Neutral processes forming large clones during colonization of new areas.

In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises due to natural selection favouring generally or locally successful genotypes in new environments. Alternatively, as we show here, this pattern may result from neutral processes during species' range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that favour sexual recruitment in the long run, colonization starts with an asexual wave. After colonization is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long-distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our model shows that neutral processes (with respect to genotype fitness) during the population expansion, such as random dispersal and demographic stochasticity, produce genotype patterns that differ from the patterns arising in a selection model. The comparison with empirical data from a post-glacially established seaweed species (Fucus radicans) shows that in this case, a neutral mechanism is strongly supported.

Preferential Sampling and Small-Scale Clustering of Gyrotactic Microswimmers in Turbulence.

Recent studies show that spherical motile microorganisms in turbulence subject to gravitational torques gather in down-welling regions of the turbulent flow. By analyzing a statistical model we analytically compute how shape affects the dynamics, preferential sampling, and small-scale spatial clustering. We find that oblong organisms may spend more time in up-welling regions of the flow, and that all organisms are biased to regions of positive fluid-velocity gradients in the upward direction. We analyze small-scale spatial clustering and find that oblong particles may either cluster more or less than spherical ones, depending on the strength of the gravitational torques.

Local adaptation and oceanographic connectivity patterns explain genetic differentiation of a marine diatom across the North Sea-Baltic Sea salinity gradient.

Drivers of population genetic structure are still poorly understood in marine micro-organisms. We exploited the North Sea-Baltic Sea transition for investigating the seascape genetics of a marine diatom, Skeletonema marinoi. Eight polymorphic microsatellite loci were analysed in 354 individuals from ten locations to analyse population structure of the species along a 1500-km-long salinity gradient ranging from 3 to 30 psu. To test for salinity adaptation, salinity reaction norms were determined for sets of strains originating from three different salinity regimes of the gradient. Modelled oceanographic connectivity was compared to directional relative migration by correlation analyses to examine oceanographic drivers. Population genetic analyses showed distinct genetic divergence of a low-salinity Baltic Sea population and a high-salinity North Sea population, coinciding with the most evident physical dispersal barrier in the area, the Danish Straits. Baltic Sea populations displayed reduced genetic diversity compared to North Sea populations. Growth optima of low salinity isolates were significantly lower than those of strains from higher native salinities, indicating local salinity adaptation. Although the North Sea-Baltic Sea transition was identified as a barrier to gene flow, migration between Baltic Sea and North Sea populations occurred. However, the presence of differentiated neutral markers on each side of the transition zone suggests that migrants are maladapted. It is concluded that local salinity adaptation, supported by oceanographic connectivity patterns creating an asymmetric migration pattern between the Baltic Sea and the North Sea, determines genetic differentiation patterns in the transition zone.

Parallel speciation or long-distance dispersal? Lessons from seaweeds (Fucus) in the Baltic Sea.

Parallel evolution has been invoked as a forceful mechanism of ecotype and species formation in many animal taxa. However, parallelism may be difficult to separate from recently monophyletically diverged species that are likely to show complex genetic relationships as a result of considerable shared ancestral variation and secondary hybridization in local areas. Thus, species' degrees of reproductive isolation, barriers to dispersal and, in particular, limited capacities for long-distance dispersal will affect demographical structures underlying mechanisms of divergent evolution. Here, we used nine microsatellite DNA markers to study intra- and interspecific genetic diversity of two recently diverged species of brown macroalgae, Fucus radicans (L. Bergström & L. Kautsky) and F. vesiculosus (Linnaeus), in the Baltic Sea. We further performed biophysical modelling to identify likely connectivity patterns influencing the species' genetic structures. For each species, we found intraspecific contrasting patterns of clonality incidence and population structure. In addition, strong genetic differentiation between the two species within each locality supported the existence of two distinct evolutionary lineages (FST  = 0.15-0.41). However, overall genetic clustering analyses across both species' populations revealed that all populations from one region (Estonia) were more genetically similar to each other than to their own taxon from the other two regions (Sweden and Finland). Our data support a hypothesis of parallel speciation. Alternatively, Estonia may be the ancestral source of both species, but is presently isolated by oceanographic barriers to dispersal. Thus, a limited gene flow in combination with genetic drift could have shaped the seemingly parallel structure.

Effects of a large northern European no-take zone on flatfish populations.

In March 2006, a 360 km² no-take zone (NTZ) was established north of Gotland in the central Baltic Sea, with the purpose to scientifically evaluate the effects of a fishing ban on flatfish populations. A monitoring programme was set up to study the populations in the NTZ and in a reference area east of Gotland where the fishing pressure was high. The programme included fishing with multimesh survey nets, modelling of potential larval export and estimation of fish consumption by large marine predators. Overall, the results showed a clear positive effect of the NTZ on turbot Scophthalmus maximus, with higher densities in the closed area compared with the fished area and also higher densities after closure compared with before. The NTZ also had older individuals and a more even sex ratio. This, in combination with a high potential for larval export from the NTZ to Gotland, shows that the marine reserve may be important for maintaining a viable S. maximus stock at Gotland. Also, for flounder Platichthys flesus, the densities were higher in the NTZ compared to the reference area and there was a net larval export to the fished area. For both species, density-dependent growth was evident, with a lower length at age in the closed area. Potential predation by grey seal Halichoerus grypus and great cormorant Phalacrocorax carbo sinesis on flatfishes, that could hamper the evaluation of the marine reserve, was also addressed. Taken together, the results show that there are clear benefits of the fishing ban for both flatfish species within the NTZ, while the net effects on fisheries are difficult to quantify.

Trophic transfer and passive uptake of a polychlorinated biphenyl in experimental marine microbial communities.

To study the uptake mechanisms of 2,2',4,4',6,6'-hexachlorobiphenyl (HxCB 153) in microplankton with different feeding strategies, two laboratory communities were used. Trophic transfer of HxCB 153 was tested in a heterotrophic microbial food web consisting of bacteria, flagellates, and ciliates. Passive uptake was tested in a community consisting of algae and bacteria. The experiments were conducted over 6 d, and samples were retrieved daily. In the heterotrophic food web, a significant increase of the HxCB 153 concentration was observed in the top predator level (ciliates, p < 0.05), where the concentration doubled during the 3 d of the experiment. A concomitant decrease was observed in the ciliate prey (flagellates), indicating that HxCB 153 was redistributed due to trophic transfer. On average, 33% of the HxCB 153 was sorbed to ciliates over the course of the experiment. In the experiment with algae and bacteria, the HxCB 153 concentrations were relatively stable over time. The largest fraction partitioned into bacteria (57%), whereas only 4% of the HxCB 153 sorbed to the algae. The uptake was 8-fold higher in ciliates than in algae, although the algal biomass was 10-fold higher. The results imply that trophic transfer may be a significant transport route of more hydrophobic organic contaminants (HOCs) in plankton communities. In transfer models, this pathway may be taken into consideration so that the transport rate of HOCs to higher trophic levels is not underestimated.

Roughness-dependent removal of settled spores of the green alga Ulva (syn. Enteromorpha) exposed to hydrodynamic forces from a water jet.

Topographic features change the hydrodynamic regime over surfaces subjected to flow. Hydrodynamic microenvironments around topographic structures may have consequences for recruitment and removal of propagules of marine benthic organisms. The settlement and adhesion of zoospores from the green alga Ulva linza (syn. Enteromorpha linza) to defined topographies was investigated. A range of topographic size scales (Rz: 25-100 microm) was manufactured from plankton nets, creating patterns with ridges and depressions. The topographic scales span a roughness similar to that of natural substrata and antifouling coatings. Spores were removed from the surfaces by a calibrated water jet. Fewer spores were removed from the smallest topographic structure tested (Rz: 25 microm) compared to both the smooth (Rz: 1) and the roughest (Rz: 100 microm) structures. Zoospores that settled in depressions were less likely to be removed compared to spores on the ridges. The results in terms of the interaction between surface topography and hydrodynamic forces have implications for both natural substrata exposed to wave action and antifouling surfaces on ships' hulls. The possible effects of topography on increasing zoospore adhesion and offering a refuge from hydrodynamic forces are discussed.