Novel responses of diatoms in neotropical mountain lakes to indigenous and post-European occupation

How Indigenous peoples and European settlers transformed terrestrial ecosystems in the Americas is well evidenced in the literature, but far less is known about how aquatic ecosystems changed. This study examined diatom records from sediment cores from paleoclimate studies in mountain lakes of Guatemala, Panama, and Colombia, in order to clarify the long-term human influences on the ecology of these lakes. We focused on two aspects of beta diversity in identified diatom zones: species turnover and variation in relative abundances of species. All records displayed a single diatom zone or cluster of zones with assemblages that had no close analogues in the past. The ages of these distinctive assemblages varied from post-European to 5000 cal yr BP. Most novel assemblages comprised moderately motile, non-planktonic, single-celled diatoms, associated with disturbed, productive environments. Archaeological data and pollen records pointed to the onset of intensification of human activities as the main driver behind the changes in diatoms. Maximum depth exerted control on how diatom communities responded to anthropogenic stressors. In shallower lakes, diatom assemblages transitioned gradually over time into their current novel composition. In deeper lakes, assemblages displayed slight fluctuations before shifting to their current condition. Differences between the two lake groups likely resulted from a more complete water-column mixing and higher rates of nutrient cycling in the shallow lakes, which led to a more rapid response of the diatoms to environmental changes. The shift to novel recent diatom assemblages suggests that these lakes now display ecosystem characteristics with no past analogues, and that they have lost their capacity to resist change or recover from stressors, jeopardizing the future of mountain water resources.