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