There seems to have been a radical change to the lake water. The Oscillatoria that arrived this spring has persisted. After arriving in February, there were just a few floating clumps during the dry period in late March and the first half of April, but the return of westerly winds on 17th April led to a visible accumulation of this blue-green beside the promenade on the public side of the lake, around the watersports jetties, dissipated again by the 19th with the return of easterlies. So, the obvious signs of this bloom migrate around the lake with the prevailing wind, but it remains there on the lake bed and floating up to the surface. The experts tell us that blooms are often poisonous with liver or nerve toxins known collectively as cyanotoxins which are typically observed in at least half of the blooms analyzed (see, for example review papers by Prof Linda A. Lawton). Determining toxicity, however, requires specialist analytical methods, inaccessible to the ordinary laboratory or citizen scientist, so one should be precautionary and assume all blooms are toxic to animals, especially dogs, but also us. Oscillatoria is the fourth species of bloom-forming blue-green bacteria found in the lake recently. It is urgent to deal with the excess nutrients in the lake to return it to better health. LB Merton and AELTC assume that the solution lies in "de-silting", but without citing any peer-reviewed science to justify this assumption. My reading of the literature suggests that the preferred method of removing sediment from the lake would make the situation much worse, rather than better. It's surely time for AELTC to spend just a little money actually studying where the excess nutrients come from and looking at the existing published studies for the soluion.
I have taken part in the citizen science Waterblitz, Big River Watch and Freshwater Watch whenever one is run. They did this on 25th-28th April and I took part. As usual the lake scored high on Phosphate at 0.35 ppm, but as unpolluted for Nitrate, also 0.35 ppm. This tells us that nitrate is the limiting nutrient and helps to explain why Blue-green bacteria do well in the lake, as they can fix atmospheric nitrogen. The Anglers have been monitoring lake water quality on and off since late 2022 and you should consult their website for their results, but suffice to say that they agree that phosphate levels are high (indeed their kit gives consistently higher phosphate than the citizen science kit). Unfortunately, the anglers' nitrate measurements, whilst agreeing that there isn't much are mainly zero, which makes no sense. I suspect their kit is not sensitive enough for nitrate. Helpfully, the anglers also measure ammonia and most of their measurements of it show no pollution.
That's the bad news, but the good news is that there's much to celebrate in the park. The native Bluebells, Dog's mercury, Lords-and-Ladies, Wild cherries, Hawthorn and Jack-by-the-hedge are flowering in the woodlands and hedgerows. Stock doves, tits, Green and Great-spotted woodpeckers, Chiff-chaffs, Starlings and Blackcaps are breeding. Bees are collecting pollen and nectar from those native plants and also from Green alkanet. None of these plants are named in the lists of "pollinator friendly" plants pushed by the popular media, but the bees have the answer. There's a pair of Mandarin ducks on the lake again this spring, and the male has been on his own recently, so the female is probably on a nest in one of the few veteran trees that AELTC have spared in their tree works. After months without any, a pair of Great-crested grebes has returned to the lake to join the Tufted ducks, Pochards, Grey herons and Mute Swans. The grebes give us hope that the fish may be breeding successfully again.
