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The excretion of P-PO₄ and N-NH₄ by snails (Lymnaea (Lymnaea) stagnalis, Lymnaea (Radix) auricularia and Planorbarius corneus) was measured in laboratory experiments. For L. (L.) stagnalis the release of these elements from decomposing animals and their faecal material were also determined. Amounts of nutrients excreted by the snails studied varied greatly in the ranges <1 to 70 μg P-PO₄ and <2 to over 200 μg N-NH₄ per individual. These amounts were very much related to the sizes of snails. The phosphorus and nitrogen contained in the tissues of L. (L.) stagnalis are released to the water very rapidly due to the rapid decomposition that the bodies of snails undergo. The release of phosphorus from freshly produced faecal matter is considerable - even greater than the excretion by live snails (expressed per unit weight). However, the limited mass of faeces produced as compared with the body weight of an snail, ensures that this still accounts for only 3% of daily excretion of phosphorus from the body of a live snail.
The research concerned the density, dominance structure and species composition of planktonic Crustacea community, as well as their diel horizontal migrations between aggregations of vegetation and open water in the littoral zone of two lakes. The Cladocera communities of sparse beds of submerged macrophyte showed a sharp domination (70%) of individuals of large-sized tycholimnetic species. In the Cladocera community in a dense beds of macrophyte also individuals of littoral species occurred in greater numbers. The filamentous algal mats were mainly dominated by Copepoda and small tycholimnetic species of Cladocera. We observed a sharp daily gradient of Crustacea density across macrophyte beds and open water. The greatest diel changes were found near the edge of the macrophyte beds (ecotone zone). Daytime density of Cladocera was several times greater in the macrophyte beds than in the open zone, while during the night-time density differences were not so marked. In contrast, in the case of filamentous algal mat, the sharp gradient of abundance remained constant by both day and night. An inverse relationship between chlorophyll a concentration and Cladocera density was noted in the transect between beds of submerged macrophytes and open water, provide to effective control of phytoplankton biomass by Cladocera during their diel migrations.
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