Comparações entre medições em tempo real da pCO2 aquática com estimativas indiretas em dois estuários tropicais contrastantes: o estuário eutrofizado da Baía de Guanabara (RJ) e o estuário oligotrófico do rio São Francisco (AL)

Abstract

publicado na web em 16/08/2016 COMPARISONS BETWEEN REAL-TIME pCO 2 MEASUREMENTS WITH INDIRECT ESTIMATES IN TWO CONTRASTING BRAZILIAN ESTUARIES: THE EUTROPHIC GUANABARA BAY (RJ) AND THE OLIGOTROPHIC SÃO FRANCISCO RIVER ESTUARY (AL). Carbon dioxide (CO 2 ) fluxes from aquatic systems are generally derived from the gradient in the partial pressure of CO 2 (pCO 2 ) between air and surface waters. In this study, we compare real-time measurements of water pCO 2 using an equilibrator and non-dispersive infrared gas detector, with calculations based on pH and total alkalinity (TA) in two contrasting Brazilian estuaries: Guanabara Bay (Rio de Janeiro) and the São Francisco River Estuary (Alagoas). In Guanabara Bay, the measured and calculated values showed an excellent agreement (R 2 = 0.95, p < 0.0001), without significant statistical differences between the two methods. In the São Francisco River Estuary, where the entire gradient from freshwaters to seawater could be sampled, important overestimates were found for the calculated pCO 2 . The overestimation was on average 71%, and reached up to 737%. This large bias in pCO 2 calculation was verified at low pH and TA concentrations in freshwaters (pH < 7.5; TA < 700 μmol kg -1 ) possibly due to the contribution of organic alkalinity, lowering the buffer capacity of the carbonate system. As such, direct measurements of pCO 2 should be considered as a priority for CO 2 studies conducted in estuarine systems, particularly tropical systems where physical and biological processes are prone to significant spatial and temporal variability.Keywords: CO 2 ; direct measurements; spatial and temporal variability; estuaries. INTRODUÇÃOO dióxido de carbono (CO 2 ) é o principal gás do efeito estufa e tem a maior contribuição no aquecimento do sistema climático global.1 Os níveis atuais alarmantes das concentrações atmosféri-cas de CO 2 e as taxas de aumento das emissões antropogênicas ao longo das últimas décadas alertam a importância da identificação, quantificação e monitoramento das fontes e sumidouros de dióxido de carbono.2 O recente processo de acidificação dos oceanos, que também é atribuído ao aumento das concentrações de CO 2 atmosfé-rico, 3 reforça a importância dos estudos da dinâmica desse gás, tanto no oceano aberto quanto em ecossistemas costeiros. Além disso, publicações recentes relataram o aumento da acidificação costeira como resultado da hipóxia e anóxia em águas costeiras em função de estágios avançados da eutrofização, especialmente em ambientes impactados por atividades antropogênicas. [4][5][6] Os oceanos abertos são importantes sumidouros de CO 2 atmosférico com fluxos na interface água-atmosfera bem quantificados, 7 enquanto águas costeiras estão sujeitas a maiores incertezas devido à elevada heterogeneidade espacial e temporal, diferentes tipologias geomorfológicas e hidrológicas (estuários dominados por águas fluviais, dominados pelo mar, com regimes de maré, ondas e ventos diferenciados). [8][9][10] Outro aspecto a ser considerado é a menor disponibilidad...

Carbon dioxide sources and sinks in the delta of the Paraíba do Sul River (Southeastern Brazil) modulated by carbonate thermodynamics, gas exchange and ecosystem metabolism during estuarine mixing

“…Continuous samplings were performed from a boat, with real-time measurements of position, pCO2, DO, pH, salinity, and temperature, at a frequency of 1 min (Frankignoulle et al, 2001, Abril et al, 2014Cotovicz et al, 2016). Briefly, a water pump placed at a depth of ~0.5 m provided continuous 6 L min −1 water flux to the boat.…”

“…The boat speed was 8-10 km h -1 during the spatial surveys. The equilibration time of the pCO2 measurement system is 2-3 min (Cotovicz et al, 2016). This gas equilibration device is ideal for pCO2 analysis in estuarine waters due to its fast response time (Frankignoulle et al, 2001, Abril et al, 2014Cotovicz et al, 2016).…”

Ocean acidification studies and the uncertainties relevance on measurements of marine carbonate system properties

“…Thus, not only improving the knowledge concerning the OA, but also increasing the visibility of Brazil in the international scientific community. It is evident, according to the last BrOA Network publication (Kerr et al, 2016), that the Brazilian OA community is improving the data quality and technical information, even considering the distinct approaches and ecosystems, which includes: evaluation of carbonate system parameters and/or acidification in rivers (Abril et al, 2014), estuaries (Cotovicz et al, 2015Cotovicz et al 2016b), lagoons (Peixoto et al, 2013), lakes (Marotta et al, 2014;Fontes et al, 2015) DIC 10 μmol kg -1 2 μmol kg -1 pCO 2 2.5 % 0.5 % Table 1. The uncertainties of the standard carbonate system parameters for each data quality goal, weather or climate, according to Newton et al (2015).…”

Alkalinity, inorganic carbon and CO2 flux variability during extreme rainfall years (2010-2011) in two polluted tropical estuaries NE Brazil

“…Recently, Noriega et al (2015) has reported values of pCO 2 in 12 estuaries of northeastern Brazil ranging from 987 to 8970 μatm. Lower pCO 2 values have been reported by Cotovicz et al (2016) for the Guanabara Bay (418±431 µatm) -an eutrophic ecosystem where the primary productivity significantly lowers the pCO 2 -and in the São Francisco estuary (1469±2948 µatm) which is oligotrophic and dominated by heterotrophy and thus presents higher pCO 2 values.…”