Abstract
Tropical high-altitude lakes are vital freshwater reservoirs in the Andean regions. They are heavily threatened by human activities that may alter their functioning and hamper the provisioning of key ecosystem services such as water supply. Despite their ecological and social relevance, we know little about these waterbodies, especially regarding the factors influencing their functioning. Here, we explored the links between several environmental variables and phytoplankton productivity, measured as chlorophyll-a concentration and total phytoplankton biovolume. For this, we sampled twenty-four tropical high-altitude lakes located over three-thousand meters above sea level in Southern Ecuador. We found that four abiotic factors combined explained 76% of the variation in chlorophyll-a concentration amongst lakes. Contrary to what studies from temperate regions suggest, taxa richness was not related to either chlorophyll-a concentrations or total phytoplankton biovolume. Moreover, phytoplankton biovolume diversity was negatively correlated to both chlorophyll-a concentrations and total phytoplankton biovolume. This was due to a very uneven distribution of productivity amongst taxa in the more productive lakes. To the best of our knowledge, this is the first attempt to explore the determinants of phytoplankton functioning in tropical high-altitude lakes. We hope that this study will help to establish a baseline for evaluating the consequences of human activities in the ecology and functioning of this vital but fragile ecosystem. Our results suggest that by modifying the abiotic and biotic parameters of tropical high-altitude lakes, human activities can indirectly impact their functioning and their capacity to provide vital ecosystem services.
Highlights
High-altitude lakes, called high-mountain lakes, are important natural freshwater reservoirs for human consumption, irrigation and hydropower production purposes in Andean regions [1,2,3,4]
The levels of chlorophyll-a measured in this group of lakes from Tres Lagunas are comparable to most previous studies in tropical high-altitude lakes (TRHALs) from Mexico, Bolivia and other parts of Ecuador [6,13,15,16,17]
Another study with similar levels of phytoplankton production involving TRHALs in Ecuador found that total nitrogen concentration was the only variable that explained some variation in chlorophyll-a amongst lakes [13]
Summary
High-altitude lakes, called high-mountain lakes, are important natural freshwater reservoirs for human consumption, irrigation and hydropower production purposes in Andean regions [1,2,3,4]. Due to their low latitudes, TRHALs have some specific features not shared by high-altitude temperate lakes This includes moderate or no seasonality, no ice cover, polymictic mixing regimes that often have a complex thermal structure, intense UV radiation through the year, high dissolved organic carbon and low UV transparency [2,3,4,5,9,10,11,12,13]. One type of TRHALs are the páramo lakes, located above the tree line between approximately from 3200 to 4500 meters above sea level (m.a.s.l.) and filled almost exclusively by rain and groundwater Another type are the glacial lakes, located at even higher altitudes and are fed directly by glacier melting waters. This study focuses on páramo lakes, located between 3288 and 3362 meters above sea level without a direct connection to glaciers
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