Tracking centennial changes in algal community composition and biomass using subfossil pigments in a high mountain lake (Sierra Nevada, Spain)

Abstract. Remote aquatic ecosystems are affected by the rapid intensification of human-driven climate change, along with increasing atmospheric nutrient deposition. However, there is a lack of studies that examine changes in the composition of the overall algal community, including cyanobacteria, over extended periods of time. This study investigates shifts in pigment assemblage composition and algal community biomass over approximately the past 430 years. We used high-resolution (2.5 mm intervals), well-dated sediment core from Borreguil Lake, a high-altitude lake in the Sierra Nevada Mountains (Southern Spain). We noted a significant change in both algal biomass and community composition throughout the core, with a notable intensification since the ca. 1970s, which appears to be linked to increasing temperatures and aerosol deposition. Algal biomass and composition exhibited two significant shifts approximately between 1740–1840 and from 1970 to the present, with the latter period reaching unprecedented increases in algal biomass. From the bottom to the top, the compositional shifts were characterized by an increase in cyanobacteria (indicated by aphanizophyll and scytonemin), cryptophytes (indicated by alloxanthin), and green algae (indicated by lutein and zeaxanthin), at the expense of diatoms (indicated by diatoxanthin). Statistical analyses revealed that both algal biomass and composition were strongly influenced by warming temperatures, reduced precipitation, and enhanced Saharan dust deposition. In particular, the increase in nitrogen-fixing cyanobacteria (indicated by aphanizophyll) since the 1970s has led to previously unrecorded nitrogen fixation in the lake. This is probably due to reduced nitrogen availability compared to phosphorus, which is brought disproportionally to this lake by Saharan dust. The observed changes in the algal community in Borreguil Lake are unprecedented in the last ∼ 400 years in the Sierra Nevada lakes and were temporally related to algal community shifts in other Mediterranean sites suggesting that rising global temperatures, aerosol deposition and enhanced Saharan dust deposition will likely continue to affect the ecological condition of these ecosystems. Projected increases in global temperatures and Saharan dust deposition will likely lead to an increase in the lake trophic status and significant changes in algal composition in Borreguil Lake.