Abstract

Spatial distribution shapes almost every aspect of the ecology of populations. Even though the importance of spatial distribution to the ecology of zooplankton is established, little is known about the factors that generate and maintain these aggregations (Megard et al . 1997). The role that environmental variables, specifically light intensity, and pond substrate coloration play on the formation and maintenance of aggregations was investigated in the freshwater calanoid Hesperodiaptomus shoshone . The spatial distribution of H. shoshone was determined using traditional methods (i.e. the Morisita's index of dispersion) and spatial methods--trend analyses and autocorrelations--with the aim of clarifying distribution dynamics during a 24hr point interval sampling period. Copepods were found to be aggregated through the day and night with the highest aggregation levels at midday. Spatial patterns were not consistent either between the two ponds studied or between sampling times of the day. Partial regression analyses were used to identify the relative contribution of environmental and spatial factors structuring H. shoshone spatial distributions. Although the results were not consistent between the two ponds, light intensity and proximity to logs were two proximate factors influencing the observed copepod heterogeneity. Furthermore results showed that background colours of pond substrate were found to influence the aggregation behaviour of copepods in the water column.