digitata in particular, and the majority of macroalgae in general. The study demonstrates the potential of integrated physical–biological approaches for the prediction of ecological changes resulting from factors such as anthropogenically induced coastal zone changes.

Coupled hydrodynamic and wastewater plume models of Belfast Lough, Northern Ireland: A predictive tool for future ecological studies

Wastewater outfalls provide a natural laboratory for the study of nutrient dynamics in coastal seas, however if properly designed and operated their impact can be difficult to detect. A model was developed and applied to investigate the effect of variation in hydrodynamic conditions on the transport and dilution of a treated wastewater plume in Belfast Lough, Northern Ireland. To validate these predictions we measured the physiochemical properties of the waters surrounding the outfall with a specific focus on inherent plume tracers likely to be relevant to the study of macroalgae (salinity, nitrogen and phosphorus). The model performed well and our data show high dilution of the plume, even under neap-tide conditions. This provides a spatially and temporally defined predictive framework for future studies investigating the compliance of Northern Irelands coastal waters with European Water Framework Directive objectives and for feasibility studies investigating macroalgal aquaculture near wastewater outfalls.

A Description of the Stationary Wave Pattern of a Marine Craft in Shallow and Intermediate Water Depths

Abstract A stationary phase model is used to study supercritical waves generated by high speed ferries. Some general relationships in terms of wave angle, propagation direction, dispersion relationship and depth wavelength relationship are explored and discussed. In particular, it is shown that the wave pattern generated by high speed craft at supercritical speeds depends mainly on the relationship of water depth and ship speed and that the wave patterns are similar in terms of location of crests and troughs for a given depth Froude number. In addition it is found that the far field wave pattern can be described adequately using a single moving point source. The theoretical model compares well with towing tank measurements and full scale data over a range of parameters and hull shapes. The paper also demonstrates that the far field wave pattern at supercritical speeds should be non-dimensionalised by water depth and not hull length unlike it is usually done for subcritical speeds.

Hierarchical structuring of genetic variation at differing geographic scales in the cultivated sugar kelp Saccharina latissima

The cultivation of macroalgae for biofuels, food and fertilisers has increased dramatically in recent years. The demand for such algal-derived products means that large scale cultivation in coastal waters will become necessary to provide sufficient algal biomass. As part of the process of establishing new macroalgal farms, the potential for gene flow between cultivated specimens and natural populations needs to be taken into consideration. Consequently, in the present study we have used a combined population genetic and hydrodynamic modelling approach to determine potential levels and patterns of gene flow in the kelp Saccharina latissima. Microsatellite analysis of 14 populations sampled across the northern part of the Irish Sea indicated four distinct genetic clusters. These were consistent with dispersal patterns indicated by the particle tracking model and show a combination of isolation by distance and genetic structuring due to local hydrodynamic conditions. At smaller scales (less than a few 10s of km), gene flow appears to be fairly extensive, with evidence of local population connectivity due to local currents. At larger scales, however, factors such as freshwater efflux and open water would appear to represent barriers to gene flow. Together, these patterns suggest that factors other than simple geographical distance and proximity need to be taken into account when planning the siting of kelp farms with the aim of minimizing gene flow to and from natural populations.