This summer the lake has suffered again from poor water quality. The graph shows water clarity declined in May and June and has remained low since late June. This is the same pattern as in 2018 and 2020, when I attributed it to increased nutrients when the water sports people cut water weed and disturbed the sediment of the lake bed. In contrast, in 2017 and 2019 the waterweed thrived and water clarity was better. The organism concerned is a blue-green bacterium, Aphanizomenon flos-cuculae, and it can be seen floating in huge numbers in the water. The greatest risk is to dogs, which may drink the water. The science of toxicity is not clear and it could well be that our bloom is not toxic, but the precautionary advice from the Environment Agency is to avoid exposure to such blooms of blue-green bacteria, but all that
Merton has done is to erect a few notices warning people to take heed.
Things don't look good, when the lake has been in good condition in summer in only two of the last five years. It's possible that things have got worse because of a change in the amount of nutrient leaching into the lake from fertiliser used at AELTC or the golf course, or from hotter summers, but the risk is that the lake will revert to the hypertrophic condition that prevailed before the return of waterweed a few years back.
The condition of the lake is one of the concerns of English Heritage that puts the landscape on their "at risk" register. I fear that EH may be attributing the problem to silt, but the science tells us that sediment removal hasn't been shown to lead to improvement of water quality. This is because the nutrients in sediment are largely locked away from the water of the lake. The science tells us that water quality depends more on nutrients coming from the catchment and disturbance of sediment by bottom-feeding fish. Our depleted fish numbers are unlikely to be the cause, leaving nutrients from the catchment as the most likely cause.
On the management of the park. I note that several items have been taped-off pending repair for well over a month: man-hole covers on the perimeter path between the Revelstoke Road car park and brook and the waterfall. Perennial problems persist: the black self-closing gate near the white pavilion still sags and so sticks open. The yellow self-closing gate at the opposite end of the children's play area has been broken again for several weeks. There seems to be no urgency in carrying out repairs even though dogs enter the area freely and the odd child escapes. The bowls grass is being spray-irrigated on hot, dry mornings, precisely the worst treatment for water conservation. Much of the water will be evaporating in the air. Studies show that such losses can be as high as 45%. The management regime of the long grass between the tube line and tennis courts has still not been changed in response to our advice. This area is being invaded by young trees of oak, ash and horse chestnut and by bramble and blackthorn suckers. If it were not for my secateurs, the invasion would be advanced this year. The solution is easy: the grass needs to be mown twice a year. That will retain flower-rich long grass. Perhaps we want natural woodland colonisation here, which is what we will get quite quickly without a management change. The vegetation under the veteran oak shows just how quickly woody species invade when mowing ceases.
There has long been an enigmatic water flow from the south-east of the waterfall plot. I deduced that this did not reflect a leak through the dam from the lake, because it ceased when the supply of water to the waterfall was closed off and the waterfall basins dried up. So, it seemed that the leak was from the upper waterfall basin. This flow has increased considerably in recent weeks and now not only drains down the west site of the footpath towards the lower waterfall basin, but also across the adjacent path and on towards the nearby brook. The flow across the path causes concern to people walking along it. I suggest that the waterfall is in need of repairs.
In my monthly bird count on the 16th, I missed out on any passage migrants. Swifts are long gone, but swallows and martins normally call in for a bit of respite. The usual pied wagtail movement was evident, however, with several birds gathering insects from the close-cropped grasslands of the stadium, great field and bowls. Grebes and cormorants seem to be geting plenty of fish, which I continue to see as good news for the anglers because it shows that natural fish reproduction is working. More of these would survive for the anglers to enjoy were the water quality better as that leads to waterweed growth providing the young fish with a safe refuge. The Black-headed gulls are giving the grebes and Cormorants a hard time as they surface with their catch, these "kleptoparastites" keep the fishing birds hungry which is not good news for the anglers.
Many plants grow from seed under the Leylandii hedge around the stadium. These originate from birds that have eaten the fruit elsewhere and defaecate there. The protection from mowing allows the plants to grow. This year there are many Ti (the Maori name for Cordyline australis, which I insist on using in preference to the colonial horticulturalists' "cabbage palm": it's actually in the Asparagaceae, so neither a cabbage nor a palm). There's also a nice plant of Black nightshade in flower there and a single Holly. Another stange colonist is a single struggling potato plant in flower (and another on the steep bank beside Home Park Road). Sadly, these little gems are rooted out by the Park maintenance team who seem to prefer scorched-earth below the trees. A species that does need removing is the Japanese knotweed, several clumps of which flourish on the tube line embankment beside the park. Most flowering is over, but there's still a little native honeysuckle in the hedge on the golf course boundary near Wimbledon Park Road.
