Environmental Investments Decreased Partial Pressure of CO in a Small Eutrophic Urban Lake: Evidence from Long-term Measurements

Inland waters emit large amounts of carbon dioxide (CO) to the atmosphere, but emissions from urban lakes are poorly understood. This study investigated seasonal and interannual variations in the partial pressure of CO (pCO) and CO 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 pCO was 1030 ± 281 μatm (mean ± standard deviation) with a mean CO flux of 1.1 ± 0.6 g m d during 2000-2015, suggesting that compared with other lakes globally, Lake Wuli was a significant source of atmospheric CO. Substantial interannual variability was observed, and the annual pCO 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 pCO (R = 0.90, p < 0.01). The lake was dominated by cyanobacterial blooms and showed nonseasonal variation in pCO. This finding was different from those of other eutrophic lakes with seasonal variation in pCO, mostly because the uptake of CO by algal-derived primary production was counterbalanced by the production of CO by algal-derived organic carbon decomposition. Our results suggested that anthropogenic activities strongly affect lake CO dynamics and that environmental investments, such as ecological restoration and reducing nutrient discharge, can significantly reduce CO 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.