Anthropocene

How did Pre-Columbian Indigenous Activities and Subsequent European Colonization Affect Tropical Mountain Lakes in the Americas?

María I. Vélez

Pre-Columbian Indigenous peoples had significant impacts on terrestrial ecosystems in the American tropics. They cleared forests, which in many cases caused rapid soil erosion, and in some areas even produced anthropogenic soil e.g. Terra Preta. When Europeans arrived, the agricultural frontier expanded. Mining and exploitation of other natural resources intensified, and new plants and animals (cows, pigs, horses), and new technologies, e.g., the efficient axe, were introduced. Whereas we know a considerable amount about how Native Americans and European colonists affected terrestrial landscapes in the American tropics, little is known about how early human activities affected the physical and chemical conditions of water bodies in the region. In a recent publication in The Anthropocene (https://www.sciencedirect.com/science/article/abs/pii/S2213305421000175), Velez et al. (2021) reported on diatom stratigraphies in sediment cores collected from eight mountain lakes in Guatemala, Panama and Colombia. The authors showed that the lakes were indeed altered by human activities. Some lakes were influenced by humans as early as the middle or late Holocene. Others, though subject to the impacts of Indigenous activities throughout the record, displayed change only after European arrival. Deforestation, agriculture and urbanization caused changes in water level and increased sediment and nutrient loads, with consequent increases in water turbidity and trophic state.

This study also demonstrated that the lakes responded differently, according to their maximum water depths. Shallow lakes (≤ 7 m) changed gradually until they reached their current condition. Deeper lakes (≥7 m), however, experienced only slight fluctuations, then shifted to their modern condition. The differential response was likely a consequence of more complete water-column mixing, higher rates of nutrient cycling, and abundance of floating or submersed aquatic vegetation in shallow lakes, which resulted in diatoms responding more rapidly to environmental changes. In the deeper lakes, the slight fluctuations were probably related to changes in water level, expansion/contraction of the littoral zone area, stable thermal stratification of the water column, and limited cycling of nutrients from deeper waters.

After human impact, diatom assemblages in the study lakes became more homogenous and were characterized by lower diversity and unprecedented species composition, quite different from the communities that had inhabited the water bodies in earlier times. These novel diatom communities indicate that the lakes have lost or are losing their capacity to resist change or recover from stressors, including human activities and climate change. It is now clear that both terrestrial and aquatic ecosystems in the American tropics have been responding to the impacts of human activities for millennia, that is, long before 1950 CE, a date proposed by some to mark the beginning of the Anthropocene.

Maria I. Vélez was part of "SAFER: Sensing the Americas’ freshwater ecosystem risk from climate change" , a research project that received funding from the IAI's Small Grants Program.