Last year I started a thread on the pH of my local lake, that I measured at 6.8 and worried that it might be too acidic. I've subsequently monitored the pH for 9 months. All through the winter it was around 8.4 - 8.6. However, since the weed has become prolific it has varied within the day, being acidic early in the morning and basic late afternoon. From my reading it looks as though this is typical of a lake in which there is insufficient calcium buffering capacity to neutralise the release of carbon dioxide in the night by the weed. The high pH in winter is due to the release of ammonia by the decaying weed. From talking to other members, it appears that the weed problem has only been critical in the last few years. The lake owner made a throw-away comment at our AGM that if we want the water level to be raised, he could again pump water from the neighbouring stream. So it looks as though his previous pumping efforts may have introduced fertilizer from the stream into the lake and encouraged the proliferation of the weed. The same thing happened at a carp lake I used to fish a few years ago. The only way I can see of reversing this change is to reduce the nutrient content by mechanically removing the weed and depositing it well away from the lake, so that the nitrogen released by the decaying weed does not get washed back into the lake. Although there is no evidence that this daily pH variation is toxic to fish or invertebrates, I'm sure it must be sub-optimal and could affect invertebrate population and health. The excessive weed is also a problem though it is being partially controlled by using a blue dye. Has anyone else experienced this and have any comments?