'Biofiltration and biofouling on artificial structures in Europe: the potential for mitigating organic impacts

Man-made structures deployed either deliberately or accidentally into the sea are subject to levels of biofouling. The resultant communities are usually dominated by sessile marine invertebrates that tend to utilize suspension-feeding for nutritional gain from the water column. Not all suspension-feeders are filtration-feeders but in general terms a large suspension-feeding community will provide varying scales of water filtration. The potential for utilizing some form of in-water biofiltration in association with localised organic enrichment has long been suggested but with few quantitative estimates of probable efficacy. The major taxa that are likely to be relevant to the process of biofiltration are discussed in relation to the functional classification of suspension-feeders. In order to generate estimates of biofiltration potential, activity rates of the major functional and taxonomic groups of suspension-feeders are derived through a review of the general mode of suspension-feeding, the predominant food sources with the size range of particles retained and individual suspension-feeding rates that are scaled up to the population level. However, any naturally occurring fouling community will consist of a number of species and so estimates of multispecies suspension-feeding, interspecific interactions, rates of biodeposition and nutrient release are derived. The rates and densities of biofouling are dependent of the abiotic and biotic characteristics of the receiving environment and the types of materials used in the provided substrate. The factors affecting biofouling are discussed in relation to existing examples of artificial structures found in European waters or waters of relevance to Europe. In-water outputs from finfish mariculture provide examples of localised point sources of organic enrichment that could benefit from the associated deployments of biological filters. Using estimates of filtration clearance rates combined with the major taxa thought most likely to dominate any filtering community, probable scales of biofiltration required in order to influence the levels of mariculture discharges are calculated. Although, in theory, biofilters in open system finfish mariculture may reduce the levels of organic impact, the scale of intervention required to make a significant effect would probably exceed any form of economic viability.