The Role of Kelp in the Marine Environment

  • Kelly E
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Kelp forests have been described as one of the most ecologically dynamic and biologically diverse habitats on the planet. In response to proposals to develop a mechanical kelp harvesting industry in Ireland, a series of expert reviews were commissioned to describe the various ecological roles of kelp in the marine environment and particularly in relation to marine productivity, flora, fauna, fish and birds. Laminaria digitata and L. hyperborea are the only species that form extended monospecific kelp beds in Ireland and constituted the primary target species of this review. The primary production of kelp per unit area is amongst the highest known in aquatic ecosystems. Kelp primary production results in the production of new biomass, detrital material shed from the blade tip, mucus and other dissolved inorganic material and spores, as well as internal respiration. The production of dissolved organic matter by kelp although very difficult to determine in the field is also considered an important part of kelp production. The importance of kelp not only as a habitat but as a food resource has been highlighted by numerous studies; in some cases, up to 60% of carbon found in coastal invertebrates is attributable to kelp productivity. It may be consumed directly or colonised by bacteria that in turn are preyed upon by consumers. The holdfast, stipe and fronds of kelp plants present available substratum for colonisation by marine flora and invertebrates. The holdfasts tend to host strongly associated communities of epiphytes and marine invertebrates. Kelp contributes directly and indirectly to the food resource of suspension and deposit feeding invertebrates that in turn serve as prey to more mobile invertebrates such as polychaetes, cnidaria and larger decapods. Kelp derived detritus on the shore is also consumed by invertebrates and bird species. The rich fauna of mobile invertebrates in kelp beds makes this an important habitat in the diet of fish species. Seasonal and temporal changes in the abundance of prey species reflected in the diet of some fish species would suggest a degree of 6 opportunism. As invertebrate abundance has also been related to the age and size of the plant within kelp species, there may be some interspecific variation between kelp species in the feeding opportunities available to fishes. Kelp forests provide a foraging habitat for birds due to the associated and diverse invertebrate and fish communities present. Three sub-components, the surface canopy, mid- and bottom- areas, and fringe areas were distinguished in this regard. Accumulations of drift kelp in open water provide a valuable roosting site for birds, particulary as they often transport potential prey items. Many marine and terrestrial bird species are directly dependant on kelp detritus washed up on the shore as wrack due to the densities of resident larvae and invertebrates. Kelp wrack also benefits birds via its role in providing organic matter to coastal marine ecosystems. Some invertebrate and fish species exhibit egg attachment and nest-building, respectively, in kelp habitats while others such as juvenile gadoids and salmon utilise kelp habitats as important nursery and refuge grounds. The under-storey habitats created by the kelp plants also give rise to microniches that support a somewhat similar community of species, particularly large decapods. The effective extension of the substratum into the water column increases shelter, or refugia, available to fishes while also providing habitat for the prey species used as a forage base by reef fishes. This contribution to diversity is more pronounced in otherwise relatively 2- dimensional environments. The potential impacts that may arise from mechanical kelp harvesting are reviewed and significant gaps in information are identified.

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