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

Abstract

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 physicalbiological approaches for the prediction of ecological changes resulting from factors such as anthropogenically induced coastal zone changes.

Genetic diversity of Saccharina latissima (Phaeophyceae) along a salinity gradient in the North Sea–Baltic Sea transition zone

“…Also, kelps are often considered poor-dispersing species due to short longevity of spores and because fertilization requires close proximity of male and female gametophytes to allow sperm-egg encounter. Thus, dependent on the current regimes, kelp spore dispersal ranges may be very low and highly skewed to within scales of meters, with only a small proportion reaching scales of kilometers (e.g., Billot et al 2003, Alberto et al 2010, Brennan et al 2014, and consequently highly shaped by selfing and inbreeding depression (Johansson et al 2013). Therefore, a patchy distribution in conjunction with constrained possibilities for spore dispersal and gamete encounters, is likely to reinforce the rate of genetic differentiation with distance within the brackish gradient region.…”

“…Levels of genetic diversity within sampled populations of S. latissima were of the same order of magnitude as other surveyed kelp species, including Saccorhiza polyschides from Portugal (Assis et al 2013) and Laminaria digitata from Brittany (Billot et al 2003, Robuchon et al 2014) and Northern Ireland (Brennan et al 2014), and in studies likewise assessing genetic diversity patterns in kelp beds (Kusumo et al 2006, Coleman 2013. The fjord population (Vejle) exhibited a markedly lower genetic diversity than all the other populations, as revealed by both lower levels of genetic polymorphism and expected heterozygosity.…”

“…Thus, dependent on the current regimes, kelp spore dispersal ranges may be very low and highly skewed to within scales of meters, with only a small proportion reaching scales of kilometers (e.g., Billot et al 2003, Alberto et al 2010Brennan et al 2014), and consequently highly shaped by selfing and inbreeding depression (Johansson et al 2013).…”

“…Suitable hard bottom habitat is scarce along this region, and macroalgal settlements are therefore limited to geographically distant stone reefs separated by sandy bottom (Moy et al 2008), and to manmade structures, such as jetties and harbors. The probability for the species to disperse and achieve genetic mixing between geographically separated populations is likely to be heavily constrained by unfavorable habitat (i.e., soft-bottom) and by poor spore dispersal (as inferred for kelp by Billot et al 2003, Brennan et al 2014, Robuchon et al 2014; also taking into account that fertilization requires very close proximity of male and female gametophytes thereby limiting the probability for sperm-egg encounter after dispersal. Rafting is known to be a means of colonization expanding the dispersal range of otherwise low-dispersal seaweeds (e.g., Hernandez-Carmona et al 2006, Neiva et al 2014, Saunders 2014), but its role in connecting populations via frequent gene flow appears to be modest at most spatial scales (Fraser et al 2009, Neiva et al 2012.…”

Phylogeographic data revealed shallow genetic structure in the kelp Saccharina japonica (Laminariales, Phaeophyta)

“…However, ocean currents facilitated dispersal over long distances for organisms such as Laminaria digitata (Hudson) J.V. Lamouroux and Macrocystis pyrifera (Linnaeus) C. Agardh [ 21 , 22 ]. Incident anthropogenic introduction of S. japonica influenced dispersal and gene flow of brown seaweed, especially economic seaweeds [ 7 , 8 , 25 ].…”