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Heliozoa; planktonic protozoa; seasonal dynamics; Lake Constance
Abstract:
Planktonic heliozoa were investigated at a mid-lake and an inshore station in Lake Constance (Germany) from April to November 1993. Integrated water samples were taken over 0 to 8 m and 8 to 20 m depth intervals at the deep mid-lake station and over 0 to 2 m depth at the shallow inshore station. Heliozoans were counted and identified to genus level in live samples. The following genera were identified: Actinophrys, Raphidocystis, Heterophrys, Chlamydaster, Choanocystis, Raphidiophrys, and Pterocystis. Small heliozoans (10 to 20 Fun, mainly Heterophrys and Choanocystis) generally dominated the community in terms of abundance. Large genera (Actinophrys, Raphidocystis) were, however, the major contributors to total biovolume. Total tell concentrations remained below detection Limits from April to mid-June. Maxima of up to 6.6 ind. ml(-1) were observed in summer; smaller peaks occurred in autumn. Heliozoan cell numbers were significantly positively correlated with chlorophyll a concentration close to the surface. Negative trends were found in relation to potential heliozoan competitors or predators such as rotifers and crustacea. Community biovolumes of up to 60 mm(3) m(-3) were recorded in mid-summer. The seasonal succession of the dominant genera was similar at both stations. The vertical distribution of heliozoans, examined on 2 occasions in summer and autumn, was positively correlated with chlorophyll a and temperature. We further studied the horizontal distribution of heliozoans at 8 stations across the northwestern part of Lake Constance on one occasion in late summer. Cell numbers recorded varied by a factor of 2. In situ growth rates of heliozoans were measured in diffusion chambers after the exclusion of larger potential predators at the mid-lake station on 2 occasions. Growth rates of distinct genera ranged from 0.06 to 0.59 d(-1), community growth rates from 0.3 to 0.5 d(-1). Based on growth rates and biomasses, we calculated the potential heliozoan production. Results suggest that during periods of their maximum abundance heliozoan community production is similar to average production estimates of ciliates and heterotrophic nanoflagellates. The seasonal mean heliozoan production is, however, equivalent to only about 1% of the combined ciliate and flagellate production
