Carbon dioxide emissions across contrasting urban freshwater ecosystems

Abstract

Carbon dioxide (CO 2 ) emissions from freshwaters represent a globally important carbon flux. However, CO 2 fluxes from urban waters remain poorly constrained, and challenges remain for reliable upscaling, partly because of the high diversity of urban aquatic ecosystems. Using floating chambers to measure fluxes instantaneously and monitor them over multiple days, we estimated CO 2 emissions at 31 sites (lakes, ponds, rivers and streams) across the city of Berlin in four seasons. We evaluated land cover, nutrient concentrations, dissolved organic matter (DOM) composition, and chlorophyll‐ a and micropollutant concentrations as potential drivers of CO 2 emissions. CO 2 fluxes from lakes and ponds averaged 0.41 g C m −2 d −1 (ranging from −0.45 to 3.38 across sites and methods), aligning with the results of previous studies. CO 2 fluxes from our urban rivers and streams averaging 0.78 g C m −2 d −1 (range from −0.13 to 7.32) were lower than typically reported for running waters, conceivably due to limited gas exchange in the channelized lowland systems. CO 2 fluxes were positively associated with DOM aromaticity, pointing to allochthonous inputs delivering resources for respiration, whether from (treated) wastewater or sealed urban surfaces. Fluxes were lower and more variable in settings favoring primary production, suggesting an impact of CO 2 drawdown. This condition coincided with a higher percentage of paved area. Extrapolation to the entire aquatic network of the city yielded an annual emission estimate of 8.8 Gg C‐CO 2 . An implication of this study is that urban lakes, ponds, rivers, and streams should be recognized as distinct systems when establishing large‐scale CO 2 emission budgets.

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