Effects of ecological interactions on drought reconstructions from prairie lakes

Background/Question/Methods

Lake sediments contain fossil records that can be used to reconstruct changes in past environmental conditions. However, disparities between sediment records of geographically-close lakes may occur when an extrinsic climate signal is modified by intrinsic ecological processes. This has been a major issue for prairie saline lakes of the Great Plains, where diatom-based drought reconstructions from lake sediments have yielded asynchronous results across sites. In order to resolve these differences, and hence, improve the accuracy of paleoclimate reconstructions in this region, it is necessary to understand how drought signals in the diatom community are altered by ecological interactions occurring within the lakes themselves. Here, we investigate whether drought-related changes in lake level alter zooplankton grazing effects on algal communities, and subsequently modify the fossil diatom record that is used to reconstruct drought.

Results/Conclusions

Cladoceran zooplankton remains were analyzed in sediment cores from Coldwater Lake and Moon Lake (North Dakota, USA) in order to assess changes in grazing pressure on algae. Key zooplankton taxa differed across lakes and through time. In Coldwater Lake, assemblages included Bosmina sp., Alona sp., and Chydorus sphaericus. While these taxa were present throughout the core, older sediments were dominated by Alona sp., with a gradual shift in more recent sediments to Bosmina sp. comprising approximately 80% of the total assemblage. Zooplankton species present in the sedimentary assemblage of Coldwater Lake are primarily small-bodied, with representatives from pelagic (Bosmina sp.) and littoral habitats (Alona sp., C. sphaericus). Moon Lake assemblages are dominated by littoral zooplankton, including Alona sp., and the larger bodied taxa Leydigia leydigi and Pleuroxus sp. L. leydigi may have particularly strong effects on the diatom record, as it can feed on organic material at the sediment-water interface. These data, in combination with lake level modeling of both basins, suggest differences in grazing pressure on diatoms between the two lakes, and will aid in assessing the accuracy of drought indicators and paleosalinity reconstructions for the Great Plains.