The conterminous US has seen many landscape changes over the past century and spans a substantial latitudinal gradient, making it an ideal area to examine the effect of latitude and historical change on freshwater biodiversity. The US Environmental Protection Agency administers a “National Lakes Assessment” program that collects both contemporary and paleolimnological samples from a large sample size of lakes, encompassing different histories of colonization and land-use. We paired deep (pre-industrial era) and surface sediments with contemporary water column samples for both diatoms and cladoceran zooplankton for ~1200 lakes from the 2007 NLA. We used overlays of correlation structure to look for differences in community drivers between paleolimnological and contemporary sampling. This represents the first time that a study has addressed differences in environmental structuring patterns between paleolimnological and contemporary samples across such a large spatial scale and with such a substantial number of sites. Secondly, we tested the hypothesis that latitude will have a weaker effect on species diversity and functional diversity patterns in modern sediments as opposed to pre-1850 sediments due to the increasing presence of anthropogenic stressors. Using regression and beta-diversity approaches we examined change in relation to growth in human population, dam building and agricultural land.
Results/Conclusions
Environmental variables explained a significantly higher amount of community variation (both diatoms and crustacean zooplankton) in surface sediment samples than in contemporary water samples. This is likely because paleolimnological samples represent at least a yearly average of community composition and as such, dispersal limitation is much less important over that length of time scale. Additional work is needed to test for differences in variation partitioning between crustacean and diatom groups because of perceived differences in dispersal ability. Preliminary linear regressions of species richness, Shannon-Weiner diversity and evenness against latitude show variable patterns along different longitudinal transects. Plankton communities from both the Eastern Seaboard and the mid-East do not show the traditional pattern of decreasing species richness with increasing latitude. This may be a result of highly productive lakes in the Florida panhandle as well as around the Great Lakes region. Temporal beta-diversity, turnover between bottom and surface sediments, is also higher for lakes in the eastern part of the United States, suggesting anthropogenic disturbance has affected these lakes on a longer time scale than lakes in western locations.