RESUMEN
Algal and cyanobacterial blooms in reservoirs are driven by nutrient enrichment and may present economic and conservation challenges for water managers. Current approaches such as suppression of algal growth with barley straw, ferric dosing or manipulation of fish stocks have not yielded long term successes. A possibility that has sparked growing interest is the encouragement and cultivation of natural filter feeders, such as mussels, which remove suspended matter from the water and reduce nutrient levels through biodeposition and assimilation. This review focusses on the zebra mussel (Dreissena polymorpha) as a tool for enhancement of water quality in reservoirs. Native to the Ponto-Caspian region, this species has invaded many lakes and reservoirs across North America and Western Europe, where it occurs in very high densities. While purposeful introduction of a non-native species into new sites is socially unacceptable, we investigate the possible benefits of encouraging increased abundance of zebra mussels in sites where the species is already established. We estimate that the annual nitrogen and phosphorus input into a large UK reservoir (Grafham Water) could be assimilated into zebra mussel biomass by encouraging settlement onto 3075 m and 1400 m of commercial mussel ropes, respectively. While zebra mussel cultivation has an incredible capacity to push eutrophic systems towards a clear water state, there are many risks associated with encouraging an invasive species, even within sites where it has already established. The zebra mussel is a prominent biofouler of native unionid mussels and raw water pipes, it changes the physical characteristics of the places it inhabits, in sites low in phosphorus it can be responsible for toxic cyanobacterial blooms, it alters nutrient cycling and community structure and it can have negative impacts on amenity value. Increased propagule pressure from elevated numbers of veliger larvae in the water column may increase the risk of spread to other locations. This may render some reservoir systems, such as dammed rivers which have outflows to downstream watercourses, unsuitable for cultivation. Such reservoirs are especially common in North America. We consider the practicalities of putting a zebra mussel cultivation system into place and identify gaps in knowledge. We conclude that zebra mussel cultivation offers an attractive tool for managing nutrient-enriched reservoirs, but that the benefits and costs must be balanced on a site-by-site basis.
Asunto(s)
Dreissena/crecimiento & desarrollo , Especificidad del Huésped , Especies Introducidas , Calidad del Agua , Abastecimiento de Agua , Animales , EcosistemaRESUMEN
Cordylophora caspia is a hydrozoan which causes biofouling in power plants and is an increasing problem in UK drinking water treatment works. Thermal control is not usually feasible without a ready source of hot water so laboratory experiments were conducted to assess whether using pulsed doses of chlorine is an alternative solution. C. caspia polyps disintegrated after a single 20 min dose (the length of one backwash cycle in water treatment work filter beds) of 2.5 ppm chlorine. Without further treatment colonies regenerated within 3 days, but repeated dosing with chlorine for 20 min each day inhibited this regeneration. The resistance of surviving colonies to chlorine increased over time, although colony size and polyp regeneration continued to fall. These results suggest pulsed treatment with chlorinated backwashes at 2 ppm could be used to control C. caspia biofouling in rapid gravity filters and this may have relevance to other settings where thermal control is not feasible.
Asunto(s)
Incrustaciones Biológicas/prevención & control , Cloro/farmacología , Desinfectantes/farmacología , Hidrozoos/efectos de los fármacos , Purificación del Agua/métodos , Animales , Cloro/administración & dosificación , Desinfectantes/administración & dosificación , Relación Dosis-Respuesta a Droga , Agua Potable , Agua Dulce , Hidrozoos/crecimiento & desarrollo , Centrales EléctricasRESUMEN
In recent years biofouling from native (bryozoans, sponges) and non-native (Cordylophora) animals has increased in UK water treatment works (WTW). A survey of six UK water companies and eight WTWs revealed that these taxa were more widespread and abundant than previously recognised. Primary problems related to the occlusion of underfloor nozzles and tailpipes in rapid gravity filter beds (RGFs). These cost the UK water industry pound 1.49 m between 2005 and 2009. Additional impacts came from skin irritation to operatives from sponge spicules and the potential for elevated bacterial pathogen levels. Sponges penetrated the furthest through the water treatment process, reaching the point of final chlorination at one WTW. A monitoring plate study showed pronounced seasonality in fouling, with most taxa peaking in mid to late summer before a winter die-off. Control options, including the use of chlorine, and the importance of resistant stages for each taxon are discussed.