Air-water CO2 exchange in five hypereutrophic lakes in Bangalore, India

Abstract:

Inland water bodies play a significant role in terrestrial carbon cycling, rather than being just conduits for the transport of terrestrial carbon to the oceans. Recent syntheses estimate that freshwaters emit substantial amounts of CO₂ (1.4 Pg C yr^-1) (Tranvik et al. 2009) and CH₄ (0.65 Pg C yr^-1) (Bastviken et al. 2011), which are similar in magnitude to the global terrestrial carbon sink (2.5 ± 1.7 Pg C yr^-1) (IPCC 2013). However, eutrophic waters, which constitute the majority of the global freshwater supply (ILEC/UNEP 1994, Liu et al. 2012, Carpenter et al. 1998), are vastly underrepresented in these estimates. These waters, due to high primary productivity leading to CO₂ undersaturation, can act as sinks rather than sources of CO₂, thus reversing the role of lakes in the carbon cycle (Balmer and Downing 2011, Pacheco et al. 2013). We are investigating the air-water CO₂ exchange of five hypereutrophic lakes in urban Bangalore using a novel Non-Dispersive Infrared (NDIR)-based CO₂ sensor installed in flux chambers that can be used to measure CO₂ exchange in lakes in situ. This work is a part of a larger study called Bangalore Carbon Mapping Study that aims to track the spatial flows of carbon in an urban area of a developing country.

Preliminary observations reveal that these lakes absorb CO₂ during the photosynthetic hours, at an average rate of 3.4 mg C m^-2 h^-1. The ongoing study will characterize the complete diurnal cycle of CO₂ exchange, its variation over different seasons, and its relationships with various limnological and catchment characteristics. The flux estimates thus produced will also be compared with those predicted by the current models for air-water gas exchange based on wind speed.