Two millennia of anthropogenic influence and environmental change in Lake San Pablo, Ecuador

Located in the northern Ecuadorian Andes and surrounded by the city of Otavalo, numerous smaller settlements, and intensively used agricultural land, Lake San Pablo experiences declining water quality and increasing eutrophication. The lake and its water quality are of great importance to the local population, including indigenous communities. Last year, a community-driven effort led to legal recognition of the lake under the Rights of Nature, committing public resources to restoration. Under this legal framework, community-driven initiatives such as the “Tejiendo Agua en el Territorio” network that includes local indigenous worldviews, have been started to improve links between the lake and livelihoods in its catchment. These actions are guided by repeated, but non-continuous, limnological surveys since the 1970s, but lack a long-term baseline.

The lake’s longer-term environmental history, including natural variability and human-driven eutrophication, remains poorly constrained. To address this gap, we adopt a paleolimnological approach based on multi-proxy analyses of lake sediments. We analyzed a 1.18 m long sediment core retrieved from 21 m water depth in September 2024, which spans approximately the last two millennia based on a radiocarbon age-depth model using pollen concentrates. A multi-proxy approach integrates biological indicators (pollen, microcrustaceans, chironomids, and testate amoebae) with geochemistry (LOI, TC, TN, C/N, δ13C, δ15N, XRF). Biological indicators are used to track biodiversity change and to infer trophic state, habitat structure, oxygenation, littoral/macrophyte development, and water chemistry. Geochemical analyses aim to quantify internal and external sediment sources from in-lake productivity, nitrogen cycling, soil erosion and volcanic ash input, and include loss-on-ignition to estimate organic matter, carbonate, and other detrital mass fractions, C/N to partition autochthonous versus allochthonous organic matter, and stable isotopes of bulk sediment. For an assessment of trace-metal enrichment indicative of anthropogenic inputs, we combine XRF core scanning and pXRF data. Throughout the core, the sediment is homogeneous, organic-rich, and comprises macrophyte remains. Despite sedimentological uniformity, biological and geochemical proxies vary throughout the sequence and especially in the upper part of the core (representing the last century). In this upper part, preliminary results indicate increased terrestrial organic-matter input to the lake and higher sedimentation rates consistent with intensified land use in the catchment as reconstructed from pollen data.

This long-term perspective aims to constrain the timing, sources, and drivers of eutrophication and contaminant loading to inform evidence-based lake and catchment management. Our findings will ultimately support locally grounded actions by defining realistic reference conditions.