Eutrophic lakes as CO2 sinks – A survey of 19 lakes in India

Abstract:

Inland waters emit a substantial amount of CO₂ every year, most recent syntheses estimate (Raymond et al. 2013, IPCC 2013). However, eutrophic water bodies, which constitute the majority of inland waters, are underrepresented in these syntheses and may absorb rather than emit CO₂ because of their high productivity (Balmer and Downing 2011, Pacheco et al. 2013). We did a survey of 19 urban and peri-urban lakes in India across a wide range of climates and with varying levels of eutrophication to get a snapshot of lake air-water CO₂ exchange. A majority of the lakes (12 out of 19) were undersaturated with CO₂ during daytime. Surface water pCO2 varied from 26 to 4600 ppm. Using estimates of gas transfer velocity from two different methods, we found the average daytime flux of CO₂ in these lakes to vary from -3.11 mg C m^-2d^-1 to 36 mg C m^-2 d^-1.

Weighted-averages of pCO₂ and flux using lake area were 692 ppm and 2.33 mg C m^-2 d^-1, respectively. However, these values were dominated by one large coastal lake that was saturated with CO₂. The other 18 lakes yielded averages pCO2 and flux of 282 ppm and -0.65 mg C m^-2 d^-1.

Eutrophication is one the biggest contemporary threats to the global freshwater supply, and is particularly severe in developing countries. This study, despite its limited scope, provides strong support to the fact that eutrophic lakes may act as CO₂ sinks rather than sources. Follow-up studies on the diurnal and seasonal pCO₂ trends and the metabolic characteristics of these lakes will reveal the determinants of their carbon metabolism.