The spatial patterns in which ecological phenomena occur can have profound effects on the outcomes of other ecological processes. Zooplankton, a ubiquitous component of aquatic ecosystems, are found in most aquatic habitats. Annual cycles in their population densities are typified by spring and summer population maxima. Their effects on aquatic systems can be substantial and have been well documented, propagating both top-down and bottom-up effects.
Our long-term monitoring program was used to describe zooplankton density patterns over two decades at 12 sites across 24 kilometers of Kentucky Lake, the terminal reservoir on the Tennessee River. Habitat types represent main channel, forested, and agricultural embayments. All sites are monitored at 16-day intervals March-November and 32 day intervals December-February. Zooplankton are sampled in triplicate using Schindler traps and enumerated to the lowest convenient taxon. To examine geographic structure of population maxima, we tested if habitat type or site proximity could best account for similarities in maximum density among sites based on the five dominant taxa. The timing and magnitude of maximum densities were calculated for each year. Maximum densities were then compared among habitat types and the distances between each pair of sites regardless of habitat using a mantel test.
Results/Conclusions
Categorical comparisons among habitat types did not yield significant differences in maximum densities nor the timing of annual maxima for any species. However, similarities between sites were apparent for Bosmina longirostris, Daphnia lumholtzi, and other Daphnia such that the nearer the sites were, the more similar were the maximum densities but not the timing of the peaks. In contrast, Diaphanosoma spp. and Leptodora kindtii did not show any evidence of spatially structured maximum densities nor timing. Timing of maximum densities was consistent among all sites within any given year with a trend toward earlier peak times in later years. Differences in timing among sites may have been obscured by the 16 day monitoring schedule, but the differences among years appeared to overwhelm the potential effects. In conclusion, some taxa exhibited geographic structure over the 22 km long section of Kentucky Lake while others did not. This structure was best described by proximity, not habitat type. Overall, the lack of habitat differences and comparatively small differences in geographic structure indicate that this section of the lake is acting as an ecological unit independent of landscape effects.