Comment on Bachmann et al. (2013): A nonrepresentative sample cannot describe the extent of cultural eutrophication of natural lakes in the United States

Abstract

In their recent paper, Bachmann et al. (2013) evaluate the extent to which natural lakes in the contiguous United States have been affected by cultural eutrophication since Europe-an settlement, using paleolimnological data collected during the 2007 National Lakes Assessment (NLA; U.S. Environ-mental Protection Agency [USEPA] 2009). The NLA sites were selected using a statistically valid sampling design that allows for the overall ecological condition of the nation’s lakes to be accurately characterized (USEPA 2009). Given the current consensus among limnologists regarding the prevalence of culturally eutrophic lakes (Finlayson and D’Cruz 2005; Carpenter et al. 2011), the conclusion of Bachmann et al. that ‘‘in the United States of America, the extent that natural lakes have been changed by cultural eutrophication does not seem to be large’’ (Bachmann et al. 2013, p. 950) is surprising. The findings of Bachmann et al. supporting this statement are not based on the entire NLA sample of natural lakes but rather on a subset of them. We demonstrate below that not only is this subset not representative of the entire population of natural lakes in the United States, but that it is biased toward lakes in regions with less anthropogenic activity and substantially lower nutrient concentrations. Consequently, we argue that the conclusions drawn by Bachmann et al. (2013) at the national scale are based upon a statistically flawed analysis