Eutrophication triggers diel and seasonal shifts of carbon dioxide and oxygen in tropical urban coastal waters

Abstract Coastal waters play a pivotal role in the global carbon cycle, showing increased short‐term variability of dissolved oxygen saturation (DOsat) and partial pressure of greenhouse gases like carbon dioxide ( p CO 2 ), especially in underrepresented tropical eutrophic environments. Here, we conducted high‐frequency (1‐min interval) diel measurements of surface DOsat and p CO 2 in Guanabara Bay, Brazil, a highly nutrient‐enriched coastal ecosystem. The predominant metabolic controls on p CO 2 were revealed by its strong negative correlation with DOsat. Air–sea CO 2 fluxes derived from high‐frequency diel sampling showed emissions of 533 mmol C m −2 annually. Conventional estimates based on daylight‐only measurements were ~73% and 319% higher in the morning (10:00–12:00 h and sunrise–8:00 h, respectively) or ~172% and 244% lower in the afternoon (12:00–14:00 h and 14:00–16:00 h, respectively). Our findings indicate that rapid diel shifts between CO 2 sinks and sources in eutrophic coastal waters can introduce significant uncertainty in estimating air–water CO 2 fluxes from regional to global carbon budgets.