August 2026

The drought and heat hardly went away in August. There was a heatwave on the 13th and 14th and higher than average mean temperatures afterwards, even if nothing above 25C. After no rain since June 27th, just 2mm of rain fell around the 19th August and 7.7mm between the 27th and the end of the month, barely enough to moisten the ground surface and then evaporate away within a day or so. Lake levels were also affected. They dropped below the outfall weir crest at the beginning of May and, despite 100mm of rain in June, remained below. The minimum in this drought year occurred at the end of August, just before the return of rain in early September. This was 135mm below the dam crest. For lake levels in September see the account of that month.

At the end of the month, I had still had received no detail of work carried out by LB Merton to investigate the death of many large Carp on 8th July. 

Aphanizominon can be seen with the naked eye

 

I sampled the lake water more often than usual, as the heat waves and drought might have resulted in interesting changes. On the 10th there was a big change, as the cyanobacterium that had been blooming since the end of May (Aphanizomenon) had greatly declined. Before concluding that the ultrasonic device installed to control cyanobacteria had been effective, I took a closer look at the data. First, one of my sampling points  was within a few metres of the ultrasonic kit and the two others distant. Yet, the decline was evident at all three. Second, I had observed large numbers of a predator of filamentous cyanobacteria in my sample of 29th July. This doubtless had risen to such large numbers on a diet of Aphanizomenon, so was an alternative explanation for the decline the latter. Third, and crucially, I found a bloom of a non-filamentous cyanobacterium on the 10th.

Microcystis is not readily identified by eye

This species, Microcystis aeruginosa, has gas vesicles to regulate its buoyancy, which should make it as susceptible to the ultrasound as any other planktonic cyanobacterium, yet it was abundant at all three of my sampling points. Sadly, Microcystis aeruginosa has a bad reputation for toxicity and bloomed all month, being even more abundant in my samples of August 30th. So, the kit that was supposed to kill cyanobacteria did not kill one species and the other may well have declined because it was consumed by a predator, not by the technology. It looks as if we are paying for kit that has a placebo effect, at best. A pity, as the effect of ultrasound on planktonic cyanobacteria is a nice story, it would be great if it were effective in our lake.

My bird count was on the 6th. As in July, numbers of Canada Geese were lower than normal, suggesting a possible adverse effect of heat waves. The waterweed-feeder, the Coot was also in reduced numbers. Unusually, there were no Tufted Ducks to be seen, perhaps an effect of the cyanobacteria, although this species is not a waterweed specialist like the Coot. Perhaps most of these birds went off to greener pastures. Again, as in July, the other two species of goose were as abundant as in previous years. The geese continued to feed on the Great Field in the early morning, only to retreat to the lake as dog walkers arrived, so the geese were finding some food there, perhaps a spindly crop of Perennial Ryegrass seeds.  No other species seemed affected, there being 1 Common Tern, 16 Mallards, 8 Moorhens, 2 Mute Swans, 7 Great Crested Grebes and three Grey Herons. So, there was no strong indication that any bird species was affected by whatever caused the mass death of large Carp.

With the return of the rains, a few plants that normally flourish in Spring and die back until the following year experienced a false spring. Two examples were Ivy-leaved Speedwell and Small Nettle which were unseasonally in flower beneath the athletics hedge.  

Ivy-leaved Speedwell

 

 

July 2026

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 8^th July 2026 is described. Some 150 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 30^oC, 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. A nice sighting there was a Hornet Mimic Hoverfly, a Medeterranean migrant species that lives in the nests of wasps and hornets. 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 now 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 a 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!

June 2026

The long Spring drought broke on the evening of the first day of meteorological summer and we had 2.5cm of rain over 24 hours. 

The annual hosting of the tennis queue on the Great Field of the park had even more problems this year than last, despite AELTC having agreed more extensive measures to control the crowd. 

There were three significant issues that I picked up by chance, as I went into the hedgerow that runs south from the brook on the tube line boundary and below the trees around the corner from there bordering the gardens off Home Park Road, in the course of fieldwork. Neither area was fenced.

The first issue was a long queue for the public toilets by the cafe, mostly people from the tennis queue who told me that waiting times were 45 minutes for the AELTC facilities. This competition for the public facility deprived the public using the play areas the ready use of toilets.

 

The second issue might relate to the first issue, or the third. This was extensive, fresh defecation in the two wooded areas: at least 20 places with recent use. This was obviously tennis queue-related, either direct use or displaced public from the play areas.

 

The third issue was new camping equipment hidden and stored neatly in the wooded areas: tents groundsheets, etc. A count of the number of sleeping bags gave an estimate of 15 campers. I presume that the tents were erected at night after security patrols ceased, and on the grass nearby, and tidied away before security visits the following morning. I suspect these were free campers, disliking regulation, but they just might be homeless rough sleepers. I heaped up the kit beside the play area fence and it was cleared away promptly the same morning. 

 

Sadly, it seems that the queue has turned into a poorly controlled festival. I'd like to see the queuers welcomed by being given access to shade, and information on the values of the park, but I suspect social media will spread a story of fun in the sun and cause recurring problems from huge crowds. Reluctantly, I think AELTC should be denied use of the park and the tennis queue relocated to their land. Even then we can expect recurrent problems in the park during the fortnight from overspill. The festival vibe may not fade.

 

May 2026

I did the bird count on the 5th. This confirmed that there was just the one Mute Swan on the lake. I guess that the pen and her brood were got by a fox? By the end of the month, there were a pair of swans again, so the Cob had obtained a replacement mate. Sometimes the Swan Sanctuary carries out a rescue, but I haven't heard of any recent activity there. Meanwhile, the geese all managed to hatch broods. There were three pairs of Greylags accompanying their broods, two sets of Egyptians and one Canada Goose brood. Just the one Great-crested Grebe was seen, the hope is that the other was sitting tight on a nest. Swifts were evident most days in the month, commuting to the lake, so there must be some insects emerging to make those commutes worthwhile. Swarms of non-biting midges were noted on the 30th. Blackbirds, Robins, Song Thrushes, Stock Doves, Woodpigeons and Wrens were all singing, in most cases in the lead up to a second brood, I expect. Some birds were conspicuous commuting between feeding places and their nests, with Starlings most obvious.

Hawthorns were in bloom early in the month. 

At the beginning of the month, the effects of widespread spraying with glyphosate became evident. Despite the acknowledged value of Admiral's Hedge, this herbicide was sprayed onto hedge-bottom plants, so killing off everything growing there. Hedge bottom wildflowers are valuable, but here they get no chance. The perimeter path here is separated from the hedge by a grassy strip which is mown regularly. This frequency is appropriate beside the path, but there is room for a natural hedge bottom on the other side here, instead of the scortched earth that we now have. We still don't know why natural vegetation is regarded as the enemy here .

At the same time, elsewhere in the park, spray was used around the base of trees, poles, fences, hedges and seats. I had planted out some woodland wildflowers in Cecile's hedge, hoping to establish them in the park. Unfortunately I put them near the seats, so that I knew where to look to see if they established. Sadly, almost all of them were sprayed out. Here, too, there was no need for glyphosate use, as the hedge bottom plants there were shaded by the shrubs of the hedge and so were not tall or invasive at all. 

Glyphosate is licensed for use in England, but I don't imagine parents and carers will be delighted to have their little ones investigating the bottom of the fences beside the children's play areas. Children investigate things in their natural exploratory behaviour and the chemical can take hours to be absorbed when humidity is high. It's my view that there are easy alternatives in most or all of the places that glyphosate is used. One can decide to live with, and celebrate, the wildflowers, or use a strimmer.

Painted Lady

The warm dry Spring resulted in a surge in insect populations. Late in the month I saw several Painted Lady butterflies. This species is a migrant from the Mediterranean, numbers varying between years. A conspicuous resident species was the Broad-bodied Chaser dragonfly, which I saw dipping to the surface of the lake depositing eggs. There were also many Large Red Damselflies and one of the many blue damselfly species.

Broad-bodied Chaser

 

 

The vegetation of Ricardo's Shrubbery is now dense, making it difficult to find all the species, but a survey added 10 extras and corrections to some previous identifications.

At this time of year the hedgerows and woodland edges need pruning to remove vigorous incursions into the adjacent grassland whilst retaining the flowering and fruiting branches. I did this with hand tools, as the bird breeding season has two months to run and nests are notoriously difficult to find in amongst the dense vegetation. Working by hand means that almost all nests can be seen and avoided. 

I sampled the lake water on the 27th. The chemical analysis gave low concentrations of both Phosphates and Nitrates, but the water quality was low nevertheless. The good news is that the benthic blue-green Oscillatoria had diminished, although the bottom sediment remained anoxic and most unpleasant, there were no longer globs rising to the surface with trapped photosynthetic oxygen. The bad news is that there had been an earlier than usual great growth of Aphanizomenon flos-aquae, which floats in mid-water. Roughly one bright-green cluster of filaments per cc of water. The water sports people are aware of this. Most members of the public seem unaware and I see dogs put in to swim and families feeding the birds in big number. The Waterwatch chemical monitoring shows low levels of nitrates and phosphates, perhaps because the blue-greens have taken up most of it. Whilst there is no doubt that the main cause of the blue-green bloom is excess nutrients (exacerbated by the heat wave) I'm not aware that anyone is monitoring the tributaries. That's something that AELTC could afford to do, given that the tributaries all issue from their land. I doubt that the Environment Agency will do anything. There was a big hatch of fish fry on the 27th, probably Carp. Fish-eating birds, however, are in diminished numbers, suggesting that the biomass of small fish remains low.

The Cuckoo Flower Meadow, between the tennis courts and White Pavilion was not living up to its name, with just one or two flowers at the end of the month. Perhaps it should be called Germander Speedwell Meadow, as there was a nice display of this bright blue flower there at the end of the month.