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.

The high-elevation plateaus of the inter-Andean valleys are home to shallow lakes that have become eutrophic. These lakes share similarities with shallow lakes in temperate and subtropical areas. Because native species diversity is low, invasive species dominate the fish and macrophytes communities. The study aimed to investigate the behavioral response of the local Daphnia pulex from the Andean shallow Lake Yahuarcocha to the exotic submerged macrophyte Egeria densa and the exotic fish Poecilia reticulata. Laboratory habitat choice experiments revealed that D. pulex from Lake Yahuarcocha strongly avoid E. densa, irrespective of the presence of the fish P. reticulata or chemical cues indicating fish predation on D. pulex. This observation could be explained by the fact that P. reticulata displayed a strong attraction to E. densa during the daytime, probably to avoid bird predation. D. pulex from the nearby Lake San Pablo where P. reticulata is absent but where the fish community is dominated by Oncorhynchus mykiss displayed the same avoidance behavior to submerged macrophytes as D. pulex from Lake Yahuarcocha. These results indicate that macrophytes in these high-elevation shallow lakes may not facilitate top-down control of phytoplankton, since plants do not offer refuge to D. pulex from fish predation.