Comparing two zooplankton sampling methods for long-term community and population monitoring

Background/Question/Methods

Zooplankton are a very important part of aquatic ecosystems and are the trophic link between primary producers and upper-level consumers in lake food webs. Because zooplankton population changes could strongly affect food webs, long-term monitoring is important to determining if changes have occurred. We asked whether populations and communities in Kentucky Lake, Kentucky, would show different long-term patterns using two methods, grab samples at a single depth and vertical tows. Grab samples used Schindler traps that captured zooplankton in a 15-liter volume of water at a single point in the water column. Vertical tows used Wisconsin nets that captured zooplankton throughout the water column from near the bottom to the surface. Comparisons were analyzed using samples from four of sixteen regularly sampled sites in 2008. The zooplankton from each sample were enumerated to the lowest convenient taxon using a standard protocol. 

Results/Conclusions

Enumerated zooplankton from these four sites totaled 86,430 and 14,736 individuals and 30 and 15 taxa from vertical tows and Schindler traps, respectively. Preliminary results found significant differences from vertical tows among sites for the total number of zooplankton (p=0.02) with higher densities in the channel. Although no significant differences in total zooplankton existed for Schindler traps, channel sites were often highest as well. Taxon richness did not exhibit any significant differences among sites nor were qualitative differences among sites consistent between sampling methods. Annual timing of maximum densities in Daphnia retrocurva was consistent between methods, whereas Bosmina longirostris exhibited spring and autumn peaks in vertical tows and only a spring peak in Schindler traps. The results thus far indicate that there are both similarities and important differences between the two sampling methods. Identifying more peak densities within species and more total taxa may be influenced by sampling more individuals and by sampling different levels in the water column during diel vertical migrations. For some species, the reliability of measuring and observing the zooplankton communities by one method alone may not be sufficient for understanding long-term patterns and community changes.