There are six different issues here:

1.There are very many lakes around the country that are turning into meadows! This is because the efficient exit system designed and built by Victorians, that lost the dirtiest bottom-water, has been blocked up (naturally by sticks and stones, or more often by concrete!); the cleanest water – surface water - is now lost over a weir. This results in detritus building up in the lake till it breaks the surface and starts growing grass (see picture). What people usually do then is precisely the wrong thing: they dredge! What they should do is repair or reconstruct the old exit, to use the power of the water itself to take the detritus away. I have much experience with these lakes, and will be delighted to advise (I also know builders who are happy to work with watery systems).

Pond 2.The ecosystems of lakes are best looked at with a microscope. It is difficult to catch fish, newts, frogs – sometimes terrapins recently – but it is easy to sample plants and bottom-detritus; microscopic examination by an experienced eye – mine – can tell whether a lake or pond is in good condition.

3.Ponds – from the smallest garden feature to one acre small lakes – have special requirements, e.g. pumps, filters (dwarfs and gnomes?). There are several kinds of semi-tropical fishes that do very well in temperate ponds with an anti-freeze heater: medaka, mountain minnows, even paradise fish - while larger ponds can benefit from grass carp and odd catfish! As well as koi, of course.

4.Establishing ponds and small lakes is quite difficult. Basically, they need infecting with a variety of small crustaceans, and an eye kept on the microscopic life.

5.School ponds are a special case. They are often an Edwardian relic, useless to modern teachers who don’t know an Asellus fom a Gammarus. There are two possibilities, either making them simple and decorative with a pump and fountain, or importing books like the Observer’s Book of Pond Life to involve sixth formers – and teachers.

6.Amoeba and Hydra. In the middle years of last century, when I taught Protozoa and Coelenterates, about half of ponds had amoeba and nearly all had several hydras. Only one of more than 50 local lakes has amoeba now; hydra is more common. I have been keeping Sister Monica Taylor’s original (1932-isolated) Amoeba proteus since the middle 1970’s, and am happy to infect ponds with them. I also keep several hydras, including (hopefully, as I write) a very large Hydra pseudo-oligactis from the lake at Orielton Field Station in Pembrokeshire. .
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