Multiple drivers of ecological change in Arctic lakes and ponds

Griffiths et al. (2017) analyzed several ponds and lakes from the Cape Herschel region of Ellesmere Island in order to "...explicitly examine the role of ice cover as the dominant driver of diatom assemblage change...". I reanalyze their data and suggest that their classification scheme, that they propose is due to differences in ice cover seasonality ("warm", "cool", "cold", and "oasis"), is confounded with other morphological and chemical variables that better explain the differences between the groups. The "cold" sites are the deepest (lakes) and differ from the small, shallow ponds that occasionally dry, which would therefore have different diatom assemblages and histories. The "oasis" sites are nutrient enriched and probably have more stable water supplies, thereby enabling an aquatic flora providing habitats for diatoms. A key part of their interpretation is that "warm" sites have responded more rapidly to recent climate change than "cold" or "cool" sites, but their chronologies do not allow for such a conclusion. There is no clear difference between "cool" and "warm" sites, and problems in dating the sequences means inferences about their histories are not supported by data. Their results, which are restricted to the past century, are contradicted by a Holocene sequence from the region.