Lakes & Ponds

Case study: Biomanipulation in the Norfolk Broads

The Norfolk Broads are a series of ~60 shallow lakes formed by the flooding of medieval peat diggings. They are one of Europe's most important wetlands for biodiversity and nature conservation.

However, intensification of agriculture and increases in human populations has led to eutrophication. Eutrophication is the process whereby nitrates and phosphates, especially from fertilizers (though also from detergents and human waste), run off the land into rivers and lakes. This promotes the growth of algae, which take oxygen from the water, resulting in the loss of biodiversity across all levels of the aquatic food web.

ECON have been working with the Broads Authority since 1990 to restore clear water conditions at a number of key locations throughout the Norfolk Broads. ECON's particular specialism is biomanipulation, which involves removing and/or adding species in order to alter the species composition of the ecosystem, in this case with the aim of restoring clear water conditions. This has involved the removal of undesirable zooplanktivorous fish (e.g. small Roach) in order to allow zooplankton to increase, which in turn feed on the algae reducing its abundance. It has also involved removal of benthivorous (e.g. large Bream or Carp) fish, which through their foraging action disturb sediments, releasing undesirable nutrients into the water. It has also involved stocking piscivorous species (e.g. Perch and Pike) or fish that would be seen as being part of the desired fish community (e.g. Tench and Rudd). The aim is to generate a fish community typically associated with high macrophyte biodiversity and clear water conditions, potentially including rudd, perch, pike, eel and tench.

Images 1 & 2: Algal bloom and clear water conditions following biomanipulation.

On a practical level the biomanipulation strategy is site specific, although it is likely to involve a combination of mass removals of zooplanktivorous fish, removal of fish spawn, and 'top-up' removals of young-of-the-year fish or other fish that have entered the water body. All removed fish are re-located to other water bodies.

Methods used to remove fish have included electric fishing, seine netting, and fyke nets. The exact method is designed around the site, though in general the strategy has been to target concentrations of fish to maximise catch per unit effort (cost).

Full-scale biomanipulation has been undertaken at Cockshoot Broad (1989-2008), Pound End (1990-1998) and Ormesby Broad (1995-2008). Partial biomanipulation has been undertaken at Alderfen (1994-2000). All these are isolated lakes. A number of fish exclosures have been set up and biomanipulated in connected lake systems, including at Hoveton Great Broad (1992-1995), Barton Broad (2000-present) and Ranworth Broad (2013-present).

In all instances biomanipulation has been extremely successful in reducing chlorophyll a and increasing Secchi depth, providing 'gin-clear' conditions.

Image 3: Biomanipulation has involved removing large numbers of roach.

Once chlorophyll a concentrations are reduced, an increase in macrophyte cover can occur, although the extent of cover and the timescale over which this happens is likely to be variable depending on other factors, such as the presence of a propagule bank, climactic conditions, and herbivory by waterbirds. The most rapid response was observed in the Neatishead Arm South fish exclosure at Barton where dense coverage of Chara spp. and especially Rananculus circinatus was achieved within a year. In other sites it has taken 2-8 years for macrophytes to colonize.

In terms of species richness, the highest values have occurred in Cockshoot and Ormesby (7-10 species and 10-17 species respectively), observed 15-20 years after biomanipulation.

Image 4: The location of the biomanipulated fish exclosure is obvious (photo courtesy of the Broads Authority).

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