Responses of a Submerged Macrophyte Potamogeton crispus and Epiphytic Biofilm to Humic-Substance Enrichment Coupled with Brownification in Freshwater Habitats

Brownification denotes increasing water color, partly caused by increasing dissolved organic matter of terrestrial origin in freshwater. Brownification has become a wide-spread environmental problem because water color alters the physicochemical environment and biological communities in aquatic ecosystems. However, our understanding of its ecological effects on aquatic macrophytes is limited. Here, an indoor mesocosm experiment with a common submerged macrophyte, Potamogeton crispus, along an increasing gradient of brownification was conducted over a period of 42 days. Results showed that P. crispus was able to overcome low degrees of brownification owing to the plasticity in morphological and physiological traits and P. crispus growth even benefitted from the concomitant nutrients along with brownification. However, the macrophyte growth was negatively affected by a 10-fold increase in water color beyond its current level. Additionally, collapse in antioxidant systems and potent photosynthesis inhibition implied that P. crispus could not adapt to the low-light stress generated under the high degree of brownification. Epiphytic bacteria are more sensitive to brownification than their hosts. Any degree of brownification initially caused a decrease in microbial diversity of epiphytic biofilm, whereafter the concomitant nutrients under brownification favored the growth of epiphytic microorganisms. The shading effect of a large number of epiphytic biofilms under brownification may further aggravate the low-light stress on macrophytes. Overall, the study provides new insights into the comprehensive effects and underlying mechanisms of brownification on macrophytes.