Ecosystem services and environmental impacts associated with commercial kelp aquaculture
Walls, Aimée M.
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Wild kelp forests represent one of the most productive habitats on Earth, suppling ecosystem services, including, but not limited to, biodiversity, habitat provision, food web subsidy, nutrient cycling, carbon sequestration and coastal defence. Ecosystem services associated with wild kelps have been examined by researchers for more than six decades. Currently, with the development and growth of seaweed aquaculture the ecosystem services and potential impacts associated with cultivated kelps need to be assessed. World aquaculture production continues to grow year on year with approximately 131.4 million tonnes of fish, aquatic animals and aquatic plants produced in 2014. The cultivation of aquatic plants, predominantly marine species of seaweeds, accounts for over 20 % of this total production (by weight) which too has seen rapid growth of almost 8 % per year over the past decade. Seaweed cultivation is traditionally dominated by Asian countries, however, over the last 15 years the industry has also expanded in Europe. This interest has been supported by feasibility studies and pilot-scale farms being set up to begin to develop the industry and advance cultivation techniques of kelps native to European waters. Observations of seaweed farms suggest that both the growing kelp and its associated infrastructure could provide ecosystem services, in particular, habitat for associated species altering the ecosystem structure and functioning of the surrounding area. Additionally, carbon from the highly productive kelps released as detritus could be assimilated by other species and incorporated into the food web. This primary productivity may also negatively impact the benthic environment beneath the farm causing increased organic enrichment of the seabed. Little research has been conducted into the communities associated with cultivated kelps. The habitat created by farmed kelps may act as a novel habitat and not simply an expansion of the existing habitat created by wild kelps, due to differences in kelp morphology, age and position within the water column. A comparative study between wild (benthic) and cultivated (suspended) holdfasts communities of Laminaria digitata at an experimental research site on the west coast of Ireland, demonstrated that cultivated kelps provide a distinct habitat for species compared to their wild counterparts. Subsequently, an extensive investigation of the communities associated with cultivated kelp (Alaria esculenta) was conducted at a commercial-scale seaweed farm in Ventry Harbour, Co. Kerry v (southwest coast of Ireland). These studies assessed the properties of the whole farms and the individual parts (frond and holdfast) of the kelp sporophyte as a habitat for communities. Kelps are a major source of primary productivity and using stable isotope analysis it was found that cultivated kelps (A. esculenta and Saccharina latissima) are incorporated into the food web via the suspension-feeding mollusc Mytilus edulis. Additionally, a multivariate analysis of the Ecological Status of benthic communities suggested no negative impacts from the deposition of kelp detritus on the seabed. The commercial kelp farm in Ventry Harbour was located above a Zostera marina bed which is a key habitat under the EU Habitats Directive and OSPAR Commission, yet, this macrophyte community was not adversely impacted by the presence of the farm. Results from this study show that kelp aquaculture sites provide additional ecosystem services beyond the supply of biomass. These services increase the ecological importance of farms, with multifunctional uses of farms and their infrastructure suggested. Negative impacts caused by detrital deposition on the benthos were not evident at the study site which is provisionally a very positive result for the sector as future expansion and development of the industry may not be limited by detrimental effects on the surrounding environment. However, this is one of the first studies of its kind to identify and assess the ecosystem services and impacts of commercial seaweed (kelp) farms in Europe. Many more factors such as farm scale, seaweed species, site location and hydrodynamics need to be researched in order to fully understand the ecological interactions and impacts (positive or negative) of commercial seaweed cultivation in the future.
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