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 (pCO₂), especially in underrepresented tropical eutrophic environments. Here, we conducted high-frequency (1-min interval) diel measurements of surface DOsat and pCO₂ in Guanabara Bay, Brazil, a highly nutrient-enriched coastal ecosystem. The predominant metabolic controls on pCO₂ were revealed by its strong negative correlation with DOsat. Air–sea CO₂ 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 CO₂ sinks and sources in eutrophic coastal waters can introduce significant uncertainty in estimating air–water CO₂ fluxes from regional to global carbon budgets.