Long-Term CO2 Variability in Two Shallow Tropical Lakes Experiencing Episodic Eutrophication and Acidification Events

Abstract

Here we report the long-term (13-year) dynamics of surface pCO2 and its response to episodic eutrophication and acidification events in two contrasting tropical coastal lakes, one clear-water and the other humic. A short-term nutrient addition experiment was also conducted in mesocosms in the humic lake where in situ eutrophication was moderate. Our objective was to elucidate the response of pCO2 to interannual changes in key limnological conditions, such as nutrient concentrations and pH. The humic waters showed a median pCO2 almost ninefold higher across the 13-year study than the clear waters, supporting pCO2 values about tenfold above atmospheric equilibrium. Eutrophication of the clear-water lake resulted in a decrease in pCO2 to median values below atmospheric equilibrium, producing a strong sink for atmospheric CO2. In contrast, pCO2 increased by over tenfold in both lakes during the acidification phase, resulting in very large CO2 emissions to the atmosphere. Experimental nutrient additions in the humic lake showed a strong persistence of high pCO2. The extreme variability in pCO2 observed here might be a characteristic of tropical lakes and may have important consequences for regional carbon budgets.