Extreme droughts drive tropical semi-arid eutrophic reservoirs towards CO₂ sub-saturation

Secas extremas direcionam os reservatórios eutróficos do semiárido tropical para a subsaturação de CO₂

Abstract

Aim: This study aimed to evaluate the carbon dioxide (CO₂) dynamics in tropical semi-arid reservoirs during a prolonged drought period as well as to test if the trophic state affects the CO₂ saturation.

Methods

This study was performed in four reservoirs located in the tropical semi-arid region in the northeast of Brazil. All samplings were performed between 9 and 12 am using a Van Dorn Bottle. Samples for partial pressure of carbon dioxide (pCO₂ ) measurements were taken in the sub-surface as well as samples for total phosphorus and chlorophyll-a. Correlation analysis and linear regression were used to detect relations among the calculated pCO₂, water volume and chlorophyll-a.

Results

The water level reduction due to atypical droughts caused chlorophyll-a concentrations to increase, which in turn, led to CO₂ reduction in the water. However, CO₂ concentrations were very variable and an alternation between CO₂ sub-saturation and super-saturation conditions was observed. This paper showed that water volume and chlorophyll-a were important regulators of CO₂ in the water, as well as important carbon balance predictors in the tropical semiarid reservoirs.

Conclusions

The results of this paper indicate that the eutrophication allied to drastic water level reductions lead to a tendency of autotrophic metabolism of these systems.

Keywords:
carbon dioxide partial pressure (pCO₂); carbon sink; carbon source; eutrophication; reservoirs