Some Observations of Zooplankton Swarms in Windermere

Abstract

The number of recorded direct observations of swarms of zooplankton in lakes appears to be very small. Birge (1897) observed swarms of Daphnia hyalina Sars. on three occasions; they were fairly distinct patches, differing in size but measuring tens of metres. The animals were confined to a narrow depth zone of only a few centimetres at the surface of the water. The concentrations were high, about 1000 per litre, and the animals were nearly all adults. Birge considered that swarms of this type were not very common and were only found during very calm conditions. Daphnia has been observed to be very patchy in its distribution (Ragotzkie & Bryson 1953, Southern & Gardiner 1926) but I have not been able to find any other references describing swarms of the type observed by Birge. In the evening of 24 June 1955 a small swarm of D. hyalina was seen in Mitchell Wyke, a deep, sheltered, roughly semi-circular bay in Windermere. The swarm was fairly distinct, about 1 m in diameter and confined to the top 10 cm of the water; the distribution in the surrounding area showed scattered patches of animals, which were not confined to the surface but occurred at varying depths down to about 50 cm. A number of samples were taken using a dip jar, one from the Daphnia swarm, four from the surrounding patches and eight at random. The results expressed in numbers per litre are given in Table 1. The Daphnia swarm