As the most widespread seagrass in temperate waters of the Northern Hemisphere, Zostera marina provides a unique opportunity to investigate the extent to which the historical legacy of the last glacial maximum (LGM18 000-10 000 years bp) is detectable in modern population genetic structure. We used sequences from the nuclear rDNA-internal transcribed spacer (ITS) and chloroplast matK-intron, and nine microsatellite loci to survey 49 populations (> 2000 individuals) from throughout the species' range. Minimal sequence variation between Pacific and Atlantic populations combined with biogeographical groupings derived from the microsatellite data, suggest that the trans-Arctic connection is currently open. The east Pacific and west Atlantic are more connected than either is to the east Atlantic. Allelic richness was almost two-fold higher in the Pacific. Populations from putative Atlantic refugia now represent the southern edges of the distribution and are not genetically diverse. Unexpectedly, the highest allelic diversity was observed in the North Sea-Wadden Sea-southwest Baltic region. Except for the Mediterranean and Black Seas, significant isolation-by-distance was found from ~150 to 5000 km. A transition from weak to strong isolation-by-distance occurred at ~150 km among northern European populations suggesting this scale as the natural limit for dispersal within the metapopulation. Links between historical and contemporary processes are discussed in terms of the projected effects of climate change on coastal marine plants. The identification of a high genetic diversity hotspot in Northern Europe provides a basis for restoration decisions.

Fauna associated with the common kelp along the Norwegian coast, Laminaria hyperborea, was sampled at four sites covering 1000 km of coastline. Exploitation of the kelp habitat by the fauna, and the ways in which habitat size and seasonal variations a¡ect the kelp fauna community were analysed. The study focused on mobile macrofauna, of which 238 species were found on 56 kelps sampled, with an average density of almost 8000 individuals per kelp. Amphipods and gastropods were the most diverse and abundant fauna groups. The species composition was di¡erent on the lamina, stipe (with epiphytic algae) and holdfast. A similar pattern of epiphyte-and holdfast-fauna composition was found for all regions. Lowest diversity and abundance were found on the lamina, and highest diversity in the holdfast. Highest abundance was found on the stipe in summer, but there were large variations between sites and seasons, from a few individuals to more than 80 000 animals per stipe. Neither seasonal changes nor variation in habitat volume a¡ected the number of species signi¢cantly, but abundance was signi¢cantly related to season and habitat volume. These variations were most pronounced for stipe fauna. Laminaria hyperborea o¡ers a heterogeneous habitat exploited by a diverse and abundant invertebrate community, its abundance depending on local and regional variations in kelp size.

Macrophyte systems, including kelp, seaweed and seagrasses, have revealed high diversity and abundance of associated fauna along the Norwegian coast. In the present study, data from a number of recent studies were assembled and supplemented with new data to elucidate the organisation of macrofaunal diversity on macrophytes. The aim was to compare faunal composition on macrophytes of different size, shape, longevity and function. Macrofaunal densities frequently exceed 100 000 individuals m -2 in macrophyte beds. Commonly, high densities of amphipods and gastropods are found. The faunal composition depends mainly on habitat architecture at a spatial micro-scale, while faunal abundances depend on habitat size. These 2 patterns are consistent over larger spatial scales. Most faunal species show high mobility and dispersal rates, and they colonize available habitats rapidly. Macrophyte longevity may, in some cases, influence faunal composition. The macrophytes function both as a habitat and as a food source, but the feeding behaviour of the majority of the faunal components prevents the fauna from overgrazing their habitat and thus destroying the primary producer and foundation species of the community. The perennial macrophytes are mainly consumed as particulate organic matter. A high functional redundancy in both plants and animals is most likely important for the stability of the macrophyte system. The stability and diversity of macrophyte systems are found to be threatened in various ways by overgrazing, removal by storms and commercial harvesting, eutrophication and overfishing of top predators, with concurrent challenges for management.KEY WORDS: Macroalgae · Seagrass · Fauna · Diversity · Stability Resale or republication not permitted without written consent of the publisherContribution to the Theme Section 'Marine biodiversity: current understanding and future research'