Humans have had a significant influence on estuaries through land use change and increased use of fertilizers, causing proliferation of algal blooms, hypoxia, and presence of harmful microbes. Now, acidification due to myriad processes has been identified as a potential threat to many estuaries. In Texas estuaries for example, short-term acidification as a result of episodic hypoxia is a well-documented phenomenon. Unfortunately, a longer-term trend toward chronic acidification (decreasing alkalinity, pH) has now been observed. The alkalinity decrease is likely caused by a reduction in riverine alkalinity export due to precipitation declines under drought conditions and freshwater diversions for human consumption.

Based on our existing long-term data, we hypothesize that hydrology acts as a switch, where increased river flows cause hypoxia and short-term acidification due to increased loads of organic matter, whereas prolonged low flows cause long-term acidification due to reduced loads of riverine alkalinity and calcification. In urbanized, wastewater-influenced systems, we hypothesize that reduced flows out of the watershed may lead to long-term acidification and chronic hypoxia due to reduced loads of riverine alkalinity and presence of low pH, high nutrient/organic matter wastewater.

To test our hypotheses, field and modeling studies are proposed to examine the relationships between estuarine acidification and other stressors (i.e., reduced freshwater inflow, hypoxia, and nutrient loading). Analysis of changes in ecosystem health and model calibration will be conducted based on long-term data. Mechanistic linkages between acidification, eutrophication and flow will be quantified through a field campaign. Chemical markers of organic matter sources fueling hypoxia will be determined. Future ecological states of the estuaries will be predicted using ecosystem models that account for projected changes in aforementioned parameters and ocean conditions based on IPCC estimates. The combination of prediction and consequence will be useful to multiple stakeholder groups.

This project will provide time-series observations of coastal ocean pH and carbon system properties, along with other variables that affect carbon transformations, in the northern Gulf of Mexico in support of goals elucidated in the NOAA Ocean and Great Lakes Acidification Research Implementation Plan. This project most directly addresses Theme 1: Develop the monitoring capacity to quantify and track ocean acidification in open-ocean, coastal, and Great Lake systems, but also addresses the educational objectives of Theme 6. USM will maintain a 3- m discus buoy in the northern Gulf of Mexico with a PMEL MAPCO2 system that includes a CTD, dissolved oxygen, and pH sensors. Meteorological sensors on the buoy will be utilized for computing air-sea fluxes of CO2. Water samples and continuous vertical profiles will be taken at the buoy site during quarterly cruises. Water samples will be analyzed for DIC, TA, pH, dO, S, NUTS and chlorophyll a. Analyzed water samples and profile data will be submitted to NODC through standard NOAA OAP submission spreadsheets containing both data and associated metadata.

While this work is focused on the Gulf of Mexico additional time-series sites in the South Atlantic Bight and Gulf of Maine can provide a comparison over a wide range of coastal and latitudinal regimes. The northern Gulf of Mexico, Florida and South Atlantic Bight regions are all commonly influenced by one contiguous western boundary current system, which originates with the Loop Current in the Gulf of Mexico and then becomes the Gulf Stream along the southeastern U.S. continental shelf. The Gulf of Mexico observations will be compared with the other western boundary current influenced site in the South Atlantic Bight maintained by the University of Georgia (UGA) and the high latitude site in the Gulf of Maine maintained by the University of New Hampshire (UNH). 

Analysis of the data collected during the first (2007) and the second (2012) Gulf of Mexico and East Coast Carbon (GOMECC) cruises showed measurable temporal pH and aragonite saturation state (ΩAr) changes along the eight major transects. However, it is challenging to determine how much of this temporal change between the two cruises is due to ocean acidification and how much is due to variability on seasonal to interannual scales. Indeed, the expected 2% average decrease in ΩAr due to increasing atmospheric CO2 levels over the 5-year period was largely overshadowed by local and regional variability from changes in ocean circulation, remineralization/respiration and riverine inputs (Wanninkhof et al., 2015). Therefore, in order to provide useful products for the ocean acidification (OA) research community and resource managers, it is important to filter out seasonal cycles and other variability from the multi-annual trend. Here, we propose to use a high-resolution regional ocean-biogeochemistry model simulation for the period of 1979 - present day (real-time run) to fill the temporal gap between the 1st and 2nd GOMECC cruise data. In addition we will fine-tune and validate the model by using extensive surface water pCO2 observations from the ships of opportunity in the coastal region (SOOP-OA), and using the carbon observations from the East Coast Ocean Acidification Cruises (ECOA-1) and OAP mooring stations and from remotely sensed data. Then, we will use the real-time model run to estimate the 5-year trends (2012 – 2007) of OA and the carbon and biogeochemical variables along the East and Gulf coasts of the U.S. We will also examine the future OA variability in the East and Gulf coasts of the U.S. by downscaling the future climate projections under different emission scenarios developed for the IPCC-AR5. Based on the results obtained from the proposed model simulations, we will contribute to an observational strategy suitable for elucidating multi-annual trend of carbon and biogeochemical variables along the East and Gulf coasts of the U.S.

NOAA operates the largest ship of opportunity (SOOP) effort for surface CO2 observations in the world. The objective of the ocean acidification (OA) monitoring effort in the coastal ocean on NOAA fisheries ships Gordon Gunter and Henry B. Bigelow is to obtain data for a data-based ocean acidification product suite for the East Coast and Gulf Coast. The ship of opportunity (SOOP) in support of OA monitoring (SOOP-OA) is in direct response to the needs expressed in the NOAA OA strategic plan, national and international program documentation, to understand how the rates and magnitude of acidification will vary across time and space, as a consequence of local and regional geochemical, hydrological, and biological variability and trends. The core of understanding rests upon monitoring the carbon system and related physical and biogeochemical parameters that are used to characterize the state of the coastal ocean in the project area. 

The NOAA fisheries ships Gunter and Bigelow provide regular cruise tracks used in stock assessments such that over time correlations and causality can be obtained between OA and fisheries interests. The repeatability also provides good snapshots of change. As there are robust correlations between surface CO2 levels and remotely sensed parameters, these data are critical for the mapping of OA parameters. The development of algorithms to perform this mapping is done from support measurements on the SOOP-OA, other SOOP data under our purview, and from the dedicated research cruises.

Dedicated research cruises are used to obtain subsurface measurements and a comprehensive suite of biogeochemical observations to gain a process level understanding of OA. OAP provides funds to carry out the Gulf of Mexico and East Coast Carbon (GOMECC) research cruises every 5 years. These cruises provide a data set of unprecedented quality of physical and chemical coastal ocean parameters that is used both for improved spatial understanding of OA and also to provide a general understanding of changing patterns over time by comparison with previous cruises. The monitoring component is an essential part of the OAP, providing a long-term assessment of changes of biogeochemistry and ecology in response to increasing CO2 atmospheric levels and large-scale changes in coastal dynamics. 

The climate quality data from the research cruises provide an important link to the Global Ocean Acidification Network (GOAN) effort, and contribute to a long-term record of dynamics and processes controlling OA on the coastal shelves. The data are used for validation measurements of autonomous assets, applying the data for algorithm development utilizing remotely sensed signals that are used to characterize saturation states, and to project the future state of ocean acidification in the project area. The GOMECC research cruises have now been divided into two cruises, one focused on the east coast, the “East Coast Ocean Acidification” (ECOA) cruise and the other covering the Gulf of Mexico, the “Gulf of Mexico Ecosystems and Carbon Cycle” (GOMECC) cruise.