Dave Dawson’s natural history notes for 21st August 2018:-

With the recent rainfall, the level of the lake had risen back to that regulated by the weir. From being dominated by small pondweed, the lake has switched over to dominance by two Cyanobacteria (a Blue-green bacteria bloom). The main species is glorified by the name Aphanizomenon flos-aquae (see the photograph Aphanizomenon flos-aquae). It had been in the lake alongside the pondweed since May, but has now come to dominate. There was also another, smaller blue-green species, perhaps a species of Sphaerospermopsis. Some blue-greens can produce toxins which rm dogs that drink from the water, or even swimmers. One wonders why no water testing is undertaken by the Watersports people? This bloom is indicative of high nutrient levels, so I fear the lake may be returning to the murky state that prevailed before the return of waterweeds a few years back. That waterweed, Rigid hornwort, is still to be seen, but not in the previous abundance. The drought seems to have caused more Canada geese to stay at the lake after their visit to moult. 130 birds is the second highest August count in the last 30 years. Two pairs of Mute swans have raised five goslings each, but the total number of swans remains lower than in the previous two summers. It was nice to see a Common sandpiper which had called in on its migration south to Africa. As is often the case in the early morning, there was a Grey wagtail in the brook near to the toilet block. The wagtails move off to less disturbed places around the lake shore when the public arrive. Watercress and Great Willowherb in the brook were flowering. Haws in the hedgerows were ripe. The planter near Wimbledon Park Road has a good growth of flowers and also four wildflowers: White deadnettle, Large bindweed, Ribwort plantain and Chinese mugwort. The woodland planting at Horse Close Wood has survived the drought remarkably well, but there are a few larger gaps that could be filled in future winters. The replacement Woodland Trust Hornbeam seems to have survived the drought. A good crop of acorns was beginning to fall, but most were affected by the Knopper gall. There were two clumps of white-flowered Cyclamen in the wood. Haws in the hedgerows were ripe and the Autumnal hawkbit was still flowering beside the tennis courts.