A strong CO<sub>2</sub> sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)

Abstract

Abstract. In contrast to its small surface area, the coastal zone plays a disproportionate role in the global carbon cycle. Carbon production, transformation, emission and burial rates at the land-ocean interface are significant at the global scale but still poorly known, especially in tropical regions. Surface water pCO 2 and ancillary parameters were monitored during nine field campaigns between April 2013 and April 2014 in Guanabara Bay, a tropical eutrophic to hypertrophic semienclosed estuarine embayment surrounded by the city of Rio de Janeiro, southeast Brazil. Water pCO 2 varied between 22 and 3715 ppmv in the bay, showing spatial, diurnal and seasonal trends that mirrored those of dissolved oxygen (DO) and chlorophyll a (Chl a). Marked pCO 2 undersaturation was prevalent in the shallow, confined and thermally stratified waters of the upper bay, whereas pCO 2 oversaturation was restricted to sites close to the small river mouths and small sewage channels, which covered only 10 % of the bay's area. Substantial daily variations in pCO 2 (up to 395 ppmv between dawn and dusk) were also registered and could be integrated temporally and spatially for the establishment of net diurnal, seasonal and annual CO 2 fluxes. In contrast to other estuaries worldwide, Guanabara Bay behaved as a net sink of atmospheric CO 2 , a property enhanced by the concomitant effects of strong radiation intensity, thermal stratification, and high availability of nutrients, which promotes phytoplankton development and net autotrophy. The calculated CO 2 fluxes for Guanabara Bay ranged between −9.6 and −18.3 mol C m −2 yr −1 , of the same order of magnitude as the organic carbon burial and organic carbon inputs from the watershed. The positive and high net community production (52.1 mol C m −2 yr −1 ) confirms the high carbon production in the bay. This autotrophic metabolism is apparently enhanced by eutrophication. Our results show that global CO 2 budgetary assertions still lack information on tropical, marine-dominated estuarine systems, which are affected by thermal stratification and eutrophication and behave specifically with respect to atmospheric CO 2 .

Cyanobacterial carbon concentrating mechanisms facilitate sustained CO<sub>2</sub> depletion in eutrophic lakes

“…CCMs may provide a competitive advantage to cyanobacteria when high primary productivity depletes ambient CO 2 . This mechanism may allow blooms to be sustained for weeks to months at a time with negligible concentrations of CO 2 in the water column (Cotovicz et al, 2015). While nutrient reduction is ultimately critical in the prevention of blooms (Heisler et al, 2008;Rigosi et al, 2014), the mechanism presented here provides insight into the causes of bloom duration and intensity at high nutrient concentrations.…”

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…Furthermore, previous estuarine CO 2 studies have focused on riverdominated estuaries, whereas there has been limited research on CO 2 dynamics in large estuaries and bays (Joesoef et al, 2015), i.e., marine-dominated systems, including the areas of mixing at sea (outer estuaries or river plumes) . A comparative study by Jiang et al (2008) revealed large differences in CO 2 degassing between nonriverine and river-dominated estuaries and, more recently, Koné et al (2009), Maher andEyre (2012), and Cotovicz Jr. et al (2015) reported low CO 2 uptake by strongly stratified and/or marine-dominated systems. On the US east coast, the ratio of non-riverine (flushed by tidal action and receiving minimum freshwater inputs) to river-dominated estuaries is nearly 1 : 1, demonstrating the geographic importance of coastal estuaries and bays on the eastern seaboard of North America (Cai, 2011).…”

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…The production of CO 2 and CH 4 is modulated by various physical features resulting from estuarine geomorphology such as water residence time (Borges et al, 2006;Joesoef et al, 2017), tidal amplitude and vertical stratification (Borges, 2005;Koné et al, 2009;Crosswell et al, 2012;Joesoef et al, 2015), and connectivity with tidal flats and salt marshes (Middelburg et al, 2002;Cai, 2011). Highly eutrophic (Cotovicz Jr. et al, 2015) or strongly stratified estuarine systems (Koné et al, 2009) can exceptionally act as sinks of CO 2 due to high carbon sequestration, although high organic matter sedimentation can concomitantly lead to high CH 4 production and emission to the atmosphere (Koné et al, 2010;Borges and Abril, 2011).…”