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Fingerprinting ocean acidification (OA) in U.S. West Coast waters is extremely challenging due to the large magnitude of natural carbonate chemistry variations common to these regions. Additionally, quantifying a change requires information about the initial conditions, which is not readily available in most coastal systems. In an effort to address this issue, we have collated high-quality, publicly-available data to characterize the modern seasonal carbonate chemistry variability in marine surface waters of the Pacific Northwest. Underway ship data from Version 4 of the Surface Ocean CO2 Atlas, discrete observations from various sampling platforms, and sustained measurements from regional moorings were incorporated to provide ~ 100,000 inorganic carbon observations from which modern seasonal cycles were estimated. Underway ship and discrete observations were merged and gridded to a 0.1° × 0.1° scale. Eight unique regions were identified and seasonal cycles from grid cells within each region were averaged. Data from nine surface moorings were also compiled and used to develop robust estimates of mean seasonal cycles for comparison with the eight regions. This manuscript describes our methodology and the resulting mean seasonal cycles for multiple OA metrics in an effort to provide large-scale, environmental context for ongoing research, adaptation, and management efforts throughout the Pacific Northwest. Major findings include the identification of unique chemical characteristics across the study domain. There is a clear increase in the ratio of dissolved inorganic carbon (DIC) to total alkalinity (TA) and in the seasonal cycle amplitude of carbonate system parameters when moving from the open ocean North Pacific into the Salish Sea. Due to the logarithmic nature of the pH scale (pH = −log10[H+], where [H+] is the hydrogen ion concentration), lower annual mean pH values (associated with elevated DIC : TA) coupled with larger magnitude seasonal pH cycles results in seasonal [H+] ranges that are ~ 27 times larger in Hood Canal than in the neighboring North Pacific open ocean. Organisms living in the Salish Sea are thus exposed to much larger seasonal acidity changes than those living in nearby open ocean waters. Additionally, our findings suggest that lower buffering capacities in the Salish Sea make these waters less efficient at absorbing anthropogenic carbon than open ocean waters at the same latitude. All data used in this analysis are publically available at the following websites: • Surface Ocean CO2 Atlas Version 4 coastal data, doi:10.1594/PANGAEA.866856; • National Oceanic and Atmospheric Administration (NOAA) West Coast Ocean Acidification cruise data, doi:10.3334/CDIAC/otg.CLIVAR_NACP_West_Coast_Cruise_2007; doi:10.3334/CDIAC/OTG.COAST_WCOA2011; doi:10.3334/CDIAC/OTG.COAST_WCOA2012; doi:10.7289/V5C53HXP; • University of Washington (UW) and Washington Ocean Acidification Center cruise data, doi:10.5281/zenodo.1184657; • Washington State Department of Ecology seaplane data, 10.5281/zenodo.1184657; • NOAA Moored Autonomous pCO2 (MAPCO2) Buoy data, doi:10.3334/CDIAC/OTG.TSM_LAPUSH_125W_48N; doi:10.3334/CDIAC/OTG.TSM_WA_125W_47N; doi:10.3334/CDIAC/OTG.TSM_DABOB_122W_478N; doi:10.3334/CDIAC/OTG.TSM_TWANOH_123W_47N; • UW Oceanic Remote Chemical/Optical Analyzer Buoy data, doi:10.5281/zenodo.1184657; • NOAA Pacific Coast Ocean Observing System cruise data, doi:10.5281/zenodo.1184657.

Marine sediments account for up to 66% of the loss of nitrogen load to coastal areas. Sedimentary denitrification is the main sink for fixed nitrogen in the global nitrogen budget, and thus it is important to understand the structure and composition of denitrifying communities. To understand the structure and composition of denitrifying communities, the diversity of nitrite reductase (nirS) genes from sediments along the Gulf of Mexico was examined using a PCR-based cloning approach. Sediments were collected at three different depths (0-0.5, 4-5 and 19-21 cm). Geochemical analysis revealed decreasing nitrate and oxygen concentrations with increasing sediment depth. This trend coincided with the decrease in diversity of denitrifying bacteria. LIBSHUFF analysis indicated that the clone library in the shallowest sediment (depth, 0-0.5 cm) was significantly different from that in the deepest sediment (depth, 19-21 cm), and that the deeper sediments (depths of 4-5 and 19-21 cm) were significantly similar. Community structural shifts were evident between the shallowest (oxic zone) and deepest (anoxic zone) sediments. Community changes within the deepest sediments were more subtle, with the presence of different nirS clone sequences gradually becoming dominant or, alternatively, decreasing with depth. The changes in community structure at this depth are possibly driven by nutrient availability, with lower quality sources of carbon and energy leading to the disappearance of nirS sequences common in the top layer. The majority of recovered nirS sequences were phylogenetically divergent relative to known denitrifying bacteria in the database.