Sources of nutrients behind recent eutrophication of Lago de Tota, a high mountain Andean lake

Lago de Tota, the largest lake in Colombia, is the primary source of water for 250,000 people and a focus of regional economic activity in agriculture, aquaculture, and tourism. Recently, agencies and stakeholders report a shift from the naturally oligotrophic state toward eutrophy. However, the relative contributions of different inputs, including agricultural runoff, aquaculture and municipal wastewaters are unknown, hampering efforts to mitigate nutrient loading. We examined spatial and temporal variation in the trophic state of the lake over one year, as well as stable C and N isotopic profiles of aquatic producers and consumers in relation to two main potential sources (fertilizer and trout feed). We found that Lago de Tota is moderately eutrophic (average chlorophyll-a: 6.4 µg/L, TN: 1.5 mg/L and TP: 0.06 mg/L) with a 32% reduction of transparency over the last 15 years. δ15N and δ13C of aquatic organisms and surface sediments were enriched relative to prehistoric sediments, indicating that human sources dominate the C and N cycles of the lake. δ15N of macrophytes (15.7‰), particulate organic matter (12.5‰), and invertebrates (20.2‰) were enriched relative to trout food (4.6‰), but similar to chicken manure (13.7‰), suggesting that farming in the watershed may be a more important source of N than aquaculture. Our results indicate that Lago de Tota is on a trajectory toward eutrophication with potentially severe consequences for water resources in a rapidly developing mountain region.