Air-water CO2 Fluxes Research Articles

Hydrologic controls on CO2 chemistry and flux in subtropical lagoonal estuaries of the northwestern Gulf of Mexico

Estuaries are generally considered a source of CO2 to the atmosphere, although with significant uncertainties in magnitude and controlling factors between and within estuaries. We studied four northwestern Gulf of Mexico estuaries that experience extreme hydrologic conditions between April 2014 and February 2017 to determine the role of dry/wet cycle on estuarine CO2 system. Annual air–water CO2 flux ranged from 2.7 to 35.9 mol·C·m−2·yr−1; CO2 flux declined by approximately an order of magnitude along with declining river discharge. Episodic flooding made CO2 flux differ between dry (−0.7 to 20.9 mmol·C·m−2·d−1) and wet (11.6–170.0 mmol·C·m–2·d–1) conditions. During wet condition, increases in dissolved inorganic carbon (DIC) and total alkalinity (TA) significantly elevated CO2 degassing. Furthermore, ventilation of river‐borne CO2 strengthened degassing when estuaries became overwhelmingly river‐dominated. During flood relaxation, all estuaries experienced heightened productivity, evidenced by DIC and TA consumption in the mid‐salinity range (10–30). When prolonged drought led to hypersalinity (>36.5), biogeochemical and evaporative effects enhanced DIC and TA consumption and CO2 degassing. Due to flooding and high wind speeds, these estuaries were a strong CO2 source during spring and summer. Then they transitioned to a weak CO2 source or sink during the fall. Low temperatures further depressed CO2 efflux during winter except when a pulse of freshwater input occurred. This study demonstrates that changes in the hydrologic condition of estuaries, such as dry/wet cycle and river discharge gradient, will greatly alter air–water CO2 flux and estuarine contribution to the global carbon budget.

Carbon dioxide efflux and ecosystem metabolism of small forest lakes

Small lakes are numerous in the landscape and closely connected to the terrestrial environment, which strongly influences the system scale carbon cycling. However, despite their importance in large-scale carbon budgets, small lakes remain understudied compared to large lakes. We investigated oxygen and carbon dynamics in four small Danish forest lakes over a year. Continuous pH measurements were used to establish a high-frequency time series of CO2 partial pressure and 1169 direct measurements of air–water CO2 flux were performed using floating chambers. Net ecosystem oxygen production (NEP) was derived from free-water oxygen measurements in order to quantify the contribution of in-lake aerobic metabolism to the air–water CO2 flux. We found that the forest lakes, on average, were tenfold CO2 supersaturated. The two most intensively studied lakes had mean CO2 effluxes of 36.3 mol m−2 year−1. The CO2 effluxes exceeded NEP during all months implying that the CO2 generated by aerobic respiration alone could not account for the observed CO2 efflux. The observed discrepancy is likely promoted by a hydrologic CO2 input and/or anaerobic sediment processes generating CO2 without a concurrent consumption of O2. A broad-scale national analysis showed that the CO2 efflux increased as lake size decreased despite lower gas exchange velocity. Small lakes also showed higher excess CO2 efflux compared to O2 influx. Overall, small lakes and forest lakes in particular have high CO2 effluxes and high CO2 supersaturation. However, the contribution of hydrological inputs and anaerobic sediment processes to the net CO2 efflux remains elusive.

Contrasting intra-annual inorganic carbon dynamics and air–water CO2 exchange in Dhamra and Mahanadi Estuaries of northern Bay of Bengal, India

Surface water partial pressure of carbon dioxide [pCO2 (water)], total alkalinity (TA), dissolved inorganic carbon (DIC), and air–water CO2 flux were measured in two estuaries of the Bay of Bengal namely Mahanadi and Dhamra. Though the annual average air–water CO2 fluxes at the Mahanadi and the Dhamra Estuaries were − 3.9 ± 21.4 (mean ± standard deviation) µmol m−2 h−1 and − 2.9 ± 11.6 µmol m−2 h−1, respectively, the intra-annual variation of air–water CO2 fluxes in the two estuaries was contrasting. Nonetheless, from the perspective of net primary productivity, the surface water of both the estuaries were found autotrophic throughout the study period with varying rates at different seasons and highest during summer months. Mahanadi Estuary acted as a CO2 source toward atmosphere during monsoon months, whereas, Dhamra Estuary acted as a source during pre-monsoon months. On the contrary, Mahanadi and Dhamra Estuaries acted as CO2 sink during pre-monsoon months and monsoon months, respectively. The salinity in Mahanadi Estuary was much lower compared to Dhamra, which indicated significant freshwater discharge rich in organic carbon, and the remineralization of this carbon to DIC during summer and monsoon months explained the CO2 source character. Whereas, in Dhamra, reduced freshwater flow and high turbidity were held accountable for net heterotrophic character of the water column during the post-monsoon months. The annual data set of air–water CO2 fluxes from these two estuaries produced from this study could be utilized in future to fill the data gap and upscale the Indian estuaries scenario from the perspective of blue carbon budgeting.

Applicability and consequences of the integration of alternative models for CO<sub>2</sub> transfer velocity into a process-based lake model

Abstract. Freshwater lakes are important in carbon cycling, especially in the boreal zone where many lakes are supersaturated with the greenhouse gas carbon dioxide (CO2) and emit it to the atmosphere, thus ventilating carbon originally fixed by the terrestrial system. The exchange of CO2 between water and the atmosphere is commonly estimated using simple wind-based parameterizations or models of gas transfer velocity (k). More complex surface renewal models, however, have been shown to yield more correct estimates of k in comparison with direct CO2 flux measurements. We incorporated four gas exchange models with different complexity into a vertical process-based physico-biochemical lake model, MyLake C, and assessed the performance and applicability of the alternative lake model versions to simulate air–water CO2 fluxes over a small boreal lake. None of the incorporated gas exchange models significantly outperformed the other models in the simulations in comparison to the measured near-surface CO2 concentrations or respective air–water CO2 fluxes calculated directly with the gas exchange models using measurement data as input. The use of more complex gas exchange models in the simulation, on the contrary, led to difficulties in obtaining a sufficient gain of CO2 in the water column and thus resulted in lower CO2 fluxes and water column CO2 concentrations compared to the respective measurement-based values. The inclusion of sophisticated and more correct models for air–water CO2 exchange in process-based lake models is crucial in efforts to properly assess lacustrine carbon budgets through model simulations in both single lakes and on a larger scale. However, finding higher estimates for both the internal and external sources of inorganic carbon in boreal lakes is important if improved knowledge of the magnitude of CO2 evasion from lakes is included in future studies on lake carbon budgets.

CO2 emissions from hydroelectric reservoirs in the Tigris River basin, a semi-arid region of southeastern Turkey

Three hydroelectric reservoirs (Kralkızı, Dicle and Batman) in the Tigris River basin (Turkey) were sampled monthly during one year in order to reveal spatial and seasonal changes in aqueous partial pressure of CO2 (pCO2) and to estimate diffusive fluxes of CO2 from the reservoirs’ surface water. pCO2 concentrations did not show significant spatial differences, while they showed significant seasonal variations. Temperature, precipitation and biological CO2 uptake through photosynthesis controlled pCO2 seasonality in the reservoirs, with maximal concentrations in the winter (ranging from 516.9 µatm in Kralkızı to 1299.2 µatm in Dicle) and minimal concentrations in the spring (ranging from 47.7 µatm in Batman to 140.7 µatm in Kralkızı). Most studies reported that reservoirs worldwide are net sources of CO2 to the atmosphere. However, the reservoirs in this study were sinks for atmospheric CO2 during the spring, summer and autumn seasons, while they were CO2 sources to the atmosphere during the winter. Air-water CO2 fluxes in Kralkızı, Dicle and Batman dam reservoirs were 2.39, 32.88 and 8.12 mmol/m2/day in the winter, respectively. On an annual basis, all three reservoirs acted as a sink for atmospheric CO2. These estimated CO2 fluxes were in the lower range for temperate reservoirs, despite the potential for winter conditions that shifted the reservoirs from sink to net source for atmospheric CO2.

Local Variability of CO2 Partial Pressure in a Mid-Latitude Mesotidal Estuarine System (Tagus Estuary, Portugal)

Estuaries play a crucial role in regional carbon cycling. Until now, accurate estimations of the impact of environmental variables on estuarine air–water CO2 fluxes have been mostly characterized by a low spatial-temporal sampling resolution. This study reports on the variations of CO2 partial pressure (pCO2) and related environmental parameters, at both tidal and seasonal temporal scales, in the surface seawater of a station located in the lower section of the Tagus estuary, Portugal. The study was carried out from February to December 2007. Air–water CO2 fluxes suggest that the lower estuary acted as a relatively weak source of CO2 to the atmosphere, with an average rate of 7.2 mol∙m−2∙year−1, with highest fluxes occurring in winter. Over a tidal cycle, pCO2 was mainly influenced by tidal-induced mixing. Results suggest an influence of upper and central estuary inputs with higher pCO2 values. pCO2 varied seasonally, with values decreasing from ~890 µatm in winter to ~400 µatm in summer and increasing again to ~990 µatm in autumn. The generalized linear model (GLM) applied to the data set explained 69.3% of the pCO2 variability, pointing to the thermodynamic effect of temperature and biological activity as the most relevant processes in CO2 dynamics. Tidal variation of pCO2 corresponded to ~35% of its seasonal variability, denoting the importance of tide conditions on the dynamics of inorganic carbon. Results showed distinct patterns in the dynamics of CO2 at the tidal scale. This outcome suggests that disregarding tidal variability in the use of seasonal data sets may lead to significant errors in annual carbon budget estimations.

CO2 fluxes from aquaculture ponds of a tropical wetland: Potential of multiple lime treatment in reduction of CO2 emission

Partial pressure of CO2 in water [pCO2(water)] and air-water CO2 flux were estimated in two aquaculture ponds (one received no lime treatment (NTP) and in the other lime treatment was performed (LTP) four times a year) every month throughout an annual cycle, situated in East Kolkata Wetlands, a Ramsar Site in eastern India. It was hypothesized that lime treatment can potentially lower the pCO2(water) in aquaculture ponds and hence make these aquatic bodies sinks for CO2. The results portrayed that NTP acted as a source of CO2 throughout the year (annual mean: 1929 ± 1397 μmol m−2 h−1), whereas, LTP acted as CO2 sinks post lime addition (monthly mean ranged from −366 ± 16 to −449 ± 32 μmol m−2 h−1), though the effect of lime addition was found to diminish by the next month and it acted as source for CO2 in the months when no lime treatment was done (LTP annual mean: 1010 ± 1617 μmol m−2 h−1). Lime treatment increased the pH level and reduced the turbidity which facilitated optimum photosynthesis and the productivity increased rapidly. Beyond the critical pH value of 8.9–9.0, the pCO2(water) values became under-saturated with respect to atmospheric CO2 concentration. The effect of lime treatment was not found to prevail in the following months as a steady source of sewage from the Kolkata metropolis which feeds these aquaculture constantly bring in a huge carbon source both in inorganic and organic form. As soon as the flocculation effect of the lime dies off, the water column starts becoming turbid again which aids in converting the system into a net heterotrophic one from a net autotrophic. Based on the results we could successfully accept our hypothesis that lime treatment can not only reduce the CO2 emission but also make the system a CO2 sink.

Improved Post-processing of Eddy-Covariance Data to Quantify Atmosphere–Aquatic Ecosystem CO2 Exchanges

The capture of carbon by aquatic ecosystems and its sequestration in sediments has been studied as a potential method for mitigating the adverse effects of climate change. However, the evaluation of in situ atmospheric CO2 fluxes is challenging because of the difficulty in making continuous measurements over areas and for periods of time that are environmentally relevant. The eddy covariance method for estimating atmospheric CO2 fluxes is the most promising approach to address this concern. However, methods to process the data obtained from eddy covariance measurements are still being developed, and the estimated air-water CO2 fluxes have large uncertainties and differ from those obtained using conventional methods. In this study, we improved the post-processing procedure for the eddy covariance method to reduce the uncertainty in the measured air-water CO2 fluxes. Our procedure efficiently removes low-quality fluxes using a combination of filtering methods based on the received signal strength indicator of the eddy covariance sensor, the normalized standard deviation of atmospheric CO2 and water vapor concentrations, and a high-pass filter. The improved eddy covariance fluxes revealed diurnal and semi-diurnal cycles and a significant relationship with water fCO2, patterns that were not observed from the results before filtering. Although there were still differences with indirect conventional measurements like the bulk formula method, the methods used in this study should improve the accuracy of carbon flow estimates at sites with complex terrains like coastal areas.

Influence of water management and natural variability on dissolved inorganic carbon dynamics in a mangrove-dominated estuary

High-resolution time series measurements of temperature, salinity, pH and pCO2 were made during the period October 2014–September 2015 at the midpoint of Shark River, a 15km tidal river that originates in the freshwater Everglades of south Florida (USA) and discharges into the Gulf of Mexico. Dissolved inorganic carbon dynamics in this system vary over time, and during this study could be classified into three distinct regimes corresponding to October 2014–February 2015 (a wet to dry season transition period), March–May 2015 (dry period) and July–September 2015 (wet period). Average net longitudinal dissolved inorganic carbon (DIC) fluxes and air-water CO2 fluxes from the Shark River estuary were determined for the three periods. Net DIC fluxes to the coast were estimated to vary between 23.2 and 25.4×105mold−1 with an average daily DIC flux of 24.3×105mold−1 during the year of study. CO2 emissions ranged between 5.5 and 7.8×105mold−1 with an average daily value of 6.4×105mold−1 during the year. The differences in estuarine carbon fluxes during the study period are attributed to differences in the relative importance of hydro-climatological drivers. Results suggest that, during months characterized by reduced rainfall, carbon fluxes are affected by water management via control structures in the upstream Everglades marshes. During months with high rainfall, when culverts are closed and rainfall events are more frequent, carbon fluxes depend more on other forcings, such as rainfall and groundwater discharge.

Effects of nutrients and organic matter inputs in the gases CO2 and O2: A mesocosm study in a tropical lake

Lakes are important components in the carbon cycling and threatened by direct and indirect human activities, which ultimately affect metabolic processes. We analyzed the daily dynamics of the air-water CO2 and O2 fluxes over nine days in a mesocosm experiment to determine how the inputs of inorganic nutrients (+NUTRI) and allochthonous organic matter (+OM) affected the metabolic processes. The control, representing the original state of the lake, showed low coefficient of variability among the days sampled and a predominant FCO2 to the atmosphere, but with mean values close to zero (0.2 ± 0.3 mmol m−2 d−1). In +NUTRI and +NUTRI+OM treatments mesocosms, the FCO2 and FO2 showed similar response, where the FCO2 was negative during all days (mean −1.8 ± 1.1 and −1.9 ± 1.2 mmol m−2 d−1, respectively) and FO2 was initially negative, becoming positive after day 4, and decreasing again after day 6. The +OM treatment intensified the FCO2 to the atmosphere (mean 0.4 ± 0.9 mmol m−2 d−1) with highest values at day 7. The ecosystem in +NUTRI and +NUTRI+OM treatments showed a similar recovery (8 days), while +OM treatment mesocosm was similar to the control conditions. The models showed that nutrients promoted larger overall changes and higher daily variability in both FCO2 and FO2, leading to a CO2 influx, followed by organic matter addition. In conclusion, this mesocosm experiment showed the fast response of the lakes to even small disturbances (e.g organic matter addition), which can intensify the sink or the source of carbon to the atmosphere and change the role of the lakes in the global carbon cycling.

Temporal variations of water pCO2 and the air–water CO2 flux at a coastal location in the southern California Current System: diurnal to interannual scales

The aim of this work was to identify processes that cause surface CO2 partial pressure (pCO2W) and air–water CO2 flux (FCO2) variations at a coastal location in the California Current System, from semidiurnal to interannual time scales. Such processes may include sea breeze, upwelling, El Niño/Southern Oscillation (ENSO) cycle, and “The Blob” (strong positive temperature anomalies in the NE Pacific). Sea surface temperature (SST, 2008–2014) and pCO2W (2008–2015) time series were generated with data collected from a MAPCO2 buoy anchored at 100 m depth and 5 km from Punta Banda (31.6ºN, 116.6ºW), Baja California, México. There were significant changes in SST, pCO2W, and FCO2 that lasted periods that were as short as a few hours and as large as interannual time frames. Spectral analysis showed significant semidiurnal, diurnal, ~15-d, and ~28-d components of variation. Time series were divided into periods according to the Multivariate ENSO Index and the Blob event. Bayesian t tests show that both pCO2W and FCO2 had high credibility of being different between periods. During La Niña conditions (2010–2011), maximum pCO2W and FCO2 values were higher, and the minimum values were lower, than during “normal,” El Niño, and Blob conditions. The pCO2W range during this La Niña event was from 131 to 864 µatm, and the FCO2 range was from–6.9 to 40.4 mmol C·m–2·d–1. The FCO2 range during the Blob was from near equilibrium to ~2.5 mmol C·m–2·d–1. The extreme values for the FCO2 integral (± standard error), for all periods, were –57.0 ± 0.01 mmol C·m–2 for El Niño, and 257.0 ± 0.03 mmol C·m–2 for La Niña. Themean FCO2 value (± standard error) was 0.04 ± 0.02 mol C·m–2·yr–1. The buoy location was found to be a very weak source of CO2 during thestudy period.

Air–water CO2 flux in an algae bloom year for Lake Hongfeng, Southwest China: implications for the carbon cycle of global inland waters

The carbon cycle of global inland waters is quantitatively comparable to other components in the global carbon budget. Among inland waters, a significant part is man-made lakes formed by damming rivers. Man-made lakes are undergoing a rapid increase in number and size. Human impacts and frequent algae blooms lead to it necessary to make a better constraint on their carbon cycles. Here, we make a primary estimation on the air–water CO2 transfer flux through an algae bloom year for a subtropical man-made lake—Hongfeng Lake, Southwest China. To do this a new type of glass bottles was designed for content and isotopic analysis of DIC and other environmental parameters. At the early stage of algae bloom, CO2 was transferred from the atmosphere to the lake with a net flux of 1.770 g·C·m−2. Later, the partial pressure (pCO2) of the aqueous CO2 increased rapidly and the lake outgassed to the atmosphere with a net flux of 95.727 g·C·m−2. In the remaining days, the lake again took up CO2 from the atmosphere with a net flux of 14.804 g·C·m−2. As a whole, Lake Hongfeng released 4527 t C to the atmosphere, accounting for one-third of the atmosphere/soil CO2 sequestered by chemical weathering in the whole drainage. With an empirical mode decomposition method, we found air temperature plays a major role in controlling water temperature, aqueous pCO2 and hence CO2 flux. This work indicates a necessity to make detailed and comprehensive carbon budgets in man-made lakes.

Responses of carbonate system and CO2 flux to extended drought and intense flooding in a semiarid subtropical estuary

AbstractGlobally, estuaries are considered important CO2 sources to the atmosphere. However, estuarine water carbonate chemistry and CO2 flux studies have focused on temperate and high latitude regions, leaving a significant data gap in subtropical estuaries. In this study, we examined water column carbonate system and air–water CO2 flux in the Mission‐Aransas Estuary, a subtropical semiarid estuary in the northwestern Gulf of Mexico, by collecting samples at five System Wide Monitoring Program stations from 05/2014 to 04/2015. The carbonate system parameters (total alkalinity [TA], dissolved inorganic carbon [DIC], pH, CO2 partial pressure [pCO2], and carbonate saturation state with respect to aragonite [ΩAr]) and air–water CO2 flux all displayed substantial seasonal and spatial variations. Based on freshwater inflow conditions, a drought period occurred between 05/2014 and 02/2015, while a flooding period occurred from 03/2015 to 04/2015. Average DIC was 2194.7 ± 156.8 μmol kg−1 and 2132.5 ± 256.8 μmol kg−1, TA was 2497.6 ± 172.1 μmol·kg−1 and 2333.4 ± 283.1 μmol kg−1, pCO2 was 477 ± 94 μatm and 529 ± 251 μatm, and CO2 flux was 28.3 ± 18.0 mmol C·m−2·d−1 and 51.6 ± 83.9 mmol·C·m−2·d−1 in the drought and flooding period, respectively. Integrated annual air–water CO2 flux during our studied period was estimated to be 12.4 ± 3.3 mol·C·m−2·yr−1, indicating that this estuary was a net CO2 source. High wind speed, warm climate, riverine input, and estuarine biogeochemical processes all contributed to the high CO2 efflux despite the modest pCO2 levels year round.

Carbon budget of a shallow, lagoonal estuary: Transformations and source‐sink dynamics along the river‐estuary‐ocean continuum

AbstractA comprehensive carbon budget was constructed to quantify carbon flows through the freshwater‐marine continuum of a temperate, microtidal estuary. We performed coordinated measurements of dissolved inorganic carbon and total organic carbon fluxes to resolve spatial variability between and along the channel and shoals and diel variability across the entire estuary for 2 yr. Net ecosystem metabolism (NEM) was the most significant control on carbon flow within estuary regions. However, metabolic rates were spatially coupled such that counteracting fluxes across the channel‐shoal gradient or along the river‐ocean gradient resulted in system‐wide NEM that was closely in balance (–3.0 ± 3.3 to 1.1 ± 4.4 molC m−2 yr−1). Similarly, large diel and seasonal variability in air–water CO2 fluxes were observed during 72 spatial surveys, but these short‐term variations generally cancelled out when aggregated to annual budget terms. Although atmospheric exchanges were small (–0.2 ± 0.1 to 2.0 ± 0.4 molC m−2 yr−1), they were subject to large errors (± 4 molC m−2 yr−1) if diel variability was neglected. Internal mechanisms that maintained balanced carbon flows were strongly impacted by river discharge and were only apparent by separately quantifying channel and shoal fluxes. Notably, metabolic responses of the shoal to river forcing outweighed the responses of the channel, and the net impact was contrary to prior relationships derived from synthesis of lower‐resolution carbon budgets. Our budget demonstrates that resolution of carbon fluxes at appropriate scales, including channel‐shoal and diel variability, is critical to characterizing ecosystem function and the fate of carbon within the river‐ocean continuum.

Effect of hydroperiod on CO2fluxes at the air-water interface in the Mediterranean coastal wetlands of Doñana

Wetlands are productive ecosystems that play an important role in the Earth´s carbon cycle and thus global carbon budgets. Climate variability affects amount of material entering and the metabolic balance of wetlands, thereby modifying carbon dynamics. This study presents spatiotemporal changes in air-water CO2 exchange in the vast wetlands of Donana (Spain) in relation to different hydrological cycles. Water sources feeding Donana, including groundwater and streams, ultimately depend on the fluctuating balance between annual precipitation and evapotranspiration. Hence, in order to examine the contribution of the rainfall pattern to the emission/capture of CO2 by a range of aquatic habitats in Donana, we took monthly measurements during severely wet, dry and normal hydrological years (2010-2013). During wet hydrological cycles, CO2 outgassing from flooded marshes markedly decreased in comparison to that observed during subsequent dry-normal cycles, with mean values of 25.84±19 and 5.2±8 mmol m-2 d-1, respectively. Under drier meteorological conditions, air-water CO2 fluxes also diminished in permanent floodplains and ponds, which even behaved as mild sinks for atmospheric CO2 during certain periods. Increased inputs of dissolved CO2 from the underground aquifer and the stream following periods of high rainfall are believed to be behind this pattern. Large lagoons with a managed water supply from an adjacent estuary took up atmospheric CO2 nearly permanently. Regional air-water carbon transport was 15.2 GgC y-1 under wet and 1.24 GgC y-1 under dry meteorological conditions, well below the estimated net primary production for Donana wetlands, indicating that the ecosystem acts as a large CO2 sink

English

English

Air–Water CO2 Fluxes and Net Ecosystem Production Changes in a Baja California Coastal Lagoon During the Anomalous North Pacific Warm Condition

The present study examines the temporal variability of air–water CO2 fluxes (FCO2) and seawater carbonate chemistry in a Baja California coastal lagoon during an exceptionally warm anomaly that was developed in Northeast Pacific coasts during 2014. This oceanographic condition led to a summer-like season (weak upwelling condition) during the study period, which reached a maximum surface temperature anomaly of 2 °C in September 2014. San Quintin Bay acts as a source of CO2 to the atmosphere in 2014 (3.3 ± 4.8 mmol C m−2 day−1) with the higher positive fluxes mainly observed in summer months (9.0 ± 5.3 mmol C m−2 day−1). Net ecosystem production (NEP) switched seasonally between net heterotrophy and net autotrophy during the study period, with an annual average of 2.2 ± 7.1 mmol C m−2 day−1, which indicates that San Quintin Bay was a net autotrophic system during the atypical warm oceanographic condition in 2014. This pattern of seasonal variations in the carbon balance at San Quintin Bay appears to be linked to the life cycle of benthic communities, which play an important role in the whole-ecosystem metabolism. Under the limited input from external sources coupled with an increase in seawater temperatures, the recycled benthic carbon and nutrient fluxes play a major role to sustain water-column processes within the bay. Since the upwelling condition may influence the magnitude of the air–water CO2 fluxes, our results clearly indicated that San Quintin Bay is a net source of carbon to the atmosphere regardless of the adjacent oceanic conditions. Our study sheds light on the carbon dynamics and its metabolic implications in a shallow coastal ecosystem under a regional warm anomaly and contributes potentially relevant information in view of the likely future scenario of global climate change.

Groundwater, Acid and Carbon Dioxide Dynamics Along a Coastal Wetland, Lake and Estuary Continuum

Coastal wetlands are hotspots for biodiversity and biological productivity, yet the hydrology and carbon cycling within these systems remains poorly understood due to their complex nature. By using a novel spatiotemporal approach, this study quantified groundwater discharge and the related inputs of acidity and CO2 along a continuum of a modified coastal acid sulphate soil (CASS) wetland, a coastal lake and an estuary under highly contrasting hydrological conditions. To increase the resolution of spatiotemporal data and advance upon previous methodologies, we relied on automated observations from four simultaneous time-series stations to develop multiple radon mass balance models to estimate groundwater discharge and related groundwater inputs of acidity and dissolved inorganic carbon (DIC), along with surface water to atmosphere CO2 fluxes. Spatial surveys indicated distinct acid hotspots with minimum surface water pH of 2.91 (dry conditions) and 2.67 (flood conditions) near a non-remediated (drained) CASS area. Under flood conditions, groundwater discharge accounted for ∼14.5 % of surface water entering the lake. During the same period, acid discharge from the acid sulphate soil section of the continuum produced ∼4.8 kg H2SO4 ha−1 day−1, a rate much higher than previous studies in similar systems. During baseflow conditions, the low pH water was rapidly buffered within the estuarine lake, with the pH increasing from 4.22 to 6.07 over a distance of ∼250 m. The CO2 evasion rates within the CASS were extremely high, averaging 2163 ± 125 mmol m−2 day−1 in the dry period and 4061 ± 259 mmol m−2 day−1 under flood conditions. Groundwater input of DIC could only account for 0.4 % of this evasion in the dry conditions and ∼5 % during the flood conditions. We demonstrated that by utilising a spatiotemporal (multiple time-series stations) approach, the study was able to isolate distinct zones of differing hydrology and biogeochemistry, whilst providing more reasonable groundwater acid input estimates and air–water CO2 flux estimates than some traditional sampling designs. This study highlights the notion that modified CASS wetlands can release large amounts of CO2 to the atmosphere because of high groundwater acid inputs and extremely low surface water pH.

Air–water CO2 outgassing in the Lower Lakes (Alexandrina and Albert, Australia) following a millennium drought

Lakes are an important source and sink of atmospheric CO2, and thus are a vital component of the global carbon cycle. However, with scarce data on potentially important subtropical and tropical areas for whole continents such as Australia, the magnitude of large-scale lake CO2 emissions is unclear. This study presents spatiotemporal changes of dissolved inorganic carbon and water — to — air interface CO2 flux in the two of Australia's largest connected, yet geomorphically different freshwater lakes (Lake Alexandrina and Lake Albert, South Australia), during drought (2007 to September—2010) and post-drought (October 2010 to 2013). Lake levels in the extreme drought were on average approximately 1m lower than long-term average (0.71m AHD). Drought was associated with an increase in the concentrations of dissolved inorganic species, organic carbon, nitrogen, Chl-a and major ions, as well as water acidification as a consequence of acid sulfate soil (ASS) exposure, and hence, had profound effects on lake pCO2 concentrations. Lakes Alexandrina and Albert were a source of CO2 to the atmosphere during the drought period, with efflux ranging from 0.3 to 7.0mmol/m2/d. The lake air–water CO2 flux was negative in the post-drought, ranging between −16.4 and 0.9mmol/m2/d. The average annual CO2 emission was estimated at 615.5×106mol CO2/y during the drought period. These calculated emission rates are in the lower range for lakes, despite the potential for drought conditions that shift the lakes from sink to net source for atmospheric CO2. These observations have significant implications in the context of predicted increasing frequency and intensity of drought as a result of climate change. Further information on the spatial and temporal variability in CO2 flux from Australian lakes is urgently warranted to revise the global carbon budget for lakes.

Air–water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability

Abstract. Distributions of surface water partial pressure of carbon dioxide (pCO2) were measured on nine cruises in the Delaware Estuary (USA). The Delaware River was highly supersaturated in pCO2 with respect to the atmosphere during all seasons, while the Delaware Bay was undersaturated in pCO2 during spring and late summer and moderately supersaturated during mid-summer, fall, and winter. While the smaller upper tidal river was a strong CO2 source (27.1 ± 6.4 mol-C m−2 yr−1), the much larger bay was a weak source (1.2 ± 1.4 mol-C m−2 yr−1), the latter of which had a much greater area than the former. In turn, the Delaware Estuary acted as a relatively weak CO2 source (2.4 ± 4.8 mol-C m−2 yr−1), which is in great contrast to many other estuarine systems. Seasonally, pCO2 changes were greatest at low salinities (0 ≤ S < 5), with pCO2 values in the summer nearly 3-fold greater than those observed in the spring and fall. Undersaturated pCO2 was observed over the widest salinity range (7.5 ≤ S < 30) during spring. Near to supersaturated pCO2 was generally observed in mid- to high-salinity waters (20 ≤ S < 30) except during spring and late summer. Strong seasonal trends in internal estuarine production and consumption of CO2 were observed throughout both the upper tidal river and lower bay. Positive correlations between river-borne and air–water CO2 fluxes in the upper estuary emphasize the significance of river-borne CO2 degassing to overall CO2 fluxes. While river-borne CO2 degassing heavily influenced CO2 dynamics in the upper tidal river, these forces were largely compensated for by internal biological processes within the extensive bay system of the lower estuary.