SEASONAL DEVELOPMENT OF HYPOLIMNETIC CILIATE COMMUNITIES IN A EUTROPHIC POND

The ciliated protozoan communities in the hypolimnion of a highly prod uctive pond were investigated over two years. Three physiological grou ps could be distinguished: (a) microaerobic ciliates which tracked the oxic-anoxic boundary within the stratified water column; (b) anaerobi c ciliates with endosymbiotic methanogens; and (c) anaerobes without e ndosymbiotic methanogens. Both groups of anaerobes were confined to th e anoxic zone of the hypolimnion. Community biomass was dominated by m icroaerobic ciliates which had on average 20 times larger cells than a naerobic ciliates. Abundance and biomass of microaerobic ciliates decr eased over the summer, while anaerobic ciliates increased. This reflec ted a spatial shift in the availability of inorganic nutrients and, as a result, of ciliate food from the epi- and metalimnion to the hypoli mnion. The low biomass production of anaerobic ciliates was consistent with the low theoretical growth efficiency of anaerobic metabolism. C iliate species displayed characteristic spatial and seasonal distribut ion patterns within the water column which were similar in both years investigated. Spatial and temporal distribution was mainly governed by two factors: (1) the distribution of dissolved oxygen; and (2) the av ailability of food. Distribution patterns were not related to chemical gradients other than the oxygen gradient, but they were correlated wi th the distribution of major food sources.