pCO2 Variability Research Articles

Effects of seasonality, trophic state and landscape properties on CO2 saturation in low-latitude lakes and reservoirs

The role of tropical lakes and reservoirs in the global carbon cycle has received increasing attention in the past decade, but our understanding of its variability is still limited. The metabolism of tropical systems may differ profoundly from temperate systems due to the higher temperatures and wider variations in precipitation. Here, we investigated the spatial and temporal patterns of the variability in the partial pressure of carbon dioxide (pCO2) and its drivers in a set of 102 low-latitude lakes and reservoirs that encompass wide gradients of precipitation, productivity and landscape properties (lake area, perimeter-to-area ratio, catchment size, catchment area-to-lake area ratio, and types of catchment land use). We used multiple regressions and structural equation modeling (SEM) to determine the direct and indirect effects of the main in-lake variables and landscape properties on the water pCO2 variance. We found that these systems were mostly supersaturated with CO2 (92% spatially and 72% seasonally) regardless of their trophic status and landscape properties. The pCO2 values (9–40,020 μatm) were within the range found in tropical ecosystems, and higher (p < 0.005) than pCO2 values recorded from high-latitude ecosystems. Water volume had a negative effect on the trophic state (r = −0.63), which mediated a positive indirect effect on pCO2 (r = 0.4), representing an important negative feedback in the context of climate change-driven reduction in precipitation. Our results demonstrated that precipitation drives the pCO2 seasonal variability, with significantly higher pCO2 during the rainy season (F = 16.67; p < 0.001), due to two potential main mechanisms: (1) phytoplankton dilution and (2) increasing inputs of terrestrial CO2 from the catchment. We conclude that at low latitudes, precipitation is a major climatic driver of pCO2 variability by influencing volume variations and linking lentic ecosystems to their catchments.

Daily pCO2 and CO2 flux variations in a subtropical mesotrophic shallow lake

Carbon dioxide (CO2) emissions from lakes have been proven to be an important component of the continental carbon balance, but most CO2 evasion estimates ignore daily variability of partial pressure of CO2 (pCO2). To discern the variability of pCO2 and its effect on CO2 outgassing estimations, we conducted in-situ biweekly pCO2 measurements during daylight from November 2017 to June 2018 at 7:00, 10:00, 14:00, and 17:00 Central Standard Time of the United States (CST) in a subtropical shallow lake in Louisiana, USA. Daytime pCO2 varied largely from 154 to 1698 μatm with an average of 736 μatm, while daytime CO2 flux ranged from −43 to 284 mmol m2 h−1 averaging at 44 mmol m2 h−1. Significant decreases in pCO2 and CO2 fluxes from mornings to late afternoons were observed throughout the study period. Specifically, in morning hours average pCO2 and CO2 fluxes were 940 μatm and 83 mmol m2 h−1, respectively, while in afternoon hours average pCO2 and CO2 fluxes were only 410 μatm and 6 mmol m2 h−1, respectively. Relationships between pCO2 and ambient factors reveal that solar radiation, lake trophic status and water temperature play a major role in pCO2 and CO2 dynamics in lake water. We argue that uncertainties may exist in the current regional and global CO2 evasion estimates due to this high daily pCO2 variability.

Cave dripwater isotopic signals related to the altitudinal gradient of Mount-Lebanon: implication for speleothem studies

An important step in paleoclimate reconstructions based on vadose cave carbonate deposits or speleothems is to evaluate the sensitivity of the cave environment and speleothems to regional climate. Accordingly, we studied four caves, located at different altitudes along the western flank of Mount-Lebanon (Eastern Mediterranean). The objectives of this study are to identify the present-day variability in temperature, pCO2, and water isotopic composition and to assess the possible influence of the altitudinal gradient on cave drip waters and cave streams. We present here an overview of the spatial variability of rainwater based on local and regional data, and we compare these data with our results, i.e., temperature, air pCO2, and the isotopic composition of cave water and modern cave calcite collected in 2011 and 2014. The results show that the rainwater isotopic signal is generally preserved in the cave dripwater isotopic composition with some exceptions in large caves with high ceilings where evaporation effects may influence its isotopic composition. The altitude effect observed in rainwater isotopic composition seems to be transferred to the cave dripwater. Different δ18O/100 m gradients between dripwater and rainwater (0.13‰ and 0.21‰, respectively) are noted. This is mainly attributed to the δ18O/100 m value of the dripwater which is site-specific and dependent on i) local processes within the epikarst/soil, ii) the relation to the precipitation altitude gradient and iii) the extension of the defined infiltration basin.

Diel Variability of pCO2 and CO2 Outgassing from the Lower Mississippi River: Implications for Riverine CO2 Outgassing Estimation

Carbon dioxide (CO2) outgassing from river surface waters is an important component of the global carbon cycle currently not well constrained. To test the hypothesis that riverine partial pressure of CO2 (pCO2) and CO2 outgassing rates differ between daylight and darkness, we conducted in-situ pCO2 and ambient water measurements over four 24-h periods in the spring and summer of 2018 in the Lower Mississippi River under varying flow regimes. We hypothesized that diel pCO2 variation will correlate inversely with solar radiation due to light-induced photosynthesis. Despite differing ambient conditions between seasons, we found a consistent diel cycle of riverine pCO2, with highest values before sunset and lowest values during peak daylight. Recorded pCO2 measurements varied by 206–607 µatm in spring and 344–377 µatm in summer, with significantly lower records during daylight in summer. CO2 outgassing was significantly lower during daylight in both seasons, with diel variation ranging between 1.5–4.4 mmol m−2 h−1 in spring and 1.9–2.1 mmol m−2 h−1 in summer. Daily outgassing rates calculated incorporating diel variation resulted in significantly greater rates (26.2 ± std. 12.7 mmol m−2 d−1) than calculations using a single daily pCO2 value. This study suggests a likely substantial underestimation of carbon outgassed from higher order rivers that make up a majority of the global river water surface. The findings highlight the need for high temporal resolution data and further research on diel CO2 outgassing in different climate regions to constrain uncertainties in riverine flux estimation.

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.

Dynamic Greenland ice sheet driven by pCO2 variations across the Pliocene Pleistocene transition

It is generally considered that the perennial glaciation of Greenland lasting several orbital cycles began around 2.7 Ma along with the intensification of Northern Hemisphere glaciation (NHG). Both data and model studies have demonstrated that a decline in atmospheric pCO2 was instrumental in establishing a perennial Greenland ice sheet (GrIS), yet models have generally used simplistic pCO2 constraints rather than data-inferred pCO2 evolution. Here, using a method designed for the long-term coupling of climate and cryosphere models and pCO2 scenarios from different studies, we highlight the pivotal role of pCO2 on the GrIS expansion across the Plio-Pleistocene Transition (PPT, 3.0–2.5 Ma), in particular in the range between 280 and 320 ppm. Good qualitative agreement is obtained between various IRD reconstructions and some of the possible evolutions of the GrIS simulated by our model. Our results underline the dynamism of the GrIS waxing and waning under pCO2 levels similar to or lower than today, which supports recent evidence of a dynamic GrIS during the Plio-Pleistocene.

Drivers of CO2 along a mangrove-seagrass transect in a tropical bay: Delayed groundwater seepage and seagrass uptake

Water-to-air carbon dioxide fluxes from tropical coastal waters are an important but understudied component of the marine carbon budget. Here, we investigate drivers of carbon dioxide partial pressure (pCO2) in a relatively pristine mangrove-seagrass embayment on a tropical island (Bali, Indonesia). Observations were performed over eight underway seasonal surveys and a fixed location time series for 55 h. There was a large spatial variability of pCO2 across the continuum of mangrove forests, seagrass meadows and the coastal ocean. Overall, the embayment waters surrounded by mangroves released CO2 to the atmosphere with a net flux rate of 18.1 ± 5.8 mmol m−2 d−1. Seagrass beds produced an overall CO2 net flux rate of 2.5 ± 3.4 mmol m−2 d−1, although 2 out of 8 surveys revealed a sink of CO2 in the seagrass area. The mouth of the bay where coral calcification occurs was a minor source of CO2 (0.3 ± 0.4 mmol m−2 d−1). The overall average CO2 flux to the atmosphere along the transect was 9.8 ± 6.0 mmol m−2 d−1, or 3.6 × 103 mol d−1 CO2 when upscaled to the entire embayment area. There were no clear seasonal patterns in contrast to better studied temperate systems. pCO2 significantly correlated with antecedent rainfall and the natural groundwater tracer radon (222Rn) during each survey. We suggest that the CO2 source in the mangrove dominated upper bay was associated with delayed groundwater inputs, and a shifting CO2 source-sink in the lower bay was driven by the uptake of CO2 by seagrass and mixing with oceanic waters. This differs from modified landscapes where potential uptake of CO2 is weakened due to the degradation of seagrass beds, or emissions are increased due to drainage of coastal wetlands.

Modelling the mechanisms and drivers of the spatiotemporal variability of pCO2 and air–sea CO2 fluxes in the Northern Humboldt Current System

We use a coupled physical–biogeochemical model to investigate the drivers and mechanisms responsible for the spatiotemporal variability of the partial pressure of carbon dioxide in seawater (pCO2) and associated air–sea CO2 fluxes in the Northern Humboldt Current System (NHCS). Simulated pCO2 is in good agreement with available observations with an average absolute error of, approximately, 24 µatm. The highly productive upwelling region, 300 km from the shore and between 5 and 17 °S, is shown to be a strong CO2 source with an averaged flux of 5.60 ± 2.94 mol C m−2 year−1, which represents an integrated carbon flux of 0.028 ± 0.015 Pg C year−1. Through a series of model experiments we show that the high pCO2 is primarily the result of coastal upwelling, which is incompletely compensated by biology. Specifically, the supply of dissolved inorganic carbon (DIC)-rich waters to the surface pushes pCO2 up to levels around 1100 µatm. Even though biological production is high, it reduces pCO2 only by about 300 µatm. We show that this relatively low degree of biological compensation, which implies an inefficient biological pump in the nearshore domain, results from a spatiotemporal decoupling between the counteracting effects of biological production and the transport and mixing of DIC. The contribution of the outgassing and the processes affecting CO2 solubility, associated with the seasonal cycle of heating and cooling, are minor. Across the whole domain, the balance of mechanisms is similar, but with smaller amplitudes. We demonstrate that seawater pCO2 is more sensitive to changes in DIC and sea surface temperature, while alkalinity plays a minor role.

Seasonal Asymmetry in the Evolution of Surface Ocean pCO2 and pH Thermodynamic Drivers and the Influence on Sea‐Air CO2 Flux

AbstractIt has become clear that anthropogenic carbon invasion into the surface ocean drives changes in the seasonal cycles of carbon dioxide partial pressure (pCO2) and pH. However, it is not yet known whether the resulting sea‐air CO2 fluxes are symmetric in their seasonal expression. Here we consider a novel application of observational constraints and modeling inferences to test the hypothesis that changes in the ocean's Revelle factor facilitate a seasonally asymmetric response in pCO2 and the sea‐air CO2 flux. We use an analytical framework that builds on observed sea surface pCO2 variability for the modern era and incorporates transient dissolved inorganic carbon concentrations from an Earth system model. Our findings reveal asymmetric amplification of pCO2 and pH seasonal cycles by a factor of two (or more) above preindustrial levels under Representative Concentration Pathway 8.5. These changes are significantly larger than observed modes of interannual variability and are relevant to climate feedbacks associated with Revelle factor perturbations. Notably, this response occurs in the absence of changes to the seasonal cycle amplitudes of dissolved inorganic carbon, total alkalinity, salinity, and temperature, indicating that significant alteration of surface pCO2 can occur without modifying the physical or biological ocean state. This result challenges the historical paradigm that if the same amount of carbon and nutrients is entrained and subsequently exported, there is no impact on anthropogenic carbon uptake. Anticipation of seasonal asymmetries in the sea surface pCO2 and CO2 flux response to ocean carbon uptake over the 21st century may have important implications for carbon cycle feedbacks.

PCO2 Dynamics of Stratified Reservoir in Temperate Zone and CO2 Pulse Emissions During Turnover Events

This study explores the dynamic changes in the partial pressure of CO2 (pCO2) with depth, and the temporal variations of CO2 net atmospheric flux (NAF) in a stratified reservoir. A total of 16 field campaigns were conducted from the summer stratification to fall turnover period in 2017. A random forest (RF) model was developed to estimate the pCO2 using concurrently measured water quality variables. The results showed that the vertical distribution of pCO2 and associated temporal variations of the NAF are closely related to the stratification strength of the reservoir. The reservoir surface pCO2 was supersaturated (1542 µatm) in summer (July 11), but this decreased to undersaturation as algae grew. Meanwhile, dissolved CO2 continuously accumulated below the reservoir mixed-layer due to the thermal stratification barrier and organic-rich floodwater intrusion. Vertical mixing began instantly as the stratification strength began to weaken in mid-October, and the surface pCO2 increased sharply up to 1934 µatm. Consequently, the NAF drastically increased to 3235 mg−CO2 m−2·day−1, which implies that the NAF changes seasonally and large CO2 pulsing occurs during the turnover events. The results provide valuable information about pCO2 variability and physical mixing processes, as well as carbon budget estimation in stratified reservoirs, and offer an improved understanding of these phenomena.

Modern and traditional approaches combined into an effective gray-box mathematical model of full-blood acid-base

BackgroundThe acidity of human body fluids, expressed by the pH, is physiologically regulated in a narrow range, which is required for the proper function of cellular metabolism. Acid-base disorders are common especially in intensive care, and the acid-base status is one of the vital clinical signs for the patient management. Because acid-base balance is connected to many bodily processes and regulations, complex mathematical models are needed to get insight into the mixed disorders and to act accordingly. The goal of this study is to develop a full-blood acid-base model, designed to be further integrated into more complex human physiology models.ResultsWe have developed computationally simple and robust full-blood model, yet thorough enough to cover most of the common pathologies. Thanks to its simplicity and usage of Modelica language, it is suitable to be embedded within more elaborate systems. We achieved the simplification by a combination of behavioral Siggaard-Andersen’s traditional approach for erythrocyte modeling and the mechanistic Stewart’s physicochemical approach for plasma modeling. The resulting model is capable of providing variations in arterial pCO2, base excess, strong ion difference, hematocrit, plasma protein, phosphates and hemodilution/hemoconcentration, but insensitive to DPG and CO concentrations.ConclusionsThis study presents a straightforward unification of Siggaard-Andersen’s and Stewart’s acid-base models. The resulting full-blood acid-base model is designed to be a core part of a complex dynamic whole-body acid-base and gas transfer model.

Effects of Biological Production and Vertical Mixing on Sea Surface pCO2 Variations in the Changjiang River Plume During Early Autumn: A Buoy‐Based Time Series Study

AbstractA monthlong, high‐resolution buoy time series from the surface ocean of the Changjiang River plume in early autumn 2013 (30‐min sampling frequency) shows great variability in the partial pressure of carbon dioxide (pCO2) and other physical and biogeochemical parameters. Early in the deployment, surface pCO2 decreased by ~117 μatm in a single day (11–12 September, from an initial value of ~527 μatm); a similar decline of 62 μatm occurred five days later (to ~378 μatm). Both drawdown events were associated with strong vertical stratification, high chlorophyll a concentrations, and oxygen supersaturation. A one‐dimensional mass balance model suggests that biological production was responsible for more than half the pCO2 decrease observed during 10–23 September. Subsequently, in association with strong winds, the mixed layer rapidly deepened and surface pCO2 increased sharply (by about 108 μatm in late September and again in early October). Vertical mixing accounted for more than half of this pCO2 increase, which offset more than the earlier biologically driven CO2 drawdown. In the presence of such strong temporal variations of pCO2, sampling frequency exerts a substantial influence on air‐sea CO2 flux calculations for the Changjiang River plume and similar coastal areas. Compared to daily sampling, even weekly sampling would result in a bias of up to ±4.7 mmol C · m−2 · day−1 or ±63% error.

CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; partial pressure and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emission along the lower Red River (Vietnam)

Abstract. The Red River (Vietnam) is representative of a south-east Asian river system, strongly affected by climate and human activities. This study aims to quantify the spatial and seasonal variability of CO2 partial pressure and CO2 emissions of the lower Red River system. Water quality monitoring and riverine pCO2 measurements were carried out for 24 h at five stations distributed along the lower Red River system during the dry and the wet seasons. The riverine pCO2 was supersaturated relative to the atmospheric equilibrium (400 ppm), averaging about 1589±43 ppm and resulting in a water–air CO2 flux of 530.3±16.9 mmol m−2 d−1 for the lower Red River. pCO2 and CO2 outgassing rates were characterized by significant spatial variation along this system, with the highest values measured at Hoa Binh station, located downstream of the Hoa Binh Dam, on the Da River. Seasonal pCO2 and CO2 outgassing rate variations were also observed, with higher values measured during the wet season at almost all sites. The higher river discharges, enhanced external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetland were responsible for higher pCO2 and CO2 outgassing rates. The difference in pCO2 between the daytime and the night-time was not significant, suggesting weak photosynthesis processes in the water column of the Red River due to its high sediment load.

Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air–sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize available pCO2 observations to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long-term pCO2 trends, as evident through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes both seasonally and annually. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. An increase in winter data would aid in reduction of uncertainty levels. On average over the period 2002–2016, data show that carbon uptake has strengthened with annual surface ocean pCO2 trends in the Drake Passage and the broader subpolar Southern Ocean less than the global atmospheric trend. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers away from the region. We also compare DPT data from 2016 and 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017 compared to the Drake Passage Time-series, their pCO2 estimates fall within the range of underway observations given the uncertainty on the estimates. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment assessment of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

Effects of high pCO2 on the northern krill Thysanoessa inermis in relation to carbonate chemistry of its collection area, Rijpfjorden

Polar oceans are predicted to be the first marine environments affected by ocean acidification (OA). Thysanoessa inermis is one of the most abundant krill species in northern waters of the Atlantic and a key species in the food web of this ecosystem. Yet, we know very little about potential OA effects on this species. We studied the effects of elevated pCO2 on T. inermis in a laboratory experiment by exposing individuals for 11 weeks to low and high pCO2 (450 and 1200 µatm, respectively, n = 12 per pCO2 treatment). Survival, growth, and moulting frequency was monitored during the experiment, and feeding and oxygen consumption rates (n = 3–5 per pCO2 treatment) were measured at the end of the experiment. No significant effects of high pCO2 on survival, growth, moulting, oxygen consumption, and feeding rate were observed, indicating that T. inermis is tolerant to predicted high OA levels. We also explored physical and chemical properties of waters near the collection area of krill, Rijpfjorden (Svalbard 80° North) during the polar summer (July–August). In situ measurements showed large temperature and salinity gradients from surface to bottom and pCO2 and pH ranged, respectively, 161–417 µatm and 7.99–8.37. Even though substantial spatial variability in pCO2 could be observed, krill in this area is not confronted yet with the investigated high pCO2 levels.

Air-sea CO2 fluxes and spatial distribution of seawater pCO2 in Yongle Atoll, northern-central South China Sea

Yongle Atoll is a complex atoll composed of 13 separated small atolls/reefs in the Xisha Islands, northern-central South China Sea. Wide, deep channels connect the Yongle Atoll lagoon with the open ocean. The sea surface partial pressure of CO2 (pCO2) and sea–air CO2 flux was measured at different geomorphological locations on the atoll. Atmospheric pCO2 generally showed small diurnal variations, while seawater pCO2 showed large diurnal variations. Lingyang Reef lagoon, an atoll within Yongle Atoll, was used to check the spatial difference in seawater pCO2 and air–sea CO2 flux. The diurnal variation in seawater pCO2 was greater in the lagoon (258–748 μatm) than outside the lagoon area (354–476 μatm). Differences were also observed in the air–sea CO2 fluxes depending on geomorphological zone. The mean seawater pCO2 at Yongle Atoll is in the order of Lingyang Reef lagoon > inner reef slope of the Yongle Atoll > Yongle Atoll channel slope ≈ outer reef slope of the Yongle Atoll. The degree mixing between the lagoon and open sea through channels may have a key role in the horizontal distribution of seawater pCO2 in the Yongle Atoll. Additional analyses indicated that the daily variations in surface seawater pCO2 are significantly influenced by biological metabolic processes. The mean air–sea CO2 flux in the Yongle Atoll was ~ 3.2 mmol CO2 m−2 d−1, indicating that the Yongle Atoll acts as a net source of CO2 to the atmosphere.

Geographical CO2 sensitivity of phytoplankton correlates with ocean buffer capacity.

Accumulation of anthropogenic CO2 is significantly altering ocean chemistry. A range of biological impacts resulting from this oceanic CO2 accumulation are emerging, however, the mechanisms responsible for observed differential susceptibility between organisms and across environmental settings remain obscure. A primary consequence of increased oceanic CO2 uptake is a decrease in the carbonate system buffer capacity, which characterizes the system's chemical resilience to changes in CO2 , generating the potential for enhanced variability in pCO2 and the concentration of carbonate [ ], bicarbonate [ ], and protons [H+ ] in the future ocean. We conducted a meta-analysis of 17 shipboard manipulation experiments performed across three distinct geographical regions that encompassed a wide range of environmental conditions from European temperate seas to Arctic and Southern oceans. These data demonstrated a correlation between the magnitude of natural phytoplankton community biological responses to short-term CO2 changes and variability in the local buffer capacity across ocean basin scales. Specifically, short-term suppression of small phytoplankton (<10μm) net growth rates were consistently observed under enhanced pCO2 within experiments performed in regions with higher ambient buffer capacity. The results further highlight the relevance of phytoplankton cell size for the impacts of enhanced pCO2 in both the modern and future ocean. Specifically, cell size-related acclimation and adaptation to regional environmental variability, as characterized by buffer capacity, likely influences interactions between primary producers and carbonate chemistry over a range of spatio-temporal scales.

Carbon dioxide dynamics in a lake and a reservoir on a tropical island (Bali, Indonesia).

Water-to-air carbon dioxide fluxes from tropical lakes and reservoirs (artificial lakes) may be an important but understudied component of global carbon fluxes. Here, we investigate the seasonal dissolved carbon dioxide (CO2) dynamics in a lake and a reservoir on a tropical volcanic island (Bali, Indonesia). Observations were performed over four seasonal surveys in Bali’s largest natural lake (Lake Batur) and largest reservoir (Palasari Reservoir). Average CO2 partial pressures in the natural lake and reservoir were 263.7±12.2 μatm and 785.0±283.6 μatm respectively, with the highest area-weighted partial pressures in the wet season for both systems. The strong correlations between seasonal mean values of dissolved oxygen (DO) and pCO2 in the natural lake (r2 = 0.92) suggest that surface water metabolism was an important driver of CO2 dynamics in this deep system. Radon (222Rn, a natural groundwater discharge tracer) explained up to 77% of the variability in pCO2 in the shallow reservoir, suggesting that groundwater seepage was the major CO2 driver in the reservoir. Overall, the natural lake was a sink of atmospheric CO2 (average fluxes of -2.8 mmol m-2 d-1) while the reservoir was a source of CO2 to the atmosphere (average fluxes of 7.3 mmol m-2 d-1). Reservoirs are replacing river valleys and terrestrial ecosystems, particularly throughout developing tropical regions. While the net effect of this conversion on atmospheric CO2 fluxes remains to be resolved, we speculate that reservoir construction will partially offset the CO2 sink provided by deep, volcanic, natural lakes and terrestrial environments.

Assessment of Autonomous pH Measurements for Determining Surface Seawater Partial Pressure of CO2

AbstractThe Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program currently operates &gt;80 profiling floats equipped with pH sensors in the Southern Ocean. Theoretically, these floats have the potential to provide unique year‐around estimates of pCO2 derived from pH measurements. Here, we evaluate this approach in the field by comparing pCO2 estimates from pH sensors to directly measured pCO2. We first discuss data from a ship's underway system which covered a large range in temperature (2–30°C) and salinity (33.6–36.5) over 43 days. This pH sensor utilizes the same sensing technology but with different packaging than those on SOCCOM floats. The mean residual varied between −4.6 ± 4.1 and 8.6 ± 4.0 (1σ) μatm, depending on how the sensor was calibrated. However, the standard deviation of the residual, interpreted as the ability to track spatiotemporal variability, was consistently &lt; 5 μatm and was independent of the calibration method. Second, we assessed the temporal stability of this approach by comparing pCO2 estimated from four floats over 3 years to the Hawaii Ocean Time‐series. Good agreement of −2.1 ± 10.4 (1σ) µatm was observed, with coherent seasonal cycles. These results demonstrate that pCO2 estimates derived from profiling float pH measurements appear capable of reproducing spatiotemporal variations in surface pCO2 measurements and should provide a powerful observational tool to complement current efforts to understand the seasonal to interannual variability of surface pCO2 in underobserved regions of the open ocean.

Seasonal Drivers of Carbon Dioxide Dynamics in a Hydrologically Modified Subtropical Tidal River and Estuary (Caboolture River, Australia)

AbstractEstuaries are dynamic hot spots for carbon cycling and atmospheric evasion. Here we assess the dynamics and drivers of CO2 in a hydrologically modified subtropical Australian estuary. Over 1 year, 10 high‐resolution spatial surveys of pCO2, radon, chromophoric dissolved organic matter, chlorophyll a, and physicochemical parameters were conducted from the estuary mouth to a weir located 25 km upstream. The riverine respiratory quotient revealed that processes besides water column respiration were driving high CO2 within the tidal river (salinity &lt; 2) but not in the estuary (salinity &gt; 2). Conservative mixing plots for pCO2 and 222Rn implied that groundwater may be a contributing source of CO2 during most surveys, but not during dry conditions. A multiple linear regression model explained 88% of the annual pCO2 variability, indicating that mixing, metabolism, temperature, and groundwater inputs were key drivers of CO2. Inputs from an upstream wastewater outfall potentially fuel observed seasonal algal blooms, resulting in the lowest daytime CO2 emissions periods. Postbloom surveys had the highest daytime CO2 emissions. The average tidal river CO2 atmospheric flux rate was 379 ± 53 mmol m−2 day−1. The average estuarine CO2 flux was 78 ± 17 mmol m−2 day−1, equating to 28 ± 6 mol m−2 yr−1. Although the tidal river surface area was ~10 times smaller than the estuary, about one third (35%) of the CO2 emissions were derived from the tidal river. Our results suggest that CO2 emissions along the tidal river‐estuary continuum are dynamic over small temporal and spatial scales and that a combination of hydrological and biological processes is a controlling factor of this variability.