Zooplankton usually behaves in complex and dynamic ways on various temporal and spatial scales and its spatial distribution is usually highly aggregated, as it possesses a large-scale spatial variability similar to that observed in the physical environment in general. We assess here the suitability of the Optical Plankton Counter for estimating zooplankton abundance, as well as the day-to-day temporal and spatial patterns in its distribution during the open water season. The influence of certain environmental variables on zooplankton abundances is also examined.

Abundances of mesozooplankton-sized particles were measured with the Optical Plankton Counter monthly at five stations in a large mesohumic lake from May to September 2005 and agreed rather well with zooplankton abundances counted using a microscope. The poorest agreement was in autumn, when the Counter overestimated the zooplankton abundance, and for some samples in July, when it underestimated the abundance. Fluorescence and chlorophyll a had a significant effect on the Counter readings. Both the intercept and the slope of chlorophyll a varied randomly between depths, because the chlorophyll a decreased much more markedly in deeper waters than did the Counter particle counts. This emphasized the stratified nature of sampling.

In addition to seasonal variations, there were also marked short-interval variations (day-to-day) in zooplankton abundances in all the sampling periods. These patterns are highly dynamic and can on some occasions change rapidly in response to fluctuations in the weather. There were no clear differences in zooplankton abundance between the sampling stations. The horizontal heterogeneity was less pronounced than the seasonal and short-interval heterogeneity, possibly because there were no trophic gradients in the basin.

While the Optical Plankton Counter provided a rapid assessment of temporal and spatial patterns of zooplankton abundances, it does have limitations. At times it either overestimated abundances due to a large contribution from non-zooplankton particles, or underestimated abundances due to coincidence.

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