Lake sediment DNA reveals the response of phytoplankton to warming

Abstract

Phytoplankton can be used as an indicator of aquatic ecological health and to monitor nutrient levels, which are the basis of formulating lake management strategies. However, long-term phytoplankton succession and their response to environmental changes remain poorly understood due to the paucity of ecological monitoring data and the limitation of the traditional paleolimnological indicators used to identify non-fossilized phytoplankton groups (e.g. Cyanobacteria). In this study, we used both paleolimnological and 23S rRNA gene metabarcoding analyses of phytoplankton (eukaryotic algae and cyanobacteria) to probe the impacts of environmental changes on the phytoplankton dynamics over the past millennium in this deep, fault-bounded, eutrophic Lake Chenghai on the Yunnan–Kweichow Plateau, southwest China. The phytoplankton community succession was observed in four periods, including the Medieval Warm Periods (1000s–1400s), the Little Ice Age (1400s–1800s), the early Current Warm Period (1800s–1950s) and the Anthropocene (1950s–present). Specifically, Cyanobacteria dominated during the warm periods, whereas Chlorophyta was abundant during the cold period. Bloom-forming or cyanotoxin-producing taxa (e.g., Aphanizomenon) have mainly occurred in recent decades. Co-occurrence network analysis indicated that the community structure became complex and that the diversity has increased markedly during the Anthropocene. Multiple statistical analyses demonstrated that temperature was a key driver controlling phytoplankton succession on the decadal time scales and that anthropogenic warming may have led to an increase in the relative abundance of bloom-forming cyanobacterial taxa in recent decades. These findings further suggest that any efforts to reduce nutrients—and the effects of these efforts—will be offset, and that cyanobacterial blooms may continue to increase in Lake Chenghai if anthropogenic warming conditions continue.