Rising CO(2) Levels Will Intensify Phytoplankton Blooms in Eutrophic and Hypertrophic Lakes

Harmful algal blooms threaten the water quality of many eutrophic and hypertrophic lakes and cause severe ecological and economic damage worldwide. Dense blooms often deplete the dissolved CO(2) concentration and raise pH. Yet, quantitative prediction of the feedbacks between phytoplankton growth, CO(2) drawdown and the inorganic carbon chemistry of aquatic ecosystems has received surprisingly little attention. Here, we develop a mathematical model to predict dynamic changes in dissolved inorganic carbon (DIC), pH and alkalinity during phytoplankton bloom development. We tested the model in chemostat experiments with the freshwater cyanobacterium Microcystis aeruginosa at different CO(2) levels. The experiments showed that dense blooms sequestered large amounts of atmospheric CO(2), not only by their own biomass production but also by inducing a high pH and alkalinity that enhanced the capacity for DIC storage in the system. We used the model to explore how phytoplankton blooms of eutrophic waters will respond to rising CO(2) levels. The model predicts that (1) dense phytoplankton blooms in low- and moderately alkaline waters can deplete the dissolved CO(2) concentration to limiting levels and raise the pH over a relatively wide range of atmospheric CO(2) conditions, (2) rising atmospheric CO(2) levels will enhance phytoplankton blooms in low- and moderately alkaline waters with high nutrient loads, and (3) above some threshold, rising atmospheric CO(2) will alleviate phytoplankton blooms from carbon limitation, resulting in less intense CO(2) depletion and a lesser increase in pH. Sensitivity analysis indicated that the model predictions were qualitatively robust. Quantitatively, the predictions were sensitive to variation in lake depth, DIC input and CO(2) gas transfer across the air-water interface, but relatively robust to variation in the carbon uptake mechanisms of phytoplankton. In total, these findings warn that rising CO(2) levels may result in a marked intensification of phytoplankton blooms in eutrophic and hypertrophic waters.