The big event of the month was an unprecedented fish mortality. This got extensive publicity locally. Here, I reproduce the summary of my report on the incident. I await further clarification from LB Merton, when they respond to my freedom of information request.
Meanwhile, here I reproduce the summary of my report on the incident. "Fish mortality, Wimbledon Park Lake", completed on 17th August.
The nature and cause of a mass mortality of fish on 8th July 2026 is described. Some 100 mature Carp, a few European Eels and a few other bottom-feeding fish were killed. There have been other mass mortality incidents there in the last 25 years, in which various fish species were involved and all of these have occurred close to the discharge points of the lake tributaries. In contrast, this event was lake-wide, and so was not caused by a short-lived discharge from a tributary.
None of the previous mass-mortalities corresponded to high temperatures, but the water temperature this time peaked at 30oC, or above. A shortage of oxygen (hypoxia) was suggested by the behaviour of the fish and by similar incidents elsewhere in the country, and confirmed by testing the water. Previous monitoring showed plenty of oxygen in the lake water, so hypoxia is rare here.
The owner of the lake, LB Merton proposed to prevent recurrence by controlling Blue-green algae (cyanobacteria), which were abundant at the time. But these were equally abundant earlier in the summer and in previous summers, as were heat waves. In contrast to previous heat waves, this mass mortality corresponded to record low wind speeds, so I conclude that three factors came together to kill the fish: abundant cyanobacteria, high temperatures and lack of wind. I checked for other possible correlates with the die-off and found none.
This work was impeded by the absence of published information on the lake, something that LB Merton could remedy readily. The ultimate cause of cyanobacterial abundance was eutrophication of the lake by phosphate, but the main source of this remains unknown. Candidates were: waterfowl defecation, die off of plankton, excess feeding of birds and fish, arrival down the tributaries, atmospheric deposition, and disturbance of sediment on the bottom of the lake. A special study is needed to identify the principal factors.
The best way to avoid a recurrence of mass mortality would be to reduce phosphate pollution, starting with the easy ways to do this and moving on to the less tractable sources. Meanwhile, LB Merton have trialled aerators and ultrasonic devices, methods without a convincing track record in lakes as large and shallow as ours. Given that oxygen comes back naturally, and cyanobacterial blooms die off naturally, there is a real risk that the devices will be confirmed as the cause of a recovery that was, in fact, a natural occurrence. It is vital that LB Merton should publish the minutes of meetings, studies and advice they have had, so that these, and future, events can be better understood.
I did my monthly bird count on the 16th. I was on the lookout for numbers or behaviour that were affected by the continuing drought, or the mass fish kill. The 100 Canada Geese were well above the long-term average for July, if not a record number. This was an about-turn after the low numbers in the last two years. One wonders whether the birds immigrated to the park because people feed them and their natural grassland food is desiccated in the drought. The 80 Greylags were somewhat more than expected, although not such an excess as the Canadas. Interestingly, 30 of the Greylags were out in the Great Field and making their way back to the lake before gate opening time and dog disturbance. So, they were finding some food in the grassland. As usual, there were many, now well-grown, young Greylags and a remarkably late brood of 2 recently-hatched goslings. In contrast, the 25 Egyptian Geese were around the expected number, despite their similar ecology. The resident pair of Mute Swans was still there. 10 Coots was a remarkably low count for July. Coots feed on the grassland of the golf course, which would have been poor forage in the drought, but also on waterweeds. This could indicate that waterweed growth was poor, forcing the Coots to leave for better foraging elsewhere. There were plenty of fish-feeding species: Cormorants, Grey Herons and Great-crested Grebes, an indication that the mass mortality hit the big Carp harder than it did smaller fish.
It was nice to see Wild Carrot in flower in the Revelstoke Road car park verge and a Nursery Web Spider in the Lofthouse Glade. There were many reports of the Jersey Tiger Moth on social media, and there was one on a tree in Horse Close Wood. Bramble and Elder fruits were ripening. Unlike last year, the small patch of Dog's Mercury in Horse Close Wood was withering in the drought. The Lime trees in Horse Close Wood had already shed their seeds. Squirrels and Wood Pigeons will be short of food. No water was flowing down from the lake outfall, but the surface water tributary from Home Park Road suburbia had a trickle. The park gates were opened at 07:00.
On the 29th, I saw a minor pollution incident just like one I saw in 2023. Wimbledon Park Brook flows through the public park and is fed mainly from the outflow from the lake. The brook was modified as part of the works to make the dam safe in 2022, and a new open channel was constructed to take the outflow. However, there has always been a second flow into that brook, not from the lake, but surface water drainage from the nearby Home Park Road. This enters the brook where it passes by the toilet block.
Both in 2023 and in this July, this surface water flow was milky, suggesting pollution by paint or concrete. On both occasions I reported the pollution on the Thames Water hotline. In 2023 I was telephoned two hours later with a request that I turn up to the site to show the pollution to their investigation team. By then the pipe was running clear and the milkiness had been dispersed downstream with the more voluminous lake overflow. I pointed out where it had issued and asked them to look at my photograph, which they refused, saying that their job was to evaluate what they themselves saw. So, the pollution was not recognised.
On the 29th of this month, Thames Water had an online "chat" for reporting the incident. I took a sample of the water and reported full details of the incident to the chat at 13:20. This was ignored and an bot took me through a protocol of questions that merely duplicated my original report. I asked for a reference number, so I might follow up. This request was ignored. At 14:15 their bot got back to me for a photograph of the incident (a surprise to me given their response in 2023), but I sent one. At 15:15 the bot promised a member of the team within 15 minutes. At 15:54 a new bot claimed to be from the operations team and repeated some of the questions over the following hour. Nothing happened then until 19:33 when a new bot took over and asked about recent rain. It was now 6 hours since my original full report, my frustrated reply was: "No rain for many days as you already know! How about actually investigating the incident. You already have enough information" Their bot replied: "Thanks for your message! I'll connect you to one of the team as soon as they're free. They’re available Monday to Friday, 8am until 5pm". I heard no more. My deduction is that no human saw any of my information and that their bot was designed to give the impression of action whilst kicking the can down the road. This experience was a strong disincentive to reporting pollution. Someone in residential Home Park Road has put pollution into the surface water drains and got away with it. I still have that sample of water!
