The restoration of Ormesby Broad

Introduction

Ormesby Broad is the smallest of the five lakes collectively known as the Trinity Broads. Managed in partnership with the Broads Authority, the Environment Agency and Natural England, routine monitoring within the lake revealed that water quality had deteriorated due to an increase in phosphorous, promoting the growth of algae. The diversity of submerged aquatic plants (macrophytes) was also noted to be low and the fish community dominated by large species, including the common roach (Rutilus rutilus) and bream (Abramis brama). A long-term project to restore Ormesby Broad was designed and has been on-going since 1995.

Project aims

The aims of the Ormesby restoration project are:

• To improve water clarity

• To re-establish and maintain the submerged macrophyte populations

• To shift the fish community from one dominated by roach and bream, to one more diverse; containing a high proportion of piscivores (pike and perch Perca fluviatilis) and species such as tench (Tinca tinca) and rudd (Scardinius erythrophthalmus)

• To maintain the site as a public water supply and improve water quality

• To work with the local community

• To promote quiet, sustainable recreation on site

Background

A fish community dominated by roach and bream is thought to have directly contributed to the decrease in submerged macrophytes. These large fish species are benthivorous, feeding at the bottom of water bodies, and are known to uproot seedlings when foraging for food. This foraging activity also disturbs the sediment on the lake bed, causing phosphorous to disperse into the water. Nutrient enrichment is also thought to have occurred at the Trinity Broads by diffuse nutrient, sediment and chemical pollution from the surrounding catchment, raising the concentration of nutrients in the water.

Highly nutrient-rich waters are ideal for algae, which can grow to form a mat of vegetation over the water surface, known as an algal bloom. This affects the submerged plants indirectly, by limiting the amount of light which can reach their leaves through the water and hence their ability to photosynthesize.

Water fleas (Cladocera spp.) feed on algae and depend on submerged macrophytes for protection against predation from fish. The algal blooms at Ormesby left few submerged plants for water fleas to hide amongst, causing their predation rate to increase. In such low numbers, the low water flea populations were unable to consume the surface algae.

Biomanipulation

Biomanipulation is the process of artificially altering an ecosystem, through the addition or removal of species. By removing the dominant fish species, it was hoped that the macrophyte community would recover. With decreased predation, Cladocera would be of sufficient numbers to consume the surface algae, allowing more light to travel through the water and aid macrophyte growth. With reduced numbers of large fish, the risk of uprooting would be minimised and macrophyte seedlings would be able to establish. The water quality and clarity would also improve due to the decreased disturbance of phosphorous-rich sediment by large fish and lowered egestion of nutrient-rich faeces.

Project results

Fish community
After the removal of roach and bream annually, survey results have shown a decrease in proportion of bream at Ormesby. In 2003, bream comprised 98% of the total fish population, compared to 34% in 2004 and 1% in 2006. 2007 was the first year that no bream were found within the lake. The biomanipulation project has therefore successfully reduced the adult bream population, however, young of the year bream fry were still recorded in 2007. The control of roach has been rather more challenging, with the species contributing to a greater proportion of the total fish abundance than in previous years in 2007.

Despite the persistence of the roach population, this long term project has transformed the fish community at Ormesby, from one dominated by roach and bream, to a more diverse community. Numbers of perch in the lake in 2007 were encouraging and rudd were considered well established, making up 75% of the total fish abundance. Healthy populations of pike and tench were also observed.

Macrophytes
A cyclical pattern in macrophyte diversity has been observed on a yearly basis, the reasons for which remain a mystery. The summer surveys in recent years reveal a diverse community of up to eleven species with the percentage cover of macrophytes in the Trinity Broads increasing significantly since the start of the restoration project. Late summer surveys, however, show a greater covering of algae and a decline in submerged macrophytes. This community shift is thought to be associated, in part, with the water clarity.

Water clarity
The diverse macrophyte community present in early summer has been observed when the water when the lake is clear. With the macrophytic decline repeatedly seen in late summer comes a decrease in water clarity. The initial theory that water clarity was correlated with nutrient-rich excretion and sediment disturbance by large fish remains tenuous and the partnership at the Trinity Broads will continue investigate this area of research.