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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. 

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.