Search

This perspective paper aims at presenting the current knowledge on the processes of ocean acidification (OA) and coastal acidification (CA) in the Brazilian coastal ocean. We define and differentiate the processes of OA and CA: the first driven by the actual global increase of atmospheric carbon dioxide (CO2 ); the second driven by a combination of ocean uptake of atmospheric CO2 and other local/regional chemical additions or subtractions in aquatic ecosystems at the land-ocean interface. Regarding OA, we have centered our analysis on the data available for the main water masses along the Brazilian coast: South Atlantic Central Water (SACW), Tropical Water (TW), and Coastal Water (CW). The few data available for the shallow coastal waters (< 200 m depth) of the continental shelf reveal an increase in the anthropogenic component of the total dissolved inorganic carbon (DIC) pool in the SACW, with a decline in the ocean pH (over two decades; 1993-2013), and in the saturation state of calcium carbonate (CaCO3 ) minerals. We could not find OA trends for TW and CW because no data was available. Overall, the colder water masses (SACW, Plata Plume) have lower buffering capacity and simulations show that will potentially experience earlier negative OA impacts than the warmer waters masses (TW, Amazon Plume). Regarding CA, we have identified some local/regional studies investigating the carbonate chemistry in nearshore/estuarine ecosystems, particularly on the quantification of sources and sinks of CO2 , and determining short-term variabilities. Apparently, spreading coastal eutrophication in Brazil can enhances or reduces the process of OA, depending on the net ecosystem metabolism in combination with other chemical alterations. However, we could not find medium-long term acidification trends due to the limited data. There is a limited capacity to produce long time-series of carbonate chemistry parameters in key ecosystems and regions along the Brazilian coast. This lack of past information hinders and impairs the scientific community for identifying potential patterns of acidification along the Brazilian coast. We call for an urgent action in Brazil, with emphasis on the establishment of moored buoys/stations and/or scientific programs in the long term with continuous, real-time measurements of the main carbonate chemistry parameters. O presente artigo tem como objetivo apresentar o estado atual de conhecimento dos processos de acidificação do oceano (OA) e acidificação costeira (CA) no Brasil. Definimos e diferenciamos tais processos: o primeiro ocasionado pelo aumento global da concentração de CO2 na atmosfera; o segundo ocasionado pela combinação do sequestro de CO2 pelo oceano e outras adições ou subtrações locais/regionais de compostos químicos em ecossistemas aquáticos costeiros. Considerando a OA, a análise foi centrada nas principais massas de água presentes ao longo da costa do Brasil: Água Central do Atlântico Sul (SACW), Água Tropical (TW) e Água Costeira (CW). Os poucos dados disponíveis para águas rasas costeiras (< 200 m profundidade) revelam um aumento da contribuição antropogênica nas concentrações de carbono inorgânico dissolvido (DIC) na SACW, com declínio no pH (ao longo de duas décadas – 1993-2013), e na saturação dos minerais de carbonato de cálcio (CaCO3 ). Não encontramos tendências para TW e CW devido à falta de dados disponíveis. As massas de água mais frias (SACW, Pluma do Rio da Plata) têm menor capacidade de tamponamento, e simulações mostram que irão sentir os efeitos da acidificação antes que as massas de água mais quentes (TW, Pluma do Rio Amazonas). Considerado a CA, identificamos alguns estudos locais/regionais sobre a química do sistema carbonato em ecossistemas costeiros/estuarinos centrados na quantificação de fontes e sumidouros de CO2 e determinações de variação temporal de curta escala. Aparentemente, o processo de eutrofização costeira, amplamente verificado na costa do Brasil (particularmente em regiões densamente povoadas), pode amplificar ou atenuar o processo de acidificação, dependendo do metabolismo dos ecossistemas, em combinação com outras alterações químicas. Entretanto, não encontramos tendências de acidificação de médio e longo prazo devido à falta de estudos. Existe uma restrita capacidade em produzir longas séries temporais de parâmetros do sistema carbonato em ecossistemas-chave ao longo da costa do Brasil. Dessa forma, chamamos a atenção para uma urgente ação no Brasil, com ênfase no estabelecimento de estações oceanográficas (boias oceanográficas, fundeios) e/ou programas científicos no longo prazo que contemplem medições contínuas e de alta frequência dos principais parâmetros do sistema carbonato marinho.

Tropical estuarine deltas generally present poorly buffered waters in their freshwaters. Carbonate chemistry predicts that mixture of such warm freshwater with seawater will create rapid consumption of the freshwater carbon dioxide (CO2) by the carbonate buffering capacity of the seawater. In this study, we used the Paraiba do Sul River as a laboratory to investigate how thermodynamics compare with biological processes, gas exchange, and tidal advection from mangrove. We conducted three spatial surveys covering the salinity gradient of the main channel and surrounding mangrove waters and one 24-h mooring in a mangrove creek. In the main channel, dissolved inorganic carbon (DIC) and total alkalinity (TA) showed closely conservative distribution along the salinity gradient, increasing seaward. The partial pressure of CO2 (pCO(2)) followed a bell-shaped curve predicted by carbonate chemistry for conservative mixing of river and ocean endmembers. During high river flow, pCO(2) sharply decreased between salinities 0 and 5 (1800 to 390 ppmv), a pCO(2) drawdown attributed to riverine outgassing and thermodynamics. Indeed, the mixing of TA-poor freshwater (363 +/- 16 mu mol kg(-1)) with TA-rich seawater creates a deficit of dissolved CO2 not related to biotic processes. During low river flow, the entire mixing zone was undersaturated in pCO(2) with an increasing trend seaward. However, observed pCO(2) values were slightly above those predicted by conservativity. Approximately half of this deviation was attributed to biological activity (net heterotrophy), and remaining deviation was assigned to the effects of gas exchange (18%) and water heating (36%). The effect of gas exchange was higher in fresh and low salinity waters, reflecting the higher outgassing/ingassing of CO2, and lower buffering capacity. Water heating was more important in mid- to high-salinities as a result of diel patterns of solar irradiance. Heterotrophy was slight and not able to outcompete thermodynamics and generate outgassing during estuarine mixing. Consistently, stable isotopic signatures of DIC (delta C-13-DIC) presented slight deviations below the conservative mixing, corroborating net heterotrophy in the main channel. Areas of CO2 uptake due to phytoplankton activity were identified but restricted to the freshwater endmember during low river flow, with lowest pCO(2) (up to 41 ppmv) and the highest chlorophyll a (up to 21.3 mu g L-1). The estuary was a CO2 sink during low river flow ( - 1.34 to -5.26 mmolC m(-2) d(-1)) and a source during high river flow (5.71 to 19.37 mmolC m(-2) d(-1)). In the mangrove creek, the pCO(2), DIC, delta C-13-DIC and TA presented deviations from the conservativity, with slopes between TA and DIC demonstrating organic carbon degradation mediated by aerobic respiration and sulphate reduction. Mangrove creek waters were a CO2 source (average of 134.81 mmolC m(-2) d(-1)), exhibiting high values of pCO(2) (up to 21,720 ppmv). The results reveal that the low buffering capacity in the main channel of tropical estuarine deltas can be the predominant driver of pCO(2), generating CO2 undersaturation along the mixing zone, a process overlooked in estuarine systems. Moreover, air-water CO2 exchange, thermal variability, and biological activities contribute to deviation of the carbonate system from conservative mixing in specific estuarine areas, also modulating pCO(2) variability.

International audience; Coastal-water hypoxia is increasing globallydue to global warming and urbanization, and the need todefine management solutions to improve the water quality ofcoastal ecosystems has become important. The lower tidalGaronne River (TGR; southwestern France), characterizedby the seasonal presence of a turbidity maximum zone(TMZ) and urban water discharge, is subject to episodichypoxia events during low river flow periods in the summer.Future climatic conditions (higher temperature and summerdroughts) and increasing urbanization could enhance hy-poxia risks near the city of Bordeaux in the coming decades.A 3-D model of dissolved oxygen (DO) that coupleshydrodynamics, sediment transport and biogeochemicalprocesses was used to assess the efficiency of differentmanagement solutions for oxygenation of the TGR duringsummer low-discharge periods. We ran different scenarios ofreductions in urban sewage overflows, displacement of urbandischarges downstream from Bordeaux and/or temporaryriver flow support during the summer period. The modelshows that each option mitigates hypoxia, but with variableefficiency over time and space. Sewage overflow reductionimproves DO levels only locally near the city of Bordeaux.Downstream relocation of wastewater discharges allows forbetter oxygenation levels in the lower TGR. The support ofow river flow limits the upstream TMZ propagation anddilutes the TGR water with well-oxygenated river water.Scenarios combining wastewater network management andlow-water replenishment indicate an improvement in waterquality over the entire TGR. These modelling outcomesconstitute important tools for local water authorities todevelop the most appropriate strategies to limit hypoxia inthe TGR.

We report a data set of biogeochemical variables related to carbon cycling obtained in the three branches (Mỹ Tho, Hàm Luông, Co^´ Chiên) of the Mekong delta (Be^´n Tre province, Vietnam) in December 2003, April 2004, and October 2004. Both the inner estuary (upstream of the mouth) and the outer estuary (river plume) were sampled, as well as side channels. The values of the partial pressure of CO2 (pCO2) ranged between 232 and 4085 ppm, O2 saturation level (%O2) between 63 and 114 %, and CH4 between 2 and 2217 nmol L−1, within the ranges of values previously reported in temperate and tropical meso- and macro-tidal estuaries. Strong seasonal variations were observed. In the upper oligohaline estuary, low pCO2 (479–753 ppm) and high %O2 (98–106 %) values were observed in April 2004 most probably related to freshwater phytoplankton growth owing to low freshwater discharge (1400 m3 s−1) and increase in water residence time; during the two other sampling periods with a higher freshwater discharge (9300–17 900 m3 s−1), higher pCO2 (1895–2664 ppm) and lower %O2 (69–84 %) values were observed in the oligohaline part of the estuary. In October 2004, important phytoplankton growth occurred in the offshore part of the river plume as attested by changes in the contribution of particulate organic carbon (POC) to total suspended matter (TSM) (%POC) and the stable isotope composition of POC (δ13C-POC), possibly related to low TSM values (improvement of light conditions for phytoplankton development), leading to low pCO2 (232 ppm) and high %O2 (114 %) values. Water in the side channels in the Mekong delta was strongly impacted by inputs from the extensive shrimp farming ponds. The values of pCO2, CH4, %O2, and the stable isotope composition of dissolved inorganic carbon (δ13C-DIC) indicated intense organic matter degradation that was partly mediated by sulfate reduction in sediments, as revealed by the slope of total alkalinity (TA) and DIC covariations. The δ13C-POC variations also indicated intense phytoplankton growth in the side channels, presumably due to nutrient enrichment related to the shrimp farming ponds. A data set in the mangrove creeks of the Ca Mau province (part of the Mekong delta) was also acquired in April and October 2004. These data extended the range of variability in pCO2 and %O2 with more extreme values than in the Mekong delta (Be^´n Tre), with maxima and minima of 6912 ppm and 37 %, respectively. Similarly, the maximum CH4 concentration (686 nmol L−1) was higher in the Ca Mau province mangrove creeks than in the Mekong delta (Be^´n Tre, maximum 222 nmol L−1) during the October 2004 cruise (rainy season and high freshwater discharge period). In April 2004 (dry season and low freshwater discharge period), the CH4 values were much lower than in October 2004 (average 19 ± 13 and 210 ± 158 nmol L−1, respectively) in the Ca Mau province mangrove creeks, owing to the higher salinity (average 33.2 ± 0.6 and 14.1 ± 1.2, respectively) that probably led to higher sediment sulfate reduction, leading to inhibition of sediment methanogenesis and higher anaerobic CH4 oxidation. In the inner estuarine region (three branches of the Mekong delta), CO2 emissions to the atmosphere averaged 121 mmol m−2 d−1, and the CH4 emissions averaged 118 µmol m−2 d−1. The CO2 emission to the atmosphere from the Mekong inner estuary was higher than reported in the Yangtze and Pearl river inner estuaries. This was probably due to the lower salinity in the Mekong delta branches, possibly due to different morphology: relatively linear channels in the Mekong delta versus funnel-shaped estuaries for the Yangtze and Pearl river inner estuaries.

Dissolved inorganic carbon (DIC), its stable isotope composition (delta C-13-DIC) and ancillary parameters of the water column were investigated in a eutrophic tropical marine-dominated estuary surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil). Most negative delta C-13-DIC signatures (down to -6.1 parts per thousand) were found in polluted regions affected by direct sewage discharges where net heterotrophy induces high partial pressure of CO2 (pCO(2)) and DIC concentrations. Keeling plot was applied to this polluted region and determined the delta C-13-DIC sewage signature source of -12.2 parts per thousand, which is very consistent with isotopic signature found in wastewater treatment plans. These negative delta C-13-DIC signatures (i.e., DIC depleted in C-13) were restricted to the vicinity of urban outlets, whereas in the largest area of the bay delta C-13-DIC signatures were more positive (i.e., DIC enriched in C-13). The most positive delta C-13-DIC signatures (up to 4.6 parts per thousand) were found in surface waters dominated by large phytoplankton blooms, with positive correlation with chlorophyll a (Chl a). In the largest area of the bay, the preferential uptake of the lighter stable carbon isotope (C-12) during photosynthesis followed the Rayleigh distillation, and appeared as the most important driver of delta C-13-DIC variations. This reveals an important isotopic fractionation (epsilon) by phytoplankton due to successive algal blooms that has turned the remaining DIC pool enriched with the heavier stable carbon isotope (C-13). The calculated diel apparent epsilon showed higher values in the morning (18.7 parts per thousand-21.6 parts per thousand) and decreasing in the afternoon (6.8 parts per thousand-11.1 parts per thousand). epsilon was positively correlated to the pCO(2) (R-2 = 0.88, p = 0.005) and DIC concentrations (R-2 = 0.73, p = 0.02), suggesting a decline in carbon assimilation efficiency and decreasing uptake of the lighter carbon under CO2 limiting conditions. The eutrophic coastal waters of Guanabara Bay have delta C-13-DIC signatures well above that found in estuaries, shelf and ocean waters worldwide.

At the global scale, inland waters are a significant source of atmospheric carbon (C), particularly in the tropics. The active pipe concept predicts that C emissions from streams, lakes and rivers are largely fuelled by terrestrial ecosystems. The traditionally recognized C transfer mechanisms from terrestrial to aquatic systems are surface runoff and groundwater drainage. We present here a series of arguments that support the idea that land flooding is an additional significant process that fuels inland waters with C at the global scale. Whether the majority of CO2 emitted by rivers comes from floodable land (approximately 10% of the continents) or from well-drained land is a fundamental question that impacts our capacity to predict how these C fluxes might change in the future. Using classical concepts in ecology, we propose, as a necessary step forward, an update of the active pipe concept that differentiates floodable land from drained land. Contrarily to well-drained land, many wetlands (in particular riparian and littoral wetlands) combine strong hydrological connectivity with inland waters, high productivity assimilating CO2 from the atmosphere, direct transfer of litter and exudation products to water and waterlogged soils, a generally dominant allocation of ecosystem respiration (ER) below the water surface and a slow gas-exchange rate at the water-air interface. These properties force plants to pump atmospheric C to wetland waters and, when hydrology is favourable, to inland waters as organic C and dissolved CO2. This wetland CO2 pump may contribute disproportionately to CO2 emissions from inland waters, particularly in the tropics where 80% of the global CO2 emissions to the atmosphere occur. In future studies, more care must be taken in the way that vertical and horizontal C fluxes are conceptualized along watersheds, and 2-D models that adequately account for the hydrological export of all C species are necessary. In flooded ecosystems, significant effort should be dedicated to quantifying the components of primary production and respiration by the submerged and emerged part of the ecosystem community and to using these metabolic rates in coupled hydrological-biogeochemical models. The construction of a global typology of wetlands that includes productivity, gas fluxes and hydrological connectivity with inland waters also appears necessary to adequately integrate continental C fluxes at the global scale.

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), chlorophyll a (Chl a), pheo-pigments and ancillary parameters. Highest DOC, POC and Cha concentrations were found in confined-shallow regions of the bay during the summer period with strong pCO2 undersaturation, and DOC reached 82 mg L-1 , POC 152 mg L-1 , and Chl a 800 lg L-1 . Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m-2 year-1 , representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments.

The dynamics of the aragonite saturation state (Omega(arag)) were investigated in the eutrophic coastal waters of Guanabara Bay (RJ-Brazil). Large phytoplankton blooms stimulated by a high nutrient enrichment promoted the production of organic matter with strong uptake of dissolved inorganic carbon (DIC) in surface waters, lowering the concentrations of dissolved carbon dioxide (CO2aq), and increasing the pH, Omega(arag) and carbonate ion (CO32- especially during summer. The increase of Omega(arag) related to biological activity was also evident comparing the negative relationship between the Omega(arag) and the apparent utilization of oxygen (AOU), with a very close behavior between the slopes of the linear regression and the Redfield ratio. The lowest values of Omega(arag) were found at low-buffered waters in regions that receive direct discharges from domestic effluents and polluted rivers, with episodic evidences of corrosive waters (Omega(arag) < 1). This study showed that the eutrophication controlled the variations of Omega(arag) in Guanabara Bay.

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO(2)), chlorophyll a (Chl a), pheo-pigments and ancillary parameters. Highest DOC, POC and Chl a concentrations were found in confined-shallow regions of the bay during the summer period with strong pCO(2) undersaturation, and DOC reached 82 mg L-1, POC 152 mg L-1, and Chl a 800 mu g L-1. Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m(-2) year(-1), representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments.

International audience

The increasing concentrations of methane (CH4) in the atmosphere stress the importance of monitoringand quantifying the fluxes from coastal environments. In nine sampling campaigns between 2013 and 2014,we measured the spatial CH4concentrations, identified major sources and calculated the fluxes at the air-water interface in an eutrophic tropical embayment, Guanabara Bay, Rio de Janeiro, Brazil. The bay presentedhigh spatial variability of CH4concentrations, without a significant trend with salinity, but observed theinfluence of the urban areas at its watershed. Although the more polluted sector of the bay accounts forabout 10% of the sampled surface area, it contributed to one half of the bay’s total CH4emissions. In mostcases, high CH4concentrations seemed be sustained by allochtonous sources such as the sewage network andpolluted rivers, especially under high accumulated precipitation conditions. In the most stratified area, at theinner and centre of the Bay, CH4concentrations were not significantly higher in bottom hypoxic watersthan in surface waters, suggesting that CH4diffusion from these sediments was modest, due to the preva-lence of sulphate reduction over methanogenesis. Our calculated annual air-sea fluxes (565–980 lmol m22d21) are well above those of most estuaries worldwide, showing that urban pollution can be an importantsource of CH4to the coastal waters and even more significant than the presence of organic-rich environ-ments, like salt marshes and mangroves. Comparing the greenhouse gas emissions in terms of CO2-equiva-lent, CH4emissions reduced the net CO2sink in Guanabara Bay by 16%.