Over the previous month much Fennel-leaved pondweed had been removed from the lake and I found that a blue-green bacterium, Aphanizomenon flos-aquae, had come to dominate the open water.
This last happened in 2018, when the bacterium was so abundant that it made the water murky until late August. The Long-spined waterflea was absent when the bacterium dominated, but returned in late August. I speculated that the clearance of waterweed had disturbed the sediment on the lake bottom, increasing water fertility and leading to the bacterial growth. So I have been looking closely at water clarity this year. I was disturbed to find a decline in clarity in June, but pleased that this did not continue into July, as it did in 2018. Nevertheless, I found not a single waterflea this July, so there remains a risk that water quality will decline further.
This is not a trivial matter, as blue-green bacteria can be toxic and those doing water sports, and the anglers, could be at risk.
The chart below gives the water clarity for each of the last four years. It shows the drastic reduction in clarity between June and July 2018 and the arrested decline this year.
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