Past responses of modern phytoplankton and subfossil diatoms to volcanism in an Andean lake (central Chile)

Laguna del Maule is a volcanic basin lake in the Central Chilean Andes that contains a peculiar and poorly understood algal flora from an ecological perspective. We present a comprehensive list of modern phytoplankton and limnological parameters (measured in 2011 and 2015), as well as a subfossil diatom assemblage spanning the past ∼700 years. We examined the relationship between the past assemblages and two Holocene tephra layers, pH, and the impact of recent warming. Chlorophyll values (1.3–1.8 µg l⁻¹) are indicative of an oligotrophic lake. Counts (past and modern specimens) totalled two hundred fifty-four taxa: Bacillariophyceae (239, mainly subfossil), Chlorophyta (4), Chrysophyta (3), Cryptophyta (3), Cyanophyta (3) and Dinophyta (2). Diatom groups included: twelve centric (the most abundant specimens) and two hundred twenty-seven pennate taxa; of the latter, thirty-three were araphid and one hundred and ninety-four were either monoraphid or biraphid. An increased richness was observed after deposition of the tephra layers, along with different diatom assemblages associated with each tephra. Alkaliphilous-circumneutral taxa dominated, in concordance with the alkaline basin conditions of Laguna del Maule. A recent megadrought brought a shift from planktic to benthic diatoms, as coastal areas expanded, fostering a higher diversity of biraphid genera. Records of past and present dynamics of Andean lacustrine microorganisms are necessary to further evaluate responses to critical global environmental change and for high-altitude aquatic ecosystem conservation efforts.