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The marine carbonate system is influenced by anthropogenic CO₂uptake, biogeochemical processes, and physical changes that involve freshwater input and removal. Two frequently used parameters to quantify seawater carbonate system are total alkalinity (TA) and total dissolved inorganic carbon (DIC). To account for the physical changes, both TA and DIC are usually normalized to a reference salinity (i.e., nTA and nDIC), and then the relationship between nTA and nDIC is used to identify major biogeochemical processes that regulate the carbonate system, based on process‐specific reaction stoichiometry. However, the theoretical basis of this interpretation has not been holistically examined. In this study, we validated this method under  idealized conditions and discussed the associated assumptions and limitations. Furthermore, we applied this method to interpret field TA and DIC data from a lagoonal estuary in the northwestern Gulf of Mexico. Our results demonstrated that evaluating field data that encompass multiple stations and time periods could be problematic. In addition, various combinations of biogeochemical processes can lead to the same nTA–nDIC relationship, even though the relative importance of each individual process may vary significantly. Therefore, the stoichiometric relationship relying solely on TA and DIC data is not a definitive approach for uncovering dominant biogeochemical processes. Instead, measurements of process‐specific parameters are necessary.

 

Estuarine total alkalinity (TA), which buffers against acidification, is temporally and spatially variable and regulated by complex, interacting hydrologic and biogeochemical processes. During periods of net evaporation (drought), the Mission-Aransas Estuary (MAE) of the northwestern Gulf of Mexico experienced TA losses beyond what can be attributed to calcification. The contribution of sedimentary oxidation of reduced sulfur to the TA loss was examined in this study. Water column samples were collected from five stations within MAE and analyzed for salinity, TA, and calcium ion concentrations. Sediment samples from four of these monitoring stations and one additional station within MAE were collected and incubated between 2018 and 2021. TA, calcium, magnesium, and sulfate ion concentrations were analyzed for these incubations. Production of sulfate along with TA consumption (or production) beyond what can be attributed to calcification (or carbonate dissolution) was observed. These results suggest that oxidation of reduced sulfur consumed TA in MAE during droughts. We estimate that the upper limit of TA consumption due to reduced sulfur oxidation can be as much as 4.60 × 10⁸ mol day⁻¹in MAE. This biogeochemical TA sink may be present in other similar subtropical, freshwater-starved estuaries around the world.

 

In the northwestern Gulf of Mexico (nwGOM), the coastal climate shifts abruptly from the humid northeast to the semiarid southwest within a narrow latitudinal range. The climate effect plays an important role in controlling freshwater discharge into the shallow estuaries in this region. In addition to diminishing freshwater runoff down the coast, evaporation also increases substantially. Hence, these estuaries show increasing salinity along the coastline due to the large difference in freshwater inflow balance (river runoff and precipitation minus evaporation and diversion). However, this spatial gradient can be disrupted by intense storm events as a copious amount of precipitation leads to river flooding, which can cause temporary freshening of these systems in extreme cases, in addition to freshwater-induced ephemeral stratification. We examined estuarine water aragonite saturation state (Ω arag ) data collected between 2014 and 2018, covering a period of contrasting hydrological conditions, from the initial drought to multiple flooding events, including a brief period that was influenced by a category 4 hurricane. Based on freshwater availability, these estuaries exhibited a diminishing Ω arag fluctuation from the most freshwater enriched Guadalupe Estuary to the most freshwater-starved Nueces Estuary. While Ω arag values were usually much higher than the threshold level (Ω arag = 1), brief freshwater discharge events and subsequent low oxygen levels in the lower water column led to episodic corrosive conditions. Based on previously obtained Ω arag temporal trends and Ω arag values obtained in this study, we estimated the time of emergence (ToE) for Ω arag . Not only did estuaries show decreasing ToE with diminishing freshwater availability but the sub-embayments of individual estuaries that had a less freshwater influence also had shorter ToE. This spatial pattern suggests that planning coastal restoration efforts, especially for shellfish organisms, should emphasize areas with longer ToE. 

A four-decade dataset that spans seven estuaries along a latitudinal gradient in the northwestern Gulf of Mexico and includes measurements of pH and total alkalinity was used to calculate partial pressure of CO 2 ( p CO 2 ), dissolved inorganic carbon (DIC), saturation state of aragonite (Ω Ar ), and a buffer factor (β DIC , which measures the response of proton concentration or pH to DIC concentration change) and examine long-term trends and spatial patterns in these parameters. With the notable exception of the northernmost and southernmost estuaries (and selected stations near freshwater input), these estuaries have generally experienced long-term increases in p CO 2 and decreases in DIC, Ω Ar , and β DIC , with the magnitude of change generally increasing from north to south. At all stations with increasing p CO 2 , the rate of increase exceeded the rate of increase in atmospheric p CO 2 , indicating that these estuaries have become a greater source of CO 2 to the atmosphere over the last few decades. The decreases in Ω Ar have yet to cause Ω Ar to near undersaturation, but even the observed decreases may have the potential to decrease calcification rates in important estuarine calcifiers like oysters. The decreases in β DIC directly indicate that these estuaries have experienced continually greater change in pH in the context of ocean acidification. 

East and West Flower Garden Bank (FGB) are part of Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwest Gulf of Mexico. This geographically-isolated reef system contains extensive coral communities with the highest coral cover (>50%) in the continental United States due, in part, to their remoteness and depth, and have historically exhibited low incidence of coral disease and bleaching despite ocean warming. Yet in late August 2022, disease-like lesions on seven coral species were reported during routine monitoring surveys on East and West FGB (2.1–2.6% prevalence). A series of rapid response cruises were conducted in September and October 2022 focused on 1) characterizing signs and epidemiological aspects of the disease across FGB and within long-term monitoring sites, 2) treating affected coral colonies with Base 2B plus amoxicillin, and 3) collecting baseline images through photostations and photomosaics. Marginal and/or multi-focal lesions and tissue loss were observed, often associated with substantial fish and invertebrate predation, affecting the dominant coral species Pseudodiploria strigosa (7–8% lesion prevalence), Colpophyllia natans (11–18%), and Orbicella spp. (1%). Characterizing this disease event during its early epidemic phase at East and West FGB provides a critical opportunity to observe how coral disease functions in a relatively healthy coral ecosystem versus on reefs chronically affected by various stressors (e.g., Caribbean reefs adjacent to urban centers). Insights into the etiology, spread, and impacts of the disease can ultimately inform efforts to mitigate its effects on coral communities. 

Water quality parameters from 2000 to 2020 were used to identify the spatial and temporal sulfate variations in estuaries of the northwestern Gulf of Mexico. Sulfate enrichment relative to conservative mixing was found to be associated with a low river discharge period from 2012 to 2014 in all estuaries. Based on reaction stoichiometry, sedimentary sulfide oxidation holds significant potential for reducing the alkalinity in estuarine waters. However, during this extreme drought, alkalinity enrichment was also occasionally observed in some of the southern estuaries along with sulfate enrichment, and when alkalinity depletion occurred, the magnitude of depletion was usually much less than what would be expected based on sulfide oxidation alone. This discrepancy can be partially explained by carbonate dissolution and other proton removal pathways (e.g., Fe‐oxide dissolution), and by the uncertainties in the model used to estimate alkalinity enrichment/depletion. Under a changing climate, the close coupling between river discharge variation and estuarine sulfate dynamics will significantly impact estuarine carbonate chemistry.

 

The Ecosystem Science and Modeling lab has been collecting water samples from five stations in the Mission-Aransas Estuary (MAE, Northwest Gulf of Mexico, Texas coast) for carbonate system characterization on a monthly to twice monthly basis since May 2014. This dataset includes temperature, salinity, dissolved inorganic carbon (DIC), total alkalinity (TA), calcium, and pH measurements from surface and bottom water samples in MAE from May 2014 – Feb 2017 and Dec 2018 – Feb 2020. Additional data for this estuary to fill in the Feb 2017 – Dec 2018 gap are also archived with BCO-DMO (http://www.bco-dmo.org/dataset/784673, doi:10.1575/1912/bco-dmo.784673.1). 

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.