There are two species of bird on the lake that increased greatly when there was a recovery of waterweeds some ten years ago: Mute Swans and Coots, both of which feed on the submerged waterweed. On the 3rd, I noted a third species, not observed before. In the early morning, the geese that feed on the surrounding grasslands make their way to the lake and there were Canada Geese and Greylags there. The Canadas were upending to gather a fresh growth of Fennel Pondweed and consuming it, but the Greylags weren't; an interesting difference in the feeding ecology of these two. It could be that the drought left the geese short of their early morning bite from the grassland and that those Canada Geese were making up for the shortage, but it's interesting nevertheless that the Greylags seemed to lack the ability.
The water in the brook downstream of the cafe was a series of pools with just a trickle of flow on the 4th, accounting for the Water Striders seen there, a species of calm waters. And there was a new species of plant for the park, Elegant Clarkia, growing beside the brook. This is a garden annual originating in California and it remains a mystery how it arrived in this odd situation.
There was just a little easing of the drought on the 8th when there was around 10 mm of rain, but then we were straight back into another hot dry week. Sadly, the high temperatures and nutrient pollution in the lake led to a significant bloom of blue-green bacteria and water clarity declined greatly early in the month, as has happened in previous years with blooms. I could see the floating clumps of Oscillatoria and the abundance of Aphanizomenon flos-aquae, which is dispersed through the water. Both of these are known sometimes to be toxic. At the beginning of the month the lake level was below the crest of the outfall weir, so practically no water, polluted by blue-greens or not, went down the brook from the lake. However, the old pipe that issues near the toilet block continued with a trickle. This pipe carries the surface water drainage from much of Home Park Road, so clearly there are springs or leaks in this area that provide a tiny flow even in drought times. After the 8th, water flowed down from the weir in the new course of the brook for a day or two, and it was noted that this was milky in appearance and that there were some suds where it flowed over an obstruction. This is typical of lake water with algal blooms and was, therefore, not the result of a single pollution incident, rather of an ongoing situation in the whole lake.
I did my regular lake water sampling on the 11th, when I found the cause of that colour in the brook: a bloom of a blue-green bacterium, confirmed by microscopic examination as Aphanizomenon flos-aquae, This species is unusual, as the individual filaments clump together to form shapes that are visible to the naked eye floating dispersed throughout the water body, looking much like finely-chopped grass cuttings (see the magnified image). I have found this species in a wide range of months (April to December) every year since I first noted it blooming in 2018 and it bloomed also in May and August 2019, September 2021, June 2022 and August 2024 as well as this month. On these occasions I found some pondweeds and plenty of microscopic animals, such as water boatmen, water fleas, water mites and the copepod Cyclops, nor was any unusual fish mortality observed, suggesting that the blooms of this species were not very toxic on those occasions. When Aphanizomenon runs out of nutrients or oxygen it can die off and be seen more obviously as bright green scum accompanied by a turquoise coloration in the water. That's what was seen in the incident after the 8th. Another species, Oscillatoria, bloomed this Spring, between February and April, and was still present, if in reduced numbers. Like Aphanizomenon it can be identified without a microscope as it lives in the top layer of sediment and the oxygen given off by its photosynthesis causes ugly globs of sediment to float to the surface. It has bloomed previously in the Spring of 2023.
There was a much more significant pollution of the brook with a blue-green bloom on the 23rd and 24th. This was reported in the Putney News on the 30th: https://putney.news/2025/07/30/anglers-campaigners-point-to-aeltc-after-mystery-lake-spill/?bclid=IwQ0xDSwL2pR1leHRuA2FlbQIxMQABHulJqMo3Iii5nqGBCfckoVWQiSx05tP9MFqsrh9_MqfKPsfxeHiiC9_33oxb_aem_G1OERC3RHjGQ-i_NZoyIuw&sfnsn=scwspwa
This article is remarkably good journalism, but it speculates beyond the present incident, with its headline "Many now believe the spill may have originated from drainage linked to the All England Lawn Tennis Club (AELTC), and fear it may foreshadow far more serious pollution if the club’s expansion plans are approved." We just don't know whether the cause was a spill and we have great reservations about the water quality implications of the AELTC proposals, but that is not our present concern. Here, I focus on the incident around the 24th.
Before recounting this recent incident, I repeat the occasion when a dog was left partially paralysed by exposure to a bloom in the brook in early September 2022, when I confirmed the organism to be Dolicospermum, which bloomed in August and September that year. The dog was poisoned by the contaminated water of the brook. The samples for blue-greens commissioned by LB Merton in late September, well after the incident, confirmed only Microcystis aeruginosa in high numbers. I had found a tiny amount of Microcystis in previous years and it was there in September 2022, but did not bloom. I am confident it was not the cause of the dog poisoning. It is not identifiable without a microscope. Dolicospermum also requires a microscope for determination and it has been found between June and September in other years, but not since 2022.
I was out of town on 23rd July, when there were reports of a flow of milky-grey or turquoise water, foaming where turbulent, in the whole length of the brook and of a dog under treatment as a result of exposure to the water. It was first reported where the downstream pipe discharges into the River Wandle at Earlsfield, and the Environment Agency was alerted. Photographs the following day confirmed that it came from the stilling pool which lies below the outfall weir in Ashen Grove Wood, just upstream of the public park. An angler visited the lake later on the 24th, confirming that the milky-grey water was flowing over the lake outfall weir, it was foul-smelling and there were fewer birds by the lakeside promende than usual, and he also reported to the Environment Agency. There were no signs of dead birds or fish. The Council was alerted by the Friends of Wimbledon Park, and posted extra warning notices on the 24th, and also contacted the Environment Agency.
The dog was affected by entering the polluted water of the brook and kept under observation by a veterinarian on the 24th and discharged unharmed. The dog owner had acted prudently.
Some further background.
Blue-green blooms are most numerous in summer, when light levels and water temperatures are high, but the key factor is excessive levels of plant nutrients dissolved in the water. I have compiled nutrient determinations by myself (most recently through FreshWaterWatch, a citizen science scheme) by the anglers and a very few by others. These results show that the lake has often had moderate or high phosphate concentrations, which could lead to blooms if it were not for the low concentrations of nitrogen. Many blue-greens are able to fix nitrogen, so surviving or even thriving where plants would find the low concentrations limiting. This year, phosphate measurements declined from high levels in April and May to low levels this month, so phosphate appears to have become limiting this month, perhaps causing the mass death of Aphanizomenon, but that's speculation on my part.
The breaking of the drought, accompanied by overtopping of the weir and a flow down the Brook after months of no more than a trickle, clearly explains why a bloom appeared in the Brook on the 8th. Press reports then described thunderstorms at the Wimbledon Championship. The drought returned after that, but there was much more rainfall on the 19th which sent a flood down the brook. On my return home on the 26th, my rain guage showed that 40mm of rain had fallen in the previous week. However, that flood didn't show a bloom. Photographs of the brook taken by Paul Johnson show it was running dirty, but with no colouration on the morning of the 23rd, yet the first report right down at Earlsfield that evening shows that the bloom originated that day and quickly filled the whole length of the brook. Photos also show that there was a visible bright green scum on the lake beside the promende on the 24th.
The visible bloom was over by the time I was able to take water samples on the afternoon of the 26th, but my samples confirmed that Aphanizomenon was still present dispersed throughout the water of both the lake and the brook, just no longer super-abundant, as it was on the 11th. No other blue-green species was evident then. We, and the dog, had been lucky as the bloom seems not to have been very toxic.
The origin of the nutrient pollution that is a pre-requisite of a bloom remains uncertain, it
can be excessive bird feeding, atmospheric deposition, bird defecation, and sources in the catchment coming down the tributary pipes, none of which are monitored, including the drain from the AELTC site. It's reprehensible that Merton owns and manages the site and runs the watersports yet does no substantial monitoring, and that AELTC remains silent on what goes down its drains. I, and the anglers, publish our information (me most recently as part of my representations on the AELTC overdevelopment proposals), but we have neither the access nor the resources to monitor the sources.
With a microscope, blue-green bacteria are readily monitored in water samples, as I have been doing for some eight years but, unfortunately, testing for toxicity requires specialist methods not available to the ordinary environmental scientist and an innocuous bloom can turn toxic. Effectively, we are using dogs as a test species, which is not an appropriate approach. This means that precaution is required.
Since I wrote this, it appears that Thames Water have inspected the site. The first occasion was late at night, perhaps on the 24th and certainly that week. On the second occasion, the evening of the 30th, a local lawyer spoke to the TW operatives parked up by the Home Park Road entrance to the public park, who said they were investigating the reports of pollution. I would comment, rather late. I have previously sought to help TW with their investigation of local incidents, finding them unreceptive, but I tried again, getting an incident number 01242268.
