Macrophytes as habitat for fauna

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.