Mercury distribution and reactivity in waters of a subtropical coastal lagoon, Sepetiba Bay, SE Brazil

Abstract

As concentrações de Hg total e reativo, de carbono orgânico dissolvido (COD) e carbono orgânico particulado (COP) foram medidas em diferentes massas d'água na Baía de Sepetiba, SE do Brasil, com o objetivo de avaliar mudanças na reatividade do Hg ao longo do gradiente estuarino. As concentrações de Hg reativo foram mais baixas em águas da baía franca (0,09 ± 0,06 ng L-1), seguidas pelas águas influenciadas por rios (0,19 ± 0,06 ng L-1), e pelas de manguezais (0,29 ± 0,02 ng L-1). As concentrações de Hg total foram menores em águas da baía franca (0,38 ± 0,20 ng L-1), seguidas pelas águas de manguezais (0,56 ± 0,01 ng L-1), e foram maiores nas águas influenciadas por rios (0,99 ± 0,29 ng L-1). O percentual de Hg reativo em relação à concentração total foi menor nas águas influenciadas por rios (20,7%), seguidas pelas de águas da baía franca (31,2%), e foram maiores nas águas de manguezais (53,2%). O percentual de Hg reativo em relação à concentração total apresentou significativa correlação inversa com as concentrações de Hg total. Nenhuma correlação significativa foi encontrada entre as concentrações de Hg e a salinidade, o COP e o pH. Entretanto, as concentrações de Hg reativo apresentaram uma correlação positiva, significativa com o COD, mostrando que o controle da reatividade do Hg e, eventualmente, de sua biodisponibilidade na Baía de Sepetiba, deve-se principalmente a formação de complexos com a matéria orgânica dissolvida. A survey of the reactive and total Hg concentrations in surface waters and of the quantity of particulate organic carbon (POC) and dissolved organic carbon (DOC), were performed along a transect from the major rivers mouths and mangrove dominated areas to the open bay waters of Sepetiba Bay, SE Brazil. The objective of the survey was to evaluate changes in Hg reactivity during the transport along the estuarine gradient. Reactive Hg concentrations were lowest in open bay waters, (0.09 ± 0.06 ng L-1), followed by river influenced waters (0.19 ± 0.06 ng L-1), and mangrove (0.29 ± 0.02 ng L-1). Total Hg concentrations were lowest in open bay (0.38 ± 0.20 ng L-1), followed by mangrove waters (0.56 ± 0.01 ng L-1), and river influenced waters (0.99 ± 0.29 ng L-1). The percentages of reactive Hg relative to the total Hg concentrations were lowest in river influenced waters, (20.7%), followed by open bay waters (31.2%), and mangrove waters (53.2%). The percent of reactive Hg to the total Hg concentrations showed a significant negative correlation with the total Hg concentrations following a potential equation. No relationship between reactive Hg concentrations and POC, salinity or pH were found, however, reactive Hg correlates significantly with DOC, suggesting that organo-complexes, rather than chloro-complexes, are the major controllers of Hg reactivity, and eventually bioavailability in Sepetiba Bay.

Mercury speciation changes in waters of the sepetiba Bay, SE Brazil during tidal events and different seasons

“…25 A negative correlation was also found between reactive Hg and DOC concentrations (r = -0.44, n = 101, P < 0.01) showing that increasing DOC can decrease Hg reactivity and thus influence Hg availability and reactivity in the dissolved phase, such as methylation processes. These results are in disagreement which previously results found in the internal waters of the Sepetiba Bay by Lacerda et al 41 which showed a positive correlation between these parameters, suggesting that the DOC did not control the Hg reactivity in the internal area of the bay or that this DOC was sufficiently labile to keep the Hg under reactive forms, even after complexation.…”

“…25 A negative correlation was also found between reactive Hg and DOC concentrations (r = -0.44, n = 101, P < 0.01) showing that increasing DOC can decrease Hg reactivity and thus influence Hg availability and reactivity in the dissolved phase, such as methylation processes. These results are in disagreement which previously results found in the internal waters of the Sepetiba Bay by Lacerda et al 41 which showed a positive correlation between these parameters, suggesting that the DOC did not control the Hg reactivity in the internal area of the bay or that this DOC was sufficiently labile to keep the Hg under reactive forms, even after complexation.The non-reactive Hg fraction that represents mostly the organic Hg fraction was estimated by the difference between reactive and total Hg concentrations. 51,54,[59][60][61][62][63] This fraction was always higher during the ebb tide (1.7 ± 0.8 ng L -1 , n = 40 and 3.5 ± 0.9 ng L -1 , n = 21 in dry and wet season respectively) than during the flood tide (0.7 ± 0.5 ng L -1 , n = 24 and 2.4 ± 0.8 ng L -1 , n = 23 in dry and wet season respectively) (Figure 3c).…”

“…37 In the bay's central and external portions, there is a decrease of the influence of its tributaries and a gradual dilution of Hg concentrations in sediments 37 and in waters occurs. 41,45 Figure 4 presents the non-reactive, reactive, total and particulate Hg distribution along the estuarine gradient in Sepetiba Bay, including results from earlier works 41,44,45 and those obtained in the present study.Despite the main Hg fraction in the rivers being associated with suspended particles, the dissolved Hg fraction concentrations in rivers are, in general, one order of magnitude higher than those measured in the bay. In the bay internal portion an intensive changes in the mercury partition and a large dilution of the Hg concentrations is observed.…”

“…28 Marins et al 28 showed that the direct atmospheric deposition contributes with only 5% of the total Hg input to the Sepetiba Bay while the fluvial systems contribute with 95%, between 15% to 30% of the total Hg input to the bay accumulate in bottom sediments and 70% to 85% stay cycling within the water column becoming bioavailable for methylation processes and export to the adjacent continental shelf waters. 28,36 In the last years, many studies had been made about the Hg in the sediment, 37,38 in the atmospheric deposition, 39,40 in the fresh and coastal waters [41][42][43][44][45][46] and in the biota 37,47,48 of the Sepetiba Bay, showing the Hg distribution and geochemical in different end-members.…”

“…28 Marins et al 28 showed that the direct atmospheric deposition contributes with only 5% of the total Hg input to the Sepetiba Bay while the fluvial systems contribute with 95%, between 15% to 30% of the total Hg input to the bay accumulate in bottom sediments and 70% to 85% stay cycling within the water column becoming bioavailable for methylation processes and export to the adjacent continental shelf waters. 28,36 In the last years, many studies had been made about the Hg in the sediment, 37,38 in the atmospheric deposition, 39,40 in the fresh and coastal waters [41][42][43][44][45][46] and in the biota 37,47,48 of the Sepetiba Bay, showing the Hg distribution and geochemical in different end-members.Recent studies showed that 16 to 85% of total Hg inputs through the major tributaries of Sepetiba Bay are associated with suspended particulate material (SPM). 44 Annual fluxes of total dissolved and particulate Hg and sediment from major Sepetiba Bay tributaries were estimated by Paraquetti et al 44 as 11 kg year -1 , 190 kg year -1 and 4.8 × 10 8 kg year -1 , respectively.…”