Search

Tropical estuarine systems play several ecological roles, such as acting as a nursery for biodiversity and cycling nutrients and greenhouse gases. However, the phylogenetic and metabolic diversity of estuarine microbiomes is not completely known. Furthermore, it is unclear how seasonal patterns may affect microbial diversity in these systems. The aim of the present study was to determine the metagenomic diversity and its major drivers in a large tropical estuarine system dominated by a mangrove forest in the South Atlantic around the Paraiba do Sul River. In total, 12.16 million shotgun sequences were generated (dry season: n = 8; wet season: n = 5), and water-quality parameters were evaluated for all locations. Metagenomic sequences were distributed between two patterns: (1) the dry season, in which the families Rhodobactereaceae and Flavobactereaceae increased, and (2) the wet season, in which Moraxellaceae, Pseudomonadaceae, Pseudoalteromonadaceae were more abundant. The dry season was characterized by higher salinity, nitrogen fixation, nitrification, and photosynthetic potential. In contrast, the wet season had higher carbon dioxide (CO 2 ) and methane (CH 4 ) production and a higher abundance of methanogenic, methylotrophic, and chemoorganotrophic bacteria in samples with low salinity. These findings suggest a possible relationship with the production of greenhouse gases during the wet period., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Dissolved oxygen (DO) is a fundamental parameter of coastal water quality, as it is necessary to aquatic biota, and it provides an indication of organic matter decomposition in waters and their degree of eutrophication. We present here a 7 year time series of DO concentration and ancillary parameters (river discharge, water level, turbidity, temperature, and salinity) from the MAGEST high-frequency monitoring network, at four automated stations in the central and fluvial regions of the macrotidal Gironde Estuary, one of the largest European estuaries. The spatio-temporal variability of DO at different time scales was first related to the migration and position of the maximum turbidity zone in this extremely turbid estuary. Since 2005, the Gironde Estuary has recorded several borderline hypoxic situations (DO close to 2 mg L(-1)) and a 7 day-long hypoxic event (DO < 2 mg L(-1)) in July 2006. Summer hypoxia occurred exclusively in the fluvial, low salinity, and high turbidity sections of the estuary and was significantly more pronounced in front of the large urban area of Bordeaux (715 000 inhabitants). Detailed analysis of the data at the seasonal, neap-spring and semi-diurnal tidal time scales, reveals that hypoxia in this area occurred: (i) in the maximum turbidity zone; (ii) during the spring to neap tide transition; (iii) at highest water temperature; and (iv) at lowest river discharge; there was also evidence of an additional negative impact of sewage treatment plants of Bordeaux city. Enhancement of respiration by turbidity, temperature and inputs of domestic biodegradable organic matter and ammonia, versus renewal of waters and dispersion of reduced pollutants with the river discharge, appeared as the dominant antagonist processes that controlled the occurrence of summer hypoxia. In the context of long-term environmental changes (increase in temperature and population, decrease in summer river discharge), the occurrence of severe hypoxia could not be excluded in the next decades in the upstream reach of the Gironde Estuary.

Abstract: Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40μmolL−1 h−1, dissolved inorganic carbon (DIC) production of 9–12μmolL−1 h−1, very low NH4 + and Mn2+ concentrations, and constant NO3 − production rates (0.4 - 0.7μmolL−1 h−1) due to coupled ammonification and nitrification. The beginning of anoxic periods (24h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6μmolL−1 h−1 and very fast NO3 − consumption (5.6–6.3μmolL−1 h−1) and NH4 + production (1.4–1.5μmolL−1 h−1). The latter rates were positively correlated to NO3 − concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO3 − concentrations and DIC and NH4 + productions of less than 1.3 and 0.1μmolL−1 h−1, respectively. Mn2+ and CH4 were produced at constant rates (respectively 0.3 and 0.015μmolL−1 h−1) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36h following anoxic to oxic switches) showed rapid NH4 + and Mn2+ decreases to zero (1.6 and 0.8–2μmolL−1 h−1, respectively) and very fast NO3 − production (3μmolL−1 h−1). This NO3 − production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface. [Copyright &y& Elsevier]

Abstract: In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO2 in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO2 and floating chamber flux measurements. Values of k increased with wind speed and were significantly greater when water currents and wind were in opposing directions. At low wind speeds (<1m s−1), k increased with water current velocities (0–1.5m s−1) following an exponential trend. The latter was a good proxy for the Y-intercept in a generic equation for k versus wind speed in estuaries. We also found that, in this turbid estuary, k was significantly lower at high turbidity. The presence of suspended material in great concentrations (TSS>0.2g L−1) had a significant role in attenuating turbulence and therefore gas exchange. This result has important consequences for modeling water oxygenation in estuarine turbidity maxima. For seven low turbidity estuaries previously described in the literature, the slope of the linear regression between k and wind speed correlates very well with the estuary surface area due to a fetch effect. In the Gironde Estuary, this slope follows the same trend at low turbidity (TSS<0.2g L−1), but is on average significantly lower than in other large estuaries and decreases linearly with the TSS concentration. A new generic equation for estuaries is proposed that gives k as a function of water current velocity, wind speed, estuarine surface area and TSS concentration. [Copyright &y& Elsevier]

We measured the flux of CO2 across the air-water interface using the floating chamber method in three European estuaries with contrasting physical characteristics (Randers Fjord, Scheldt, and Thames). We computed the gas transfer velocity of CO2 (k) from the CO2 flux and concomitant measurements of the air-water gradient of the partial pressure of CO2 (pCO2). There was a significant linear relationship between k and wind speed for each of the three estuaries. The differences of the y-intercept and the slope between the three sites are related to differences in the contribution of tidal currents to water turbulence at the interface and fetch limitation. The contribution to k from turbulence generated by tidal currents is negligible in microtidal estuaries such as Randers Fjord but is substantial, at low to moderate wind speeds, in macrotidal estuaries such as the Scheldt and the Thames. Our results clearly show that in estuaries a simple parameterization of k as a function of wind speed is site specific and strongly suggest that the y-intercept of the linear relationship is mostly influenced by the contribution of tidal currents, whereas the slope is influenced by fetch limitation. This implies that substantial errors in flux computations are incurred if generic relationships of the gas transfer velocity as a function of wind speed are employed in estuarine environments for the purpose of biogas air-water flux budgets and ecosystem metabolic studies. [ABSTRACT FROM AUTHOR]

Estuarine carbonate chemistry predicts that thermodynamic equilibration during the mixing of freshwater with seawater will generate a carbon dioxide (CO 2) sink in the case of warm and poorly buffered tropical rivers. The São Francisco River estuary has historically become oligotrophic after the construction of a series of hydroelectric dams in its watershed, where organic matter and nutrients are retained. During two cruises in late winter (Aug. 2014) and early summer (Nov. 2015), dissolved inorganic carbon (DIC) and total alkalinity (TA) were found to increase linearly with salinity in the main estuarine channel, where water mixing required half a day, and showed nearly conservative behaviour. In the main channel, the water partial pressure of CO 2 (p CO 2) recorded at a 1-min frequency followed an asymmetric bell-shaped trend versus salinity, similar to the curve predicted by the thermodynamic conservative mixing of freshwater DIC and TA with seawater DIC and TA. The low (0–3) salinity region was always a source of atmospheric CO 2 , where despite low chlorophyll concentrations, a p CO 2 diurnal change of approximately 60 ppmv suggested the occurrence of photosynthesis in summer. At salinities above 3, undersaturated p CO 2 values (down to 225 ppmv in winter and neap tides) and invasion of atmospheric CO 2 of 0.38–1.70 mmol m−1 h−1 occurred because of the predominating thermodynamics during estuarine mixing. In winter and neap tides, the higher river discharge, intense estuarine mixing, lower temperatures and limited tidal pumping resulted in observed p CO 2 differences from the theoretical conservative p CO 2 by less than 3 ppmv at salinities >3. Conversely, in summer and spring tides, the recorded p CO 2 values were on average + 43 ± 35 ppmv above the conservative mixing curve, when tidal pumping, CO 2 invasion and surface heating were more significant in the mixing zone but not sufficient to offset the thermodynamic uptake of atmospheric CO 2. By combining carbonate chemistry with estuarine mixing modelling and gas exchange calculations, we estimate that heating contributed to approximately 15% and gas exchange contributed to approximately 10% of the positive p CO 2 deviation from conservative mixing during summer. The remaining 75% of the deviation reached its maximum at ebb tides and within salinity ranges consistent with the occurrence of tidal pumping from marches and mangrove soils. Indeed, in the mangrove channel, water was supersaturated, with p CO 2 values of 976 ± 314 ppmv, while in the main channel, the highest positive p CO 2 deviations from conservative mixing (up to +100 ppmv for several hours) occurred at ebb tides. An important finding was that in São Francisco, the thermodynamic and biological processes compete with each other for CO 2 fluxes both at low salinities where evasion and autotrophy occur and at high salinities where invasion, heterotrophy and tidal pumping occur. Our study suggests that carbonate thermodynamics during mixing is a key process that has been overlooked in estuarine studies, although they can generate important air-water CO 2 exchange and significantly contribute to the carbon budget of estuaries and river plumes. [ABSTRACT FROM AUTHOR]