Changes in Biotic Mechanisms of Phytoplankton Biomass Stability along a Eutrophic Gradient

Understanding the biotic mechanisms of community stability in variable environments has been a focal point of fundamental ecological research. A multitude of mechanisms, encompassing compensatory dynamics arising from negative species covariance, portfolio effect linked to species richness and evenness, and dominant species stability, have been found to collectively enhance community stability. However, it is not clear how their stabilizing effects change and contribute to the maintenance of community stability along environmental gradients. We performed a ten-year investigation in a large shallow lake with a eutrophic gradient across space. With the dataset, we quantified the role of the three stability mechanisms, and their changes in effect size along the eutrophic gradient to determine their relative importance in biomass stability. Our results showed that the biomass stability shifted from one stable state at eutrophic sites to another stable state at hypertrophic sites, and biomass stability was positively correlated with composition stability. In the relatively stable state, biomass stability exhibited a closely synchronized variation along with compositional stability in response to environmental changes. Conversely, in the unstable state, biomass stability displayed weaker sensitivity to environmental changes compared to compositional stability. The effect sizes of different biotic mechanisms of biomass stability varied across the eutrophic gradient. Compensatory dynamics emerged as the primary force governing biomass stability in eutrophic waters, overshadowing the relatively weak impact of the portfolio effect, which might help resist the shift from turbid state to clear state with decreasing nutrient concentrations. However, as nutrient levels increased, the primary force shifted from compensatory dynamics toward the dominant species stability. This study improves our understanding for the biotic mechanisms of phytoplankton community responding to nutrients mitigation in eutrophic waters, which might be one of the most important ecological components for managing communities to maintain ecosystem functioning.