Ecosystem Science & Modeling

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Alkalinity distribution in the western North Atlantic Ocean margins
(2010-08-13) Cai, Wei-Jun; Hu, Xinping; Huang, Wei-Jen; Jiang, Li-Qing; Wang, Yongchen; Peng, Tsung-Hung; Zhang, Xin
Total alkalinity (TA) distribution and its relationship with salinity (S) along the western North Atlantic Ocean (wNAO) margins from the Labrador Sea to tropical areas are examined by this study. Based on the observed TA-S patterns, the mixing process that control alkalinity distribution in these areas can be categorized into a spectrum of patterns that are bracketed by two extreme mixing types, i.e., alongshore current dominated and river-dominated. Alongshore current-dominated mixing processes exhibit a segmented mixing line with a shared mid-salinity end-member. In such cases (i.e., Labrador Sea, Gulf of Maine, etc.), the y-intercept of the high salinity segment of the mixing line is generally higher than the local river alkalinity values, and it reflects the mixing history of the alongshore current. In contrast, in river-dominated mixing (Amazon River, Caribbean Sea, etc.), good linear relationships between alkalinity and salinity are generally observed, and the zero salinity intercepts of the TA-S regressions roughly match those of the regional river alkalinity values. TA-S mixing lines can be complicated by rapid changes in the river end-member value and by another river nearby with a different TA value (e.g., Mississippi-Atchafalaya/Gulf of Mexico). In the wNAO margins, regression intercepts and river end-member vale have a clear latitudinal distribution pattern, increasing from a low of ~300 mol kg-1 in the Amazon River plume to a high value between ~500-1100 mol kg-1 in the middle and high latitude margins. The highest value of ~2400 mol kg-1 is observed in the Mississippi River influenced areas. In addition to mixing control, biological processes such a calcification and benthic alkalinity production may also affect ocean margin alkalinity distribution. Therefore, deriving inorganic carbon system information in coastal oceans using alkalinity-salinity relationships, in particular, those of generic nature, may lead to significant errors.
An assessment of ocean margin anaerobic processes on oceanic alkalinity budget
(Global Biogeochem, 2011-07-08) Hu, Xinping; Cai, Wei-Jun
Recent interest in the ocean’s capacity to absorb atmospheric CO2 and buffer the accompanying “ocean acidification” has prompted discussions on the magnitude of ocean margin alkalinity production via anaerobic processes. However, available estimates are largely based on gross reaction rates or misconceptions regarding reaction stoichiometry. In this paper, we argue that net alkalinity gain does not result from the internal cycling of nitrogen and sulfur species or from the reduction of metal oxides. Instead, only the processes that involve permanent loss of anaerobic remineralization products, i.e., nitrogen gas from net denitrification and reduced sulfur (i.e., pyrite burial) from net sulfate reduction, could contribute to this anaerobic alkalinity production. Our revised estimate of net alkalinity production from anaerobic processes is on the order of 4–5 Tmol yr−1 in global ocean margins that include both continental shelves and oxygen minimum zones, significantly smaller than the previously estimated rate of 16–31 Tmol yr−1 . In addition, pyrite burial in coastal habitats (salt marshes, mangroves, and seagrass meadows) may contribute another 0.1–1.1 Tmol yr−1 , although their long‐term effect is not yet clear under current changing climate conditions and rising sea levels. Finally, we propose that these alkalinity production reactions can be viewed as “charge transfer” processes, in which negative charges of nitrate and sulfate ions are converted to those of bicarbonate along with a net loss of these oxidative anions.
Coral Energy Reserves and Calcification in a High-CO2 World at Two Temperatures
(2013-10-11) Schoepf, Verena; Grottoli, Andréa G.; Warner, Mark E.; Cai, Wei-Jun; Melman, Todd F.; Hoadley, Kenneth D.; Pettay, D. Tye; Hu, Xinping; Li, Qian; Xu, Hui; Wang, Yongchen; Matsui, Yohei; Baumann, Justin H.
Rising atmospheric CO2 concentrations threaten coral reefs globally by causing ocean acidification (OA) and warming. Yet, the combined effects of elevated pCO2 and temperature on coral physiology and resilience remain poorly understood. While coral calcification and energy reserves are important health indicators, no studies to date have measured energy reserve pools (i.e., lipid, protein, and carbohydrate) together with calcification under OA conditions under different temperature scenarios. Four coral species, Acropora millepora, Montipora monasteriata, Pocillopora damicornis, Turbinaria reniformis, were reared under a total of six conditions for 3.5 weeks, representing three pCO2 levels (382, 607, 741 µatm), and two temperature regimes (26.5, 29.0°C) within each pCO2 level. After one month under experimental conditions, only A. millepora decreased calcification (−53%) in response to seawater pCO2 expected by the end of this century, whereas the other three species maintained calcification rates even when both pCO2 and temperature were elevated. Coral energy reserves showed mixed responses to elevated pCO2 and temperature, and were either unaffected or displayed nonlinear responses with both the lowest and highest concentrations often observed at the mid-pCO2 level of 607 µatm. Biweekly feeding may have helped corals maintain calcification rates and energy reserves under these conditions. Temperature often modulated the response of many aspects of coral physiology to OA, and both mitigated and worsened pCO2 effects. This demonstrates for the first time that coral energy reserves are generally not metabolized to sustain calcification under OA, which has important implications for coral health and bleaching resilience in a high-CO2 world. Overall, these findings suggest that some corals could be more resistant to simultaneously warming and acidifying oceans than previously expected.
Organic carbon fluxes mediated by corals at elevated pCO2 and temperature
(Marine Ecology Progress Series, 2015-01-20) Levas, Stephen; Grottoli, Andréa G.; Warner, Mark E.; Cai, Wei-Jun; Bauer, James; Schoepf, Verena; Baumann, Justin H.; Matsui, Yohei; Gearing, Colin; Melman, Todd F.; Hoadley, Kenneth D.; Pettay, Daniel T.; Hu, Xinping; Li, Qian; Xu, Hui; Wang, Yongchen
Increasing ocean acidification (OA) and seawater temperatures pose significant threats to coral reefs globally. While the combined impacts of OA and seawater temperature on coral biology and calcification in corals have received significant study, research to date has largely neglected the individual and combined effects of OA and seawater temperature on coral-mediated organic carbon (OC) fluxes. This is of particular concern as dissolved and particulate OC (DOC and POC, respectively) represent large pools of fixed OC on coral reefs. In the present study, coral-mediated POC and DOC, and the sum of these coral-mediated flux rates (total OC, TOC = DOC + POC) as well as the relative contributions of each to coral metabolic demand were determined for 2 species of coral, Acropora millepora and Turbinaria reniformis, at 2 levels of pCO2 (382 and 741 µatm) and seawater temperatures (26.5 and 31.0°C). Independent of temperature, DOC fluxes decreased significantly with increases in pCO2 in both species, resulting in more DOC being retained by the corals and only representing between 19 and 6% of TOC fluxes for A. millepora and T. reniformis. At the same time, POC and TOC fluxes were unaffected by elevated temperature and/or pCO2. These findings add to a growing body of evidence that certain species of coral may be less at risk to the impacts of OA and temperature than previously thought.
Physiological response to elevated temperature and pCO2 varies across four Pacific coral species: Understanding the unique host+symbiont response
(2015-12-16) Hoadley, Kenneth D.; Pettay, D. Tye; Grottoli, Andréa G.; Cai, Wei-Jun; Melman, Todd F.; Schoepf, Verena; Hu, Xinping; Li, Qian; Xu, Hui; Wang, Yongchen; Matsui, Yohei; Baumann, Justin H.; Warner, Mark E.
The physiological response to individual and combined stressors of elevated temperature and pCO2 were measured over a 24-day period in four Pacific corals and their respective symbionts (Acropora millepora/Symbiodinium C21a, Pocillopora damicornis/Symbiodinium C1c-d-t, Montipora monasteriata/Symbiodinium C15 and Turbinaria reniformis/Symbiodinium trenchii). Multivariate analyses indicated that elevated temperature played a greater role in altering physiological response, with the greatest degree of change occurring within M. monasteriata and T. reniformis. Algal cellular volume, protein and lipid content all increased for M. monasteriata. Likewise, S. trenchii volume and protein content in T. reniformis also increased with temperature. Despite decreases in maximal photochemical efficiency, few changes in biochemical composition (i.e. lipids, proteins and carbohydrates) or cellular volume occurred at high temperature in the two thermally sensitive symbionts C21a and C1c-d-t. Intracellular carbonic anhydrase transcript abundance increased with temperature in A. millepora but not in P. damicornis, possibly reflecting differences in host mitigated carbon supply during thermal stress. Importantly, our results show that the host and symbiont response to climate change differs considerably across species and that greater physiological plasticity in response to elevated temperature may be an important strategy distinguishing thermally tolerant vs. thermally sensitive species.
Microelectrode characterization of coral daytime interior pH and carbonate chemistry
(Nature Communications, 2016-04-04) Cai, Wei-Jun; Ma, Yuening; Hopkinson, Brian M.; Grottoli, Andrea G.; Warner, Mark E.; Ding, Qian; Hu, Xinping; Yuan, Xiangchen; Schoepf, Verena; Xu, Hui; Han, Chenhua; Melman, Todd F.; Hoadley, Kenneth D.; Pettay, D. Tye; Matsui, Yohei; Baumann, Justin H.; Levas, Stephen; Ying, Ye; Wang, Yongchen
Reliably predicting how coral calcification may respond to ocean acidification and warming depends on our understanding of coral calcification mechanisms. However, the concentration and speciation of dissolved inorganic carbon (DIC) inside corals remain unclear, as only pH has been measured while a necessary second parameter to constrain carbonate chemistry has been missing. Here we report the first carbonate ion concentration ([CO3 2 ]) measurements together with pH inside corals during the light period. We observe sharp increases in [CO3 2 ] and pH from the gastric cavity to the calcifying fluid, confirming the existence of a proton (H þ ) pumping mechanism. We also show that corals can achieve a high aragonite saturation state (Oarag) in the calcifying fluid by elevating pH while at the same time keeping [DIC] low. Such a mechanism may require less H þ -pumping and energy for upregulating pH compared with the high [DIC] scenario and thus may allow corals to be more resistant to climate change related stressors.
Microelectrode characterization of coral daytime interior pH and carbonate chemistry
(2016-04-04) Cai, Wei-Jun; Ma, Yuening; Hopkinson, Brian M.; Grottoli, Andréa G.; Warner, Mark E.; Ding, Qian; Hu, Xinping; Yuan, Xiangchen; Schoepf, Verena; Xu, Hui; Han, Chenhua; Melman, Todd F.; Hoadley, Kenneth D.; Pettay, D. Tye; Matsui, Yohei; Baumann, Justin H.; Levas, Stephen; Ying, Ye; Wang, Yongchen
Reliably predicting how coral calcification may respond to ocean acidification and warming depends on our understanding of coral calcification mechanisms. However, the concentration and speciation of dissolved inorganic carbon (DIC) inside corals remain unclear, as only pH has been measured while a necessary second parameter to constrain carbonate chemistry has been missing. Here we report the first carbonate ion concentration ([CO32−]) measurements together with pH inside corals during the light period. We observe sharp increases in [CO32−] and pH from the gastric cavity to the calcifying fluid, confirming the existence of a proton (H+) pumping mechanism. We also show that corals can achieve a high aragonite saturation state (Ωarag) in the calcifying fluid by elevating pH while at the same time keeping [DIC] low. Such a mechanism may require less H+-pumping and energy for upregulating pH compared with the high [DIC] scenario and thus may allow corals to be more resistant to climate change related stressors.
Responses of carbonate system and CO2 flux to extended drought and intense flooding in a semiarid subtropical estuary
(Association for the Sciences of Limnology and Oceanography, 2017-08-14) Yao, Hongming; Hu, Xinping
Globally, 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.
Continental shelves as a variable but increasing global sink for atmospheric carbon dioxide
(2018-01-31) Laruelle, Goulven G.; Cai, Wei-Jun; Hu, Xinping; Gruber, Nicolas; Mackenzie, Fred T.; Regnier, Pierre
It has been speculated that the partial pressure of carbon dioxide (pCO2) in shelf waters may lag the rise in atmospheric CO2. Here, we show that this is the case across many shelf regions, implying a tendency for enhanced shelf uptake of atmospheric CO2. This result is based on analysis of long-term trends in the air–sea pCO2 gradient (ΔpCO2) using a global surface ocean pCO2 database spanning a period of up to 35 years. Using wintertime data only, we find that ΔpCO2 increased in 653 of the 825 0.5° cells for which a trend could be calculated, with 325 of these cells showing a significant increase in excess of +0.5 μatm yr−1 (p < 0.05). Although noisier, the deseasonalized annual data suggest similar results. If this were a global trend, it would support the idea that shelves might have switched from a source to a sink of CO2 during the last century.
Effect of hydrological variability on the biogeochemistry of estuaries across a regional climatic gradient
(Association for the Sciences of Limnology and Oceanography, 2018-08-15) Montagna, Paul A.; Hu, Xinping; Palmer, Terence A.; Wetz, Michael S.
Given projected changes in river flow to coastal regions worldwide due to climate change and increasing human freshwater demands, it is necessary to determine the role hydrology plays in regulating the biogeochemistry of estuaries. A climatic gradient exists along the Texas coast where freshwater inflow balance ranges from hydrologically positive to negative (where evaporation exceeds inflow) within a narrow latitudinal band, providing a natural experiment for examining inflow effects. Four Texas estuaries ranging from mesosaline to hypersaline were studied for 3 yr to determine how hydrological changes alter the biogeochemistry within and among the estuaries. Trends in dissolved inorganic nutrients, chlorophyll, dissolved organic matter, and carbonate chemistry indicated that these estuaries had drastically different biogeochemical signatures. Nutrients and chlorophyll patterns illustrated an emerging paradigm where phytoplankton biomass in positive estuaries is supported by “new” nitrogen from riverine input, while high concentrations of reduced nitrogen (organic, ammonium) allowed for high chlorophyll in the negative estuary. For carbonate chemistry, a positive estuary receiving river input from a limestone-dominated watershed was well-buffered under moderate to high freshwater inflow conditions. When weathering products were diluted during high-flow conditions, there is carbonate undersaturation (for aragonite) and decreases in pH. However, “acidification” was not observed in the negative estuary because evaporation concentrated the dissolved species and increased buffering capacity. Hydrological changes over spatial gradients are analogous to climatic changes over time, meaning climate change forecasts of higher temperatures and decreased precipitation can make the biogeochemistry of fresher estuaries change to the patterns of saltier estuaries.
Time of Emergence of Surface Ocean Carbon Dioxide Trends in the North American Coastal Margins in Support of Ocean Acidification Observing System Design
(2019-03-08) Turk, Daniela; Wang, Hongjie; Hu, Xinping; Gledhill, Dwight K.; Wang, Zhaohui Aleck; Jiang, Liqing; Cai, Wei-Jun
Time of Emergence (ToE) is the time when a signal emerges from the noise of natural variability. Commonly used in climate science for the detection of anthropogenic forcing, this concept has recently been applied to geochemical variables, to assess the emerging times of anthropogenic ocean acidification (OA), mostly in the open ocean using global climate and Earth System Models. Yet studies of OA variables are scarce within costal margins, due to limited multidecadal time-series observations of carbon parameters. ToE provides important information for decision making regarding the strategic configuration of observing assets, to ensure they are optimally positioned either for signal detection and/or process elicitation and to identify the most suitable variables in discerning OA-related changes. Herein, we present a short overview of ToE estimates on an OA variable, CO2 fugacity f(CO2,sw), in the North American ocean margins, using coastal data from the Surface Ocean CO2 Atlas (SOCAT) V5. ToE suggests an average theoretical timeframe for an OA signal to emerge, of 23(±13) years, but with considerable spatial variability. Most coastal areas are experiencing additional secular and/or multi-decadal forcing(s) that modifies the OA signal, and such forcing may not be sufficiently resolved by current observations. We provide recommendations, which will help scientists and decision makers design and implement OA monitoring systems in the next decade, to address the objectives of OceanObs19 (http://www.oceanobs19.net) in support of the United Nations Decade of Ocean Science for Sustainable Development (2021–2030) (https://en.unesco.org/ocean-decade) and the Sustainable Development Goal (SDG) 14.3 (https://sustainabledevelopment.un.org/sdg14) target to “Minimize and address the impacts of OA.”
Carbon cycling in the North American coastal ocean: a synthesis
(2019-03-27) Fennel, Katja; Alin, Simone; Barbero, Leticia; Evans, Wiley; Bourgeois, Timothée; Cooley, Sarah; Dunne, John; Feely, Richard A.; Hernandez-Ayon, Jose Martin; Hu, Xinping; Lohrenz, Steven; Muller-Karger, Frank; Najjar, Raymond; Robbins, Lisa; Shadwick, Elizabeth; Siedlecki, Samantha; Steiner, Nadja; Sutton, Adrienne; Turk, Daniela; Vlahos, Penny; Wang, Zhaohui Aleck
A quantification of carbon fluxes in the coastal ocean and across its boundaries with the atmosphere, land, and the open ocean is important for assessing the current state and projecting future trends in ocean carbon uptake and coastal ocean acidification, but this is currently a missing component of global carbon budgeting. This synthesis reviews recent progress in characterizing these carbon fluxes for the North American coastal ocean. Several observing networks and high-resolution regional models are now available. Recent efforts have focused primarily on quantifying the net air–sea exchange of carbon dioxide (CO2). Some studies have estimated other key fluxes, such as the exchange of organic and inorganic carbon between shelves and the open ocean. Available estimates of air–sea CO2 flux, informed by more than a decade of observations, indicate that the North American Exclusive Economic Zone (EEZ) acts as a sink of 160±80 Tg C yr−1, although this flux is not well constrained. The Arctic and sub-Arctic, mid-latitude Atlantic, and mid-latitude Pacific portions of the EEZ account for 104, 62, and −3.7 Tg C yr−1, respectively, while making up 51 %, 25 %, and 24 % of the total area, respectively. Combining the net uptake of 160±80 Tg C yr−1 with an estimated carbon input from land of 106±30 Tg C yr−1 minus an estimated burial of 65±55 Tg C yr−1 and an estimated accumulation of dissolved carbon in EEZ waters of 50±25 Tg C yr−1 implies a carbon export of 151±105 Tg C yr−1 to the open ocean. The increasing concentration of inorganic carbon in coastal and open-ocean waters leads to ocean acidification. As a result, conditions favoring the dissolution of calcium carbonate occur regularly in subsurface coastal waters in the Arctic, which are naturally prone to low pH, and the North Pacific, where upwelling of deep, carbon-rich waters has intensified. Expanded monitoring and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.
Photomineralization of organic carbon in a eutrophic, semiarid estuary
(2020-01-28) Wang, Hongjie; Hu, Xinping; Wetz, Michael S.; Hayes, Kenneth C.; Lu, Kaijun
The effect of photomineralization on the carbon cycle in a eutrophic, semiarid estuary (Baffin Bay, Texas) was investigated using closed‐system incubations. Photochemical production rate of dissolved inorganic carbon ranged from 0.16 to 0.68 μM hr−1, with a daily removal of 0.3∼1.5% of the standing stock of dissolved organic carbon (DOC). The photomineralization rate was negatively correlated with chlorophyll a concentration, suggesting that plankton‐derived DOC was less photoreactive to solar radiation. The stable carbon isotope composition (δ13C∼ −18.6‰) of degraded DOC, as calculated using the DIC “Keeling” plot, further indicated high photochemical lability of 13C‐enriched DOC in this semiarid environment. Our finding showed that photomineralization of 13C‐enriched DOC is an important component of carbon cycle in this system, and this process does not necessarily remove 13C‐depleted organic carbon as observed in other coastal systems.
Disparate responses of carbonate system in two adjacent subtropical estuaries to the influence of Hurricane Harvey - A case study
(Frontiers in Marine Science, 2020-01-31) Hu, Xinping; Yao, Hongming; Staryk, Cory J.; McCutcheon, Melissa R.; Wetz, Michael S.; Walker, Lily
Two adjacent estuaries in the northwestern Gulf of Mexico (GOM) (Mission–Aransas or MAE and Guadalupe–San Antonio or GE), despite their close proximity and similar extents of freshening caused by Hurricane Harvey, exhibited different behaviors in their post-hurricane carbonate chemistry and CO2 fluxes. The oligotrophic MAE had little change in post-Harvey CO2 partial pressure (pCO2) and CO2 flux even though the center of Harvey passed right through, while GE showed a large post-Harvey increases in both pCO2 and CO2 flux, which were accompanied by a brief period of low dissolved oxygen (DO) conditions likely due to the large input of organic matter mobilized by the hurricane. The differences in the carbonate chemistry and CO2 fluxes were attributed to the differences in the watersheds from which these estuaries receive freshwater. The GE watershed is larger and covers urbanized areas, and, as a result, GE is considered relatively eutrophic. On the other hand, the MAE watershed is smaller, much less populous, and MAE is oligotrophic when river discharge is low. Despite that Harvey passed through MAE, the induced changes in carbonate chemistry and CO2 flux there were less conspicuous than those in GE. This study suggested that disturbances by strong storms to estuarine carbon cycle may not be uniform even on such a small spatial scale. Therefore, disparate responses to these disturbances need to be studied on a case-by-case basis.
Effect of Organic Alkalinity on Seawater Buffer Capacity: A Numerical Exploration
(Springer Nature, 2020-04-20) Hu, Xinping
Organic alkalinity is a poorly understood component of total titration alkalinity in aquatic environments. Using a numerical method, the effects of organic acid (HOA) and its conjugate base (OA−) on seawater carbonate chemistry and buffer behaviors, as well as those in a hypothetical estuarine mixing zone, are explored under both closed- and open-system conditions. The simulation results show that HOA addition leads to pCO2 increase and pH decrease in a closed system when total dissolved inorganic carbon (DIC) remains the same. If opened to the atmosphere (pCO2=400 µatm), CO2 degassing and re-equilibration would cause depressed pH compared to the unperturbed seawater, but the seawater buffer to pH change [equation] indicates that weaker organic acid (i.e., higher pKa) results in higher bufer capacity (greater βDIC) than the unperturbed seawater. In comparison, OA− (with accompanying cations) in the form of net alkalinity addition leads to pCO2 decrease in a closed system. After re-equilibrating with the atmosphere, the resulting perturbed seawater has higher pH and βDIC than the unperturbed seawater. If river water has organic alkalinity, pH in the estuarine mixing zone is always lower than those caused by a mixing between organic alkalinity-free river (at constant total alkalinity) and ocean waters, regardless of the pKa values. On the other hand, organic alkalinity with higher pKa provides slightly greater βDIC in the mixing zone, and that with lower pKa either results in large CO2 oversaturation (closed system) or reduced βDIC (in mid to high salinity in the closed system or the entire mixing zone in the open system). Finally, despite the various effects on seawater buffer through either HOA or OA− addition, destruction of organic molecules including organic alkalinity via biogeochemical reactions should result in a net CO2 loss from seawater. Nevertheless, the significance of this organic alkalinity, especially that comes from organic acids that are not accounted for under the currently recognized “zero proton level” (Dickson in Deep Sea Res 28:609–623, 1981), remains unknown thus a potentially interesting and relevant research topic in studying oceanic alkalinity cycle.
Timescales and Magnitude of Water Quality Change in Three Texas Estuaries Induced by Passage of Hurricane Harvey
(Estuaries and Coasts, 2020-10-14) Walker, Lily M.; Montagna, Paul A.; Hu, Xinping; Wetz, Michael S.
Tropical cyclones represent a substantial disturbance to water quality in coastal ecosystems via storm surge, winds, and flooding. However, evidence to date suggests that the impacts of tropical cyclones on water quality are generally short-lived (days-months) and that the magnitude of the disturbance is related to proximity to storm track. Discrete and continuous water samples were collected in three Texas estuaries before and after Hurricane Harvey made landfall in 2017. Of the three estuaries, the Guadalupe Estuary and its watershed received the highest rainfall totals and wind speeds. An ephemeral increase in salinity was observed (mean of 9.8 on 24 August 2017 to a peak of 32.1 on 26 August 2017) due to storm surge and was followed by a rapid decrease to < 1 as floodwaters reached the estuary. Salinity returned to pre-storm levels within 1 month. During the low salinity period, bottom water hypoxia developed and lasted for 9 days. In all three estuaries, there was an increase in inorganic nutrients post-Harvey, but the nutrients largely returned to pre-storm baseline levels by winter. The lack of long-term water quality impacts from Harvey despite its severity corroborates previous findings that estuarine water quality tends to return to baseline conditions within days to a few months after storm passage.
Corrigendum: Disparate responses of carbonate system in two adjacent subtropical estuaries CV-5 to the influence of Hurricane Harvey - A case study
(Frontiers in Marine Science, 2021-01-25) Hu, Xinping; Yao, Hongming; Staryk, Cory J.; McCutcheon, Melissa R.; Wetz, Michael S.; Walker, Lily
Seasonal Mixing and Biological Controls of the Carbonate System in a River-Dominated Continental Shelf Subject to Eutrophication and Hypoxia in the Northern Gulf of Mexico
(Frontiers in Marine Science, 2021-03-26) Huang, Wei-Jen; Cai, Wei-Jun; Hu, Xinping
Large rivers export a large amount of dissolved inorganic carbon (DIC) and nutrients to continental shelves; and subsequent river-to-sea mixing, eutrophication, and seasonal hypoxia (dissolved oxygen < 2 mg⋅L–1) can further modify DIC and nutrient distributions and fluxes. However, quantitative studies of seasonal carbonate variations on shelves are still insufficient. We collected total alkalinity (TA), DIC, and NO3– data from nine cruises conducted between 2006 and 2010 on the northern Gulf of Mexico continental shelf, an area strongly influenced by the Mississippi and Atchafalaya Rivers. We applied a three-end-member model (based on salinity and potential alkalinity) to our data to remove the contribution of physical mixing to DIC and nitrate distribution patterns and to derive the net in situ removal of DIC and nitrate (ΔDIC and ΔNO3–, respectively). Systematic analyses demonstrated that the seasonal net DIC removal in the near-surface water was strong during summer and weak in winter. The peak in net DIC production in the near-bottom, subsurface waters of the inner and middle sections of the shelf occurred between July and September; it was coupled, but with a time lag, to the peak in the net DIC removal that occurred in the near-surface waters in June. A similar 2-month delay (i.e., January vs. November) could also be observed between their minima. A detailed examination of the relationship between ΔDIC and ΔNO3– demonstrates that net biological activity was the dominant factor of DIC removal and addition. Other effects, such as air–sea CO2 gas exchange, wetland exports, CaCO3 precipitation, and a regional variation of the Redfield ratio, were relatively minor. We suggest that the delayed coupling between eutrophic surface and hypoxic bottom waters reported here may also be seen in the carbon and nutrient cycles of other nutrient-rich, river-dominated ocean margins worldwide.
Temporal variability and driving factors of the carbonate system in the Aransas Ship Channel, TX, USA: a time series study
(Biogeosciences, 2021-08-09) McCutcheon, Melissa R.; Yao, Hongming; Staryk, Cory J.; Hu, Xinping
The coastal ocean is affected by an array of co-occurring biogeochemical and anthropogenic processes, resulting in substantial heterogeneity in water chemistry, including carbonate chemistry parameters such as pH and partial pressure of CO2 (pCO2). To better understand coastal and estuarine acidification and air-sea CO2 fluxes, it is important to study baseline variability and driving factors of carbonate chemistry. Using both discrete bottle sample collection (2014–2020) and hourly sensor measurements (2016–2017), we explored temporal variability, from diel to interannual scales, in the carbonate system (specifically pH and pCO2) at the Aransas Ship Channel located in the northwestern Gulf of Mexico. Using other co-located environmental sensors, we also explored the driving factors of that variability. Both sampling methods demonstrated significant seasonal variability at the location, with highest pH (lowest pCO2) in the winter and lowest pH (highest pCO2) in the summer. Significant diel variability was also evident from sensor data, but the time of day with elevated pCO2 and depressed pH was not consistent across the entire monitoring period, sometimes reversing from what would be expected from a biological signal. Though seasonal and diel fluctuations were smaller than many other areas previously studied, carbonate chemistry parameters were among the most important environmental parameters for distinguishing between time of day and between seasons. It is evident that temperature, biological activity, freshwater inflow, and tide level (despite the small tidal range) are all important controls on the system, with different controls dominating at different timescales. The results suggest that the controlling factors of the carbonate system may not be exerted equally on both pH and pCO2 on diel timescales, causing separation of their diel or tidal relationships during certain seasons. Despite known temporal variability on shorter timescales, discrete sampling was generally representative of the average carbonate system and average air-sea CO2 flux on a seasonal and annual basis when compared with sensor data.
Temporal variability and driving factors of the carbonate system in the Aransas Ship Channel, TX, USA: a time series study
(Copernicus Publications, 2021-08-09) McCutcheon, Melissa R.; Yao, Hongming; Staryk, Cory J.; Hu, Xinping
The coastal ocean is affected by an array of co-occurring biogeochemical and anthropogenic processes, resulting in substantial heterogeneity in water chemistry, including carbonate chemistry parameters such as pH and partial pressure of CO2 (pCO2). To better understand coastal and estuarine acidification and air-sea CO2 fluxes, it is important to study baseline variability and driving factors of carbonate chemistry. Using both discrete bottle sample collection (2014–2020) and hourly sensor measurements (2016–2017), we explored temporal variability, from diel to interannual scales, in the carbonate system (specifically pH and pCO2) at the Aransas Ship Channel located in the northwestern Gulf of Mexico. Using other co-located environmental sensors, we also explored the driving factors of that variability. Both sampling methods demonstrated significant seasonal variability at the location, with highest pH (lowest pCO2) in the winter and lowest pH (highest pCO2) in the summer. Significant diel variability was also evident from sensor data, but the time of day with elevated pCO2 and depressed pH was not consistent across the entire monitoring period, sometimes reversing from what would be expected from a biological signal. Though seasonal and diel fluctuations were smaller than many other areas previously studied, carbonate chemistry parameters were among the most important environmental parameters for distinguishing between time of day and between seasons. It is evident that temperature, biological activity, freshwater inflow, and tide level (despite the small tidal range) are all important controls on the system, with different controls dominating at different timescales. The results suggest that the controlling factors of the carbonate system may not be exerted equally on both pH and pCO2 on diel timescales, causing separation of their diel or tidal relationships during certain seasons. Despite known temporal variability on shorter timescales, discrete sampling was generally representative of the average carbonate system and average air-sea CO2 flux on a seasonal and annual basis when compared with sensor data.

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