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Within the anaerobic hypolimnion of a small eutrophic lake, a well defined and stable com- munity of large ciliates was present throughout the period of summer stratification. Peak ciliate densities occurred at the oxic/anoxic interface; ciliate numbers then declined in an approxi- mately exponential manner with increasing depth. Two thermally stratified 2 m high artificial lake ecosystems were constructed in which an anaerobic hypolimnion developed. Within these model lakes, populations of hypolimnetic ciliates were maintained for many months. Community structure, growth characteristics and spatial distribution of the ciliates were similar to those in the natural environment. in the deep water sediments during the period of winter mixing. Once thermal stratification occurs and the bottom water becomes anaerobic, these ciliates leave the sediments and move up into the water column. They then progressively colonize the hypolimnion in phase with the upward spread of anoxic conditions, returning to the sediment following autumnal destratifica- tion. A similar hypolimnetic ciliate community has been reported from other eutrophic lakes by Goulder (1972), Bark (1981) and Finlay (1981). With the primary aim of helping to understand the factors affecting ciliate distribution within the water column and to test putative correlations derived from field studies, two artificial lake systems, or microcosms, were constructed. These microcosms were designed to mimic a thermally stratified eutrophic lake as closely as possible. This paper describes the construction and operation of the microcosms. Ciliate distribution and growth rates in the artificial lakes are compared with data on population growth and distribution in Kettle Mere. METHODS Field srudj site. All field data presented here were obtained from Kettle Mere (location UK Grid Reference SJ 418341). This is one of the smaller meres with an area of only 1.7 ha. It is, however, relatively deep with a maximum depth of 7.3 m. It is one of the more productive sites and was probably formed as a kettle hole during the retreat of the late Devensian ice fronts. The mere is surrounded by trees on some 75% of its perimeter and the remaining edge is flanked by a steep slope. As a result of its extremely sheltered location Kettle Mere exhibits very stable thermal stratification. The lake normally stratifies thermally in April and does not become fully mixed again

A Comparison of the Growth Characteristics and Spatial Distribution of Hypolimnetic Ciliates in a Small Lake and an Artificial Lake Ecosystem