Small lakes and temporary ponds release CO2 even when dry

Temporary lakes and ponds emit CO₂ even when they are dry, and dry areas emit a larger amount of carbon into the atmosphere. This phenomenon, described now for the first time, could have an impact on the global carbon cycle that controls Earth's climate, according to a study led by Biel Obrador from the University of Barcelona and Núria Catalán from the Catalan Institute for Water Research (ICRA).

The new article, published Scientific Reports, challenges the classic paradigm on the role of temporary lakes and ponds as emitters of carbon into the atmosphere and their impact on the greenhouse effect.

Temporary ponds and lakes: a new view of the carbon cycle

The role of continental waters in the global carbon cycle is still unknown despite its importance, in particular in small or temporary aquatic systems with dry periods. This is one of the first published studies on carbon fluxes over the hydrological cycle of temporary water systems, with a special interest both in flooded areas and areas without water, even during summer dry phases.

According to Biel Obrador (UB), first author of the article, "up to a decade ago, it was thought that continental waters had an irrelevant role on global fluxes regarding the atmosphere, as a result of the tiny area they occupy compared to big carbon compartiments, like the oceans. Small ponds, which are not usually larger than a basketball court, are the most frequent lacustrine ecosystems in the planet, but the knowledge on the carbon cycle in freshwater ponds comes from big, permanent lakes."

Small and temporary ponds emit CO₂ all year

In the study, the experts analysed fluxes of CO₂ and methane (CH₄) in small temporary ponds in Menorca with a wide range of hydrological properties and hydroperiods that oscillate between several months and days or weeks.

The temporary ponds emit CO₂ all year, according to the study. Also, the amount of CO₂ released into the atmosphere, around two kilograms of CO₂ per square meter and year, is similar to that emitted by turbulent flux waters (rivers, creeks, streams), and this value triples the fluxes of CO₂ coming from permanent lakes, reservoirs and lagoons.

"Emissions of these gases result from the biogeochemical processes that occur in these ecosystems, in particular due the biological activity of microbial communities. According to the environmental conditions and composition of organic matter, these microorganisms produce gases such as CO₂ and CH₄ as a result of the respiration of organic matter in the sediment," says Biel Obrador, member of the Department of Evolutionary Biology, Ecology and Environmental Sciences of the UB.

Integrating the biogeochemical perspective of temporary water systems

In a world affected by climate change, the frequency and intensity of droughts could increase notably in some areas. This phenomenon could speed up the drying up and disappearance of many water systems, as seen in some lakes. In this situation, carbon emissions coming from these large areas of emerging sediments could be quite relevant regarding the global carbon cycle.

In the future, a study on the biogeochemistry of temporary water systems should be carried out from a perspective covering both dry areas and periods without water, the authors write. "The final view we can get on the functioning of the ecosystems is surprisingly different from the one we would get if we only considered flood conditions. Without this integrating perspective, studies would bring us to contrary conclusions on the role of these ecosystems as carbon emitters to the atmosphere," says Obrador.