by Trevor Moore
The report identifies a key issue for the main waterbody of the lake and the downstream channel as being the lowering of water levels in periods where there is little or no rainfall and poor water quality resulting in algal blooms. They assess that in an average year there should be a surplus of water to the lake sufficient to maintain water levels, so the suspected lowering of the levels suggests there could be seepage from the base of the lake. (As part of the Heritage Lottery funded works in 2007 the lake was drained and although the clay base was re-sealed it may not have been completely achieved.)
35percent have suggested that a stage board be installed and water levels monitored on a daily or weekly basis. These levels can then be related to rainfall to assess whether remediation work is required to stop any seepage. They warn that topping up with mains water could have a detrimental effect on water quality, so should be used sparingly.
As phosphorous levels in the lake are high, probably due to the bird and fish populations found at the lake, and potentially via surface runoff into it, they state that efforts to reduce bird feeding via bread should be continued to reduce the overall pollutant loading to the lake. Grain is sold at the café and this should be continued to encourage people not to put food waste into the lake. They also recommend regular fish surveys to ensure that fish populations do not have a negative impact on water quality - excessive numbers can lead to algal blooms.
To trap any pollutants entering the lake it is suggested that reed areas be extended to fully cover the inflow from one inlet and a reedbed be established around the second main inlet, so that pollutants are trapped before water enters the lake.
There is a recirculation system that lifts water from the outlet channel back into the lake, as well as aeration further down the channel into the so-called rivulet. They recommend an assessment of the aeration systems in relation to their ability to increase oxygen in the water. Should these prove ineffective then a new aeration system should be considered. They suggest the use of barley straw as an additional tool to inhibit algal blooms. (It seems that barley straw, when exposed to sunlight and in the presence of oxygen, produces a chemical that inhibits algal growth.)
Some years ago Dulwich Park Friends installed wire cages around new aquatic planting in the ‘swamp’ area near the boardwalk, to deter damage to new growth by geese and other wildfowl. Such is the robustness of those plantings now, 35percent report that the cages appear to be constraining the development of vegetation. A trial removal of wire-netting in some areas will therefore be undertaken, with a monitoring of the habitat to assess whether this protection is still required.
Consistent with the approach to dead wood elsewhere in the park, they recommend the development of wood decay habitat around the lake, to act as a refuge for amphibians as well as a potential breeding-ground for invertebrates such as stag beetles.
Finally, the report recommends some baseline surveys including of bats, amphibians, aquatic invertebrates and butterflies and moths, as reference points for future monitoring and input for lake management.
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