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1. Alkalinity cycling and carbonate chemistry decoupling in seagrass mystify processes of acidification mitigation

   https://doi.org/10.1038/s41598-021-92771-2  

   Miller, Cale A.; Kelley, Amanda L. (December 2021, Scientific Reports)

   Abstract The adverse conditions of acidification on sensitive marine organisms have led to the investigation of bioremediation methods as a way to abate local acidification. This phytoremediation, by macrophytes, is expected to reduce the severity of acidification in nearshore habitats on short timescales. Characterizing the efficacy of phytoremediation can be challenging as residence time, tidal mixing, freshwater input, and a limited capacity to fully constrain the carbonate system can lead to erroneous conclusions. Here, we present in situ observations of carbonate chemistry relationships to seagrass habitats by comparing dense (DG), patchy (PG), and no grass (NG) Zostera marina pools in the high intertidal experiencing intermittent flooding. High-frequency measurements of pH, alkalinity (TA), and total-CO 2 elucidate extreme diel cyclicity in all parameters. The DG pool displayed frequent decoupling between pH and aragonite saturation state (Ω arg ) suggesting pH-based inferences of acidification remediation by seagrass can be misinterpreted as pH and Ω arg can be independent stressors for some bivalves. Estimates show the DG pool had an integrated ΔTA of 550 μmol kg −1 over a 12 h period, which is ~ 60% > the PG and NG pools. We conclude habitats with mixed photosynthesizers (i.e., PG pool) result in less decoupling between pH and Ω arg . 

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2. Temporal variability and driving factors of the carbonate system in the Aransas Ship Channel, TX, USA: a time series study

   https://doi.org/10.5194/bg-18-4571-2021  

   McCutcheon, Melissa R.; Yao, Hongming; Staryk, Cory J.; Hu, Xinping (January 2021, Biogeosciences)

   Abstract. The coastal ocean is affected by an array of co-occurring biogeochemical andanthropogenic processes, resulting in substantial heterogeneity in waterchemistry, including carbonate chemistry parameters such as pH and partialpressure of CO2 (pCO2). To better understand coastal and estuarineacidification and air-sea CO2 fluxes, it is important to study baselinevariability and driving factors of carbonate chemistry. Using both discretebottle sample collection (2014–2020) and hourly sensor measurements(2016–2017), we explored temporal variability, from diel to interannualscales, in the carbonate system (specifically pH and pCO2) at theAransas Ship Channel located in the northwestern Gulf of Mexico. Using otherco-located environmental sensors, we also explored the driving factors ofthat variability. Both sampling methods demonstrated significant seasonalvariability at the location, with highest pH (lowest pCO2) in the winterand lowest pH (highest pCO2) in the summer. Significant diel variabilitywas also evident from sensor data, but the time of day with elevatedpCO2 and depressed pH was not consistent across the entire monitoringperiod, sometimes reversing from what would be expected from a biologicalsignal. Though seasonal and diel fluctuations were smaller than many otherareas previously studied, carbonate chemistry parameters were among the mostimportant environmental parameters for distinguishing between time of day andbetween seasons. It is evident that temperature, biological activity,freshwater inflow, and tide level (despite the small tidal range) are allimportant controls on the system, with different controls dominating atdifferent timescales. The results suggest that the controlling factors ofthe carbonate system may not be exerted equally on both pH and pCO2 ondiel timescales, causing separation of their diel or tidal relationshipsduring certain seasons. Despite known temporal variability on shortertimescales, discrete sampling was generally representative of the averagecarbonate system and average air-sea CO2 flux on a seasonal and annualbasis when compared with sensor data. 

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3. A decade of marine inorganic carbon chemistry observations in the northern Gulf of Alaska – insights into an environment in transition

   https://doi.org/10.5194/essd-16-647-2024  

   Monacci, Natalie M; Cross, Jessica N; Evans, Wiley; Mathis, Jeremy T; Wang, Hongjie (January 2024, Earth System Science Data)

   Abstract. As elsewhere in the global ocean, the Gulf of Alaska is experiencing the rapid onset of ocean acidification (OA) driven by oceanic absorption of anthropogenic emissions of carbon dioxide from the atmosphere. In support of OA research and monitoring, we present here a data product of marine inorganic carbon chemistry parameters measured from seawater samples taken during biannual cruises between 2008 and 2017 in the northern Gulf of Alaska. Samples were collected each May and September over the 10 year period using a conductivity, temperature, depth (CTD) profiler coupled with a Niskin bottle rosette at stations including a long-term hydrographic survey transect known as the Gulf of Alaska (GAK) Line. This dataset includes discrete seawater measurements such as dissolved inorganic carbon and total alkalinity, which allows the calculation of other marine carbon parameters, including carbonate mineral saturation states, carbon dioxide (CO2), and pH. Cumulative daily Bakun upwelling indices illustrate the pattern of downwelling in the northern Gulf of Alaska, with a period of relaxation spanning between the May and September cruises. The observed time and space variability impart challenges for disentangling the OA signal despite this dataset spanning a decade. However, this data product greatly enhances our understanding of seasonal and interannual variability in the marine inorganic carbon system parameters. The product can also aid in the ground truthing of biogeochemical models, refining estimates of sea–air CO2 exchange, and determining appropriate CO2 parameter ranges for experiments targeting potentially vulnerable species. Data are available at https://doi.org/10.25921/x9sg-9b08 (Monacci et al., 2023). 

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4. The seasonal phases of an Arctic lagoon reveal the discontinuities of pH variability and CO₂ flux at the air–sea interface

   https://doi.org/10.5194/bg-18-1203-2021  

   Miller, Cale A.; Bonsell, Christina; McTigue, Nathan D.; Kelley, Amanda L. (January 2021, Biogeosciences)

   null (Ed.)

   Abstract. The western Arctic Ocean, including its shelves and coastal habitats, has become a focus in ocean acidification research over the past decade as thecolder waters of the region and the reduction of sea ice appear to promote the uptake of excess atmospheric CO2. Due to seasonal sea icecoverage, high-frequency monitoring of pH or other carbonate chemistry parameters is typically limited to infrequent ship-based transects duringice-free summers. This approach has failed to capture year-round nearshore carbonate chemistry dynamics which is modulated by biological metabolismin response to abundant allochthonous organic matter to the narrow shelf of the Beaufort Sea and adjacent regions. The coastline of the Beaufort Seacomprises a series of lagoons that account for > 50 % of the land–sea interface. The lagoon ecosystems are novel features that cycle between“open” and “closed” phases (i.e., ice-free and ice-covered, respectively). In this study, we collected high-frequency pH, salinity,temperature, and photosynthetically active radiation (PAR) measurements in association with the Beaufort Lagoon Ecosystems – Long Term Ecological Research program – for an entire calendar yearin Kaktovik Lagoon, Alaska, USA, capturing two open-water phases and one closed phase. Hourly pH variability during the open-water phases are someof the fastest rates reported, exceeding 0.4 units. Baseline pH varied substantially between the open phase in 2018 and open phase in 2019 from ∼ 7.85to 8.05, respectively, despite similar hourly rates of change. Salinity–pH relationships were mixed during all three phases, displaying nocorrelation in the 2018 open phase, a negative correlation in the 2018/19 closed phase, and a positive correlation during the 2019 open phase. The high frequency of pH variabilitycould partially be explained by photosynthesis–respiration cycles as correlation coefficients between daily average pH and PAR were 0.46 and 0.64for 2018 and 2019 open phases, respectively. The estimated annual daily average CO2 efflux (from sea to atmosphere) was5.9 ± 19.3 mmolm-2d-1, which is converse to the negative influx of CO2 estimated for the coastal Beaufort Seadespite exhibiting extreme variability. Considering the geomorphic differences such as depth and enclosure in Beaufort Sea lagoons, furtherinvestigation is needed to assess whether there are periods of the open phase in which lagoons are sources of carbon to the atmosphere, potentiallyoffsetting the predicted sink capacity of the greater Beaufort Sea. 

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5. Re-evaluation of carbonic acid dissociation constants across conditions and the implications for ocean acidification

   https://doi.org/10.1016/j.marchem.2023.104247  

   Woosley, Ryan J.; Moon, Ji-Young (March 2023, Marine Chemistry)

   With the increasing threat of ocean acidification and the important role of the oceans in the global carbon cycle, highly precise, accurate, and intercomparable determination of inorganic carbon system parameters is required. Thermodynamic relationships enable the system to be fully constrained using a combination of direct measurements and calculations. However, calculations are complicated by many formulations for dissociation constants (over 120 possible combinations). To address these important issues of uncertainty and comparability, we evaluated the various combinations of constants and their (dis)agreement with direct measurements over a range of temperature (−1.9–40 ◦C), practical salinity (15–39) and pCO2 (150–1200 μatm). The results demonstrate that differences between the calculations and measurements are significantly larger than measurement uncertainties, meaning the oft-stated paradigm that one only needs to measure two parameters and the others can be calculated does not apply for climate quality ocean acidification research. The uncertainties in calculated pHt prevent climate quality pHt from being calculated from total alkalinity (TA) and dissolved inorganic carbon (DIC) and should be directly measured instead. However, climate quality TA and DIC can often be calculated using measured pH and DIC or TA respectively. Calculations are notably biased at medium-to-high pCO2 values (~500–800 μatm) implying models underestimate future ocean acidification. Uncertainty in the dissociation constants leads to significant uncertainty in the depth of the aragonite saturation horizon (>500 m in the Southern Ocean) and must be considered when studying calcium carbonate cycling. Significant improvements in the precision of the thermodynamic constants are required to improve pHt calculations. 

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