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A review of the current status of the London Wetland Centre and recommendations to enhance water quality.

By IWAN Jones

Abstract

The LWC is receiving a heavy loading of phosphorus from the River Thames. Mean inflow concentration is >400 µg l-1. The high nutrient loading is favouring dense growths of cyanobacteria, particularly Aphanizomenon flos-aquae. Blooms occurred in July and November 2005. A spring diatom bloom occurred in 2006 also, as a consequence of high densities of algae entering the system from the River Thames. It appears that nutrient recycling from the sediment is not a major part of the dynamics of the system to date; this may change. Macrophytes are not present in Reservoir Lagoon, but remain in Main Lake and Sheltered Lagoon. The more frequent occurrence of dense blooms in recent years is associated with an increasing fish biomass. There is a gradient of fish biomass across the LWC, highest in Reservoir Lagoon and lowest in Sheltered Lagoon. Fish appear to have removed larger benthic invertebrates and predatory zooplankton from the Reservoir Lagoon, and are now reliant upon zooplankton and chironomid prey. Fish predation on zooplankton enables the algal blooms to develop. Fish growth rate appears to have declined over time as densities have increased. Expected further increase in fish biomass will lead to further deterioration of the system. Lack of management action will result in deterioration of Reservoir Lagoon and Main Lake. Sheltered Lagoon appears to be stable and requires little management but, as it is at the hydraulic end of the system, will benefit from improvements upstream. To improve water quality in the LWC it will be necessary to manage both the supply of nutrients to the system and the fish populations.\ud It is recommended that management take action to,\ud 1. Reduce nutrient concentrations, particularly phosphorus, in the inflowing water by phosphorus stripping. This will reduce the potential for algal blooms and increase water clarity. \ud 2. Reduce fish biomass in the Reservoir Lagoon. This will reduce predation on zooplankton and benthic invertebrates, and will result in improved water clarity and increased macrophyte growth.\ud 3. Introduce piscivorous fish (preferably pike, Esox lucius) to Reservoir Lagoon, Main Lake and possibly Sheltered Lagoon. This will help control the fish populations.\ud 4. Continue to monitor the system. It would be preferable to include Total Phosphorus (after digestion), Light Attenuance (measured in situ) and benthic invertebrates in the sampling programme. This will enable any improvements to be assessed and increase the understanding of the system.\ud \ud Actions 2 and 3 may potentially have a negative impact on piscivorous birds, but the overall improvement in the quality of the site will be beneficial to wildfowl and likely to provide more food for herbivorous and invertivorous birds. However, there is also the possibility that improvements in water clarity and macrophyte abundance will result in better hunting conditions for piscivorous birds.\ud It is possible that the macrophytes in Reservoir Lagoon will need to be protected from herbivorous birds by mesh enclosures as they re-establish

Topics: Ecology and Environment
Publisher: Wildfowl and Wetlands Trust
Year: 2006
OAI identifier: oai:nora.nerc.ac.uk:5570

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