Water quality October 2020

The long-beaked Bosminid

  

The results are out for the Thames Waterblitz 2020. 

(https://freshwaterwatch.thewaterhub.org/thames-waterblitz?utm_source=fww&utm_medium=email&utm_campaign=Thames%20WaterBlitz%20Report%202020%20Email).

We have now contributed October results to this citizen-science survey for four years. As with other lakes, our results have varied greatly from year to year. They continue to show that our lake is limited by the low levels of nitrates, which have always been within the "low" range in October. This accounts for the quality of the lake water in autumn, supporting good populations of fish and other water life when things are not messed about by attempts to remove waterweed. Phosphate levels were high in 2017 and medium in the successive three years, but the lake water did not deteriorate, because blue green bacterial and algal growth was limited by the low nitrate levels. Water clarity is another good measure of lake water quality. The graph above shows that this differed between the four years, being good in the summer and autumn of 2017 and 2019, but poor in 2018 and this year, these being the two years when waterweed growth was removed from the lake, so disturbing the sediment and releasing nutrients into the water. Clearance of waterweed also denies fish fry good shelter from predators, including the cormorants that are visiting the lake in good numbers. It's surely possible to remove waterweed carefully from the parts of the lake used by watersports and for angling, whilst leaving it elsewhere to provide good natural cover for waterlife.

My monthly water sampling was completed on 23rd October. This showed that the blue-green bacterial bloom was almost gone, there being just the odd declining clump of Dolichospermum in the open water. The recovery of water clarity since the murk in summer reflected this decline. There were few of the bigger water fleas in the cooling water but it was good to see the return of the long-spined waterflea, not seen since April. There were many of a very small species, the long-beaked bosminid, much smaller than a grain of sand. There was a long list of other small animals and small amount of the waterweed, rigid hornwort and of blanket weed. A new discovery was a good number of a tiny green flatworm Dalyellia viridis. As usual, the fish supported many fish leeches.