water as everyone knows is a compound H20 (2 atoms of Hydrogen + 1 atom of Oxygen) it also contains O2 which is the oxygen dissolved in the water and the supply of oxygen that fish and other aquatic creatures use. Most algae blooms are due to an increase in Phosphorus (and possibly Carbon and Nitrates), this is a powerful nutrient which encourages the growth of all plants and particularly algae. Algae consume O2 but have a short lifespan and die and decay in large number, the decaying process also uses up O2. Given a continued supply of Phosphorus the cycle of algae growing and dying continues unabated only exponentially. As the amount of O2 decreases the organisms best suited for survival grow in enormous numbers and also are the last to succumb, in this case the algae, which is why it shows as a visible bloom. By the time this occurs the "higher" lifeforms are either dead or doomed to die unless steps are taken to re-oxygenate the water (fish tanks and ornamental ponds have fountains or pumps to force oxygen to dissolve back in to the water)
They can also produce some absolutely devastating toxins which are completely fatal to all life forms, currently Brittany in France has a major problem in some coastal areas due to the run off of phosphates from intensive pig farming where the algae growing on this nutrient is releasing the toxins in the air when decayed algae are disturbed resulting in a number of human fatalities!
Clearly the problems that came to a head at the weekend have been utterly devastating, resulting in the death of all life forms in the Mere, it is quite possible this tragedy has been a short time in reaching that point with the final moments literally moments. I believe eels like to be deep in the water, possibly in the mud? This may explain the presence of dead eels who could not escape, of course we have no way to tell how many eels DID escape once conditions became untenable. The easiest way to imagine what the fish and other creatures experienced is think of yourself in a space ship and then stepping into the vacuum of space without air.
I cant find much information about the Mere other than it has this mythical aura around it. Specifically due to the Black Mirror. It appears it had been closed for fishing and was designated a SSSI? That makes me think that the EA must have had some kind of monitoring set up, including scientific testing of water? Has anyone visited/fished it? How large was the lake? If there was any hint of poisoning or the usual culprit, industrial discharging, it would hardly help secure a prosecution for Mike Willmott to make statements regarding the matter and quoting an EA officer as to what had happened? Is this the same Mike Willmott who was caught illegally fishing the lake and fined for doing so? Allegedly!!