Contrasting responses of phytoplankton, epiphyton, and epipelon to enrichment and its interruption in a shallow lake mesocosm experiment

In a mesocosm experiment, we simultaneously evaluated the responses of phytoplankton, epiphyton, and epipelon to combined nitrogen and phosphorus enrichment and its subsequent interruption. The short-term experiment (35 days) was conducted in a shallow mesotrophic lake, with sampling performed after a phase of daily nutrient enrichment and again after a following a 20-day interruption. The enrichment phase shifted the system to eutrophic conditions, while the interruption returned it to mesotrophic status. Nutrient enrichment had a positive effect on phytoplankton biovolume, a negative effect on epiphyton, and no significant impact on epipelon. After 20 days without enrichment, phytoplankton biomass declined significantly, epipelon increased, and only minor changes were observed in epiphyton. Enrichment favored bloom-forming in the phytoplankton and species adapted to low-light conditions in both the epipelon and epiphyton. Filamentous forms, particularly Spirogyra, increased during the enrichment phase in the epiphyton and, following its interruption, in the phytoplankton. In conclusion, nutrient enrichment associated with phytoplankton blooms reduces the contribution of epipelon and epiphyton to the system, leading to loss of biomass, decreased diversity, and changes in species composition. Our findings demonstrated that nutrient enrichment could compromise the ecological function of periphyton in lake ecosystems.