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.

Tropical high-altitude lakes are vital freshwater ecosystems for the functioning and dynamics of tropical high-altitude wetlands called páramos, found at over 3300 m above sea level. They play a major role in the hydrogeological cycle and provide important hydrological services such as water storage, and yet they are understudied. Describing the patterns and processes of community composition in these lakes is required to better understand the consequences of their degradation by human activities. In this study we tested the geographical and environmental components of distance–decay relationships in the phytoplankton structure across 24 tropical high-altitude lakes from Southern Ecuador. Phytoplankton composition at the phyla level showed high among-lake variation in the tropical high-altitude lakes from Tres Lagunas. We found no links, however, between the geographic distance and phytoplankton composition. On the contrary, we observed some environmentally related patterns of community structure like redox potential, altitude, water temperature, and total phosphorus. The absence of support for the distance–decay relationship observed here can result from a conjunction of local niche-based effects and dispersal limitations. Phytoplankton community composition in the Tres Lagunas system or any other ecosystem may be jointly regulated by niche-based and neutral forces that still need to be explored. Despite not proving a mechanistic explanation for the observed patterns of community structure, we hope our findings provide understanding of these vulnerable and vital ecosystems. More studies in tropical high-altitude lakes are urgently required.

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Riparian vegetation along a stream regulates the flow of sediments and nutrients. It also provides stability to the riverbanks and controls the watershed microclimate that ultimately influences the primary production of lotic environments. The degradation of terrestrial vegetation can also have an impact on the biotic component of rivers. Periphyton communities are altered in their composition, abundance, and diversity when the natural conditions in the watershed are modified. While widely studied in Europe and North America, studies on changes in periphyton community structure in mountainous tropical ecosystems are rare. In this study, we investigated if and how alteration of the riparian vegetation results in changes in the diatom community composition of seven streams in southern Ecuador. Streams were sampled in one or 2 zones, with 5 sub-samples each. The sampled streams show different levels of degradation within their watersheds. Temperature was higher in open (i.e. deforested) areas within a stream and open areas, e.g. pastures, had higher abundance of cyanobacteria filaments. Total algal cell abundances increased from forested to open and more fully degraded zones. Species richness was lowest in the forested areas. Microbial diversity was low across all habitats, but the highest values were documented in the degraded habitats. Diatom communities dominated at all sites, followed by filamentous bacteria. Gomphonema minutum (Agardh) Agardh, G. parvulum (Kütz.) Kütz., and Achnanthidium minutissimum (Kütz.) Czarnacki were the most common and abundant species. Diatom communities from zones with different land use within a stream were as different from each other as communities from different watershed streams.