Environmental investments decreased partial pressure of CO2 in a small eutrophic urban lake: Evidence from long-term measurements

Inland waters emit large amounts of carbon dioxide (CO2) to the atmosphere, but emissions from urban lakes are poorly understood. This study investigated seasonal and interannual variations in the partial pressure of CO2 (pCO2) and CO2 flux from Lake Wuli, a small eutrophic urban lake in the heart of the Yangtze River Delta, China, based on a long-term (2000–2015) dataset. The results showed that the annual mean pCO2 was 1030 ± 281 μatm (mean ± standard deviation) with a mean CO2 flux of 1.1 ± 0.6 g m−2 d−1 during 2000–2015, suggesting that compared with other lakes globally, Lake Wuli was a significant source of atmospheric CO2. Substantial interannual variability was observed, and the annual pCO2 exhibited a decreasing trend due to improvements in water quality driven by environmental investment. Changes in ammonia nitrogen and total phosphorus concentrations together explained 90% of the observed interannual variability in pCO2 (R2 = 0.90, p < 0.01). The lake was dominated by cyanobacterial blooms and showed nonseasonal variation in pCO2. This finding was different from those of other eutrophic lakes with seasonal variation in pCO2, mostly because the uptake of CO2 by algal-derived primary production was counterbalanced by the production of CO2 by algal-derived organic carbon decomposition. Our results suggested that anthropogenic activities strongly affect lake CO2 dynamics and that environmental investments, such as ecological restoration and reducing nutrient discharge, can significantly reduce CO2 emissions from inland lakes. This study provides valuable information on the reduction in carbon emissions from artificially controlled eutrophic lakes and an assessment of the impact of inland water on the global carbon cycle.