The Distribution of Some Fresh-Water Invertebrates in Ponds in Relation to Annual Fluctuations in the Chemical Composition of the Water

Attempts have often been made to correlate the composition of the fauna of small bodies of standing water with certain chemical features of the environment. A number of interesting facts have come to light as a result of this approach. For instance, some fresh-water animals are known to be absent from ponds containing only small amounts of calcium, yet their occurrence has frequently been noted in large bodies of water of similar hardness. However, the data used in these studies have often been drawn from only one or a few visits to each locality. Since such factors as heavy rainfall have a marked effect upon the chemical constitution of the water, it seems unlikely that data of this kind give a true indication of the conditions that prevail throughout the year. This paper contains an account of a survey of sixteen ponds, extending over a period of 14 months. Since calcium is recognized as one of the most important elements for organisms living in fresh water, it was decided to investigate its importance as a factor limiting the distribution of certain invertebrate animals in soft waters. The localities studied were therefore selected so as to include a range of calcium concentrations from approximately 5 to 45 p.p.m. In a preliminary investigation a large number of ponds was examined, and fifteen of these were selected for further study. In addition, a very hard-water lake was chosen for comparison. During the period of investigation the seasonal fluctuations of a number of chemical constituents of the water were studied, and the distribution of certain invertebrate animals in the ponds was thoroughly examined. It was decided to collect molluscs, leeches and flatworms, as these animals represent the bulk of the larger members of the soft-bodied fauna inhabiting fresh water. In addition, Asellus and Gammarus were collected. It was considered that, unlike the aquatic insects, these animals would be particularly sensitive to the chemical composition of the water because their bodies are either directly exposed to the external medium, or enclosed in a casing composed largely of calcium salts.