Potential Submerged Macrophytes to Mitigate Eutrophication in a High-Elevation Tropical Shallow Lake-A Mesocosm Experiment in the Andes

Submerged macrophytes promote water clarity in shallow lakes in temperate regions via zooplankton refuge, allelopathy, and nutrient competition with phytoplankton, thereby increasing zooplankton grazing. However, in high-altitude Andean ecosystems, these interactions in shallow lakes have received far less attention. To understand the role of submerged plants in a relatively cold ecosystem (typical for the Andean region), two 100 L experiments were conducted in Yahuarcocha Lake, which has a permanent cyanobacterial bloom. In our first experiment, we evaluated the response of the cyanobacteria bloom to different concentrations of Egeria densa (15%, 35%, and 45% PVI). In the second experiment, we investigated the interactions between E. densa (35% PVI), zooplankton, and the small-sized fish Poecilia reticulata as well as their impacts on phytoplankton. We found a strong reduction in cyanobacteria in the presence of E. densa, whereas P. reticulata promoted cyanobacteria dominance and zooplankton had a null effect on phytoplankton. Remarkably, the combination of E. densa, fish, and zooplankton substantially reduced the algae. Our findings showed that the cyanobacteria bloom decreased in the presence of E. densa, thereby increasing the water clarity in the high-elevation eutrophic ecosystem in the Andes. This effect depended on the plant volume inhabited and the small-sized fish biomass.

Automated summary

Submerged macrophytes promote water clarity in high-altitude Andean shallow lakes by reducing the cyanobacteria bloom and increasing zooplankton grazing. In a series of experiments in Yahuarcocha Lake, it was found that the presence of Egeria densa resulted in a strong reduction in cyanobacteria, while the small-sized fish Poecilia reticulata promoted cyanobacteria dominance. The combination of E. densa, fish, and zooplankton substantially reduced the algae, indicating the important role of submerged plants in improving water quality in high-elevation eutrophic ecosystems.