The colonization of squares of plastic suspended in midwater

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Abstract
Summary: (1) A flat substratum hung in midwater is colonized rapidly by a number of species normally found in vegetation. Some have been recorded consistently, a greater number occasionally, but only a few have not been recorded at all. Evidently most species are moving round frequently, even those, such as snails and case‐building caddis‐larvae, that cannot swim. (2) It seems likely from this that the identity of the plant cover has less influence on the structure of a community than some have thought. (3) A comparison of flat sheet, pleated sheet and artificial Littorella suspended in midwater makes possible certain statements and deductions. (i) Leptophlebia, Enallagma and chironomid larvae are more numerous on artificial Littorella; their natural habitat is within the vegetation cover. (ii) Cloeon, Cyrnus and Holocentropus are as numerous on the sheets or more numerous than in the artificial vegetation; they inhabit the surface of the plant cover where Cloeon is protected by its speed, and where the polycentropopids, protected by their nets, can trap both the true plankton and the Cladocera which feed in the vegetation. (iii) The foot of a snail and the suckers of a leech are most effective on a flat surface, which also provides a good feeding place for a snail. The higher numbers of Lymnaea and Erpobdella on the pleated compared with the flat sheets indicates the importance of cover for these two animals. (4) Flat plastic sheets hanging in the illuminated zone would improve the production of fish a little, pleating the sheets would improve it a little more, and best results would come from hanging something with a greater resemblance to natural vegetation.

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