Small lakes and temporary ponds release CO2 into the atmosphere even when they are dry

The new article, published in the journal Scientific Reports, changes the classic paradigm on the role of temporary lakes and ponds as emitters of carbon to the atmosphere and their impact on the planet's 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 quite 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 dry phases in summer).

According to the lecturer 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." Moreover, the researcher adds that "even small ponds ─which are not usually larger than a basketball court─, are the most frequent lacustrine ecosystems in the planet, the amount of knowledge on carbon cycle in freshwater ponds comes from big permanent lakes (with water during all year)."

Small and temporary ponds emit CO₂ during the whole year

In the study, the experts analysed fluxes of CO₂ and methane (CH₄) -two gases with a powerful greenhouse effect- in small temporary ponds in Menorca- with a wide range of hydrological properties and hydroperiods (duration of wet phases) that oscillated between several months and some days or weeks.

The temporary ponds emit CO₂ during the whole 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 the one emitted by turbulent fluxes 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 global change, the frequency and intensity of droughts could increase notably in some areas of the planet. This phenomenon could speed up the drying up and disappearance of many water systems, as seen nowadays in some lakes. In this situation, carbon emissions coming from these large areas of emerging sediments could be -at least, on their first stage- 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, as warned by the authors. "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.

The new study, funded by the Institut Menorquí d'Estudis, has the participation of Lluís Gómez-Gener (University of Barcelona) and other experts from ICRA, the University of Girona, University of the Basque Country, University of Uppsala (Sweden), the Helmholtz Center for Environmental Research (Germany) and Umea University (Sweden).