Lakewide information about the planktonic ciliates of the North American Great Lakes has been lacking. Previously published data are limited spatially and temporally and based on few samples. This article focuses on ciliates sampled in lakewide longitudinal surveys conducted by Fisheries and Oceans Canada in four of the Great Lakes. Extensive sampling (n = 442) was conducted at stations distributed across Lakes Superior, Huron, Erie, and Ontario in various years from 1979–2000. The study focuses on abundance, biomass, and size spectrum. Our data demonstrate that ciliates contribute significantly to the planktonic communities of the Great Lakes. Mean abundance ranged from <1.0 to >14,000 l−1. Mean biomass ranged from <1.0 to >680 mg m-3. The mean abundance and biomass values are comparable to those reported earlier by other investigators from Great Lakes and other temperate lakes. Abundance and biomass differences, between the upper and lower Great Lakes, decreased from the 1980s compared to 1990s sampling when four- to ten-fold and two-fold decreases were recorded for the lower and upper Great Lakes, respectively. The six most common genera in these samples were Askenasia, Halteria, Balanion, Strobilidium, Strombidium, and Urotricha.

The ciliate community was dominated by cells <30 μm equivalent spherical diameter. Pre- and post-zebra mussel size spectra of ciliates were compared for Lakes Erie and Ontario. The mean equivalent spherical diameter was significantly greater in Lake Erie in the post years compared to pre-mussel period. On the other hand, in Lake Ontario the mean diameter was significantly greater in the pre-mussel period than the post. It is likely that shifts in these distributions may be due to shifts in the relative abundance of the prey species of the ciliates. Among other factors, zebra mussels may be indirectly influencing the size-frequency distribution of the ciliate community.

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