67 Ocean Acidification and Other Stressors
Description: Maps and variability of regional carbonate chemistry and other oceanographic properties
Indicator family:
Contributor(s): Grace Saba, Lori Garzio, Amy Maas, Jonathan Nash
Affiliations: Rutgers University, Bermuda Institute of Ocean Sciences – Arizona State University
67.1 Introduction to Indicator
Ocean acidification (OA) has caused measured declines in global ocean pH and is projected to continue declining by up to 0.30 pH units over the course of the 21st century if high carbon dioxide emissions continue (IPCC 2019). OA also changes the availability of minerals required by organisms to form calcified structures such as shells and other structures. Calcifying conditions in seawater can be determined by measuring aragonite saturation state (ΩArag or omega), the tendency of a common type of calcium carbonate, aragonite, to form or dissolve. When ΩArag is less than 1, shells and other calcium carbonate structures begin to dissolve. Typical surface ocean ΩArag is 2-4, but extremes can be <1 or >5. As the ocean absorbs carbon dioxide, both pH and ΩArag decrease and can cause organisms to respond with reduced survival, calcification rates, growth, and reproduction, as well as impaired development, and/or changes in energy allocation (reviewed in [92], [93]). However, sensitivity levels vary, and some organisms exhibit negative responses to calcification and other processes when ΩArag is as low as 3.
The U.S. Northeast Shelf (NES) is prone not only to global rates of ocean acidification, but also to coastal processes that can act to exacerbate acidification, including freshwater sources (primarily riverine), eutrophication and photosynthesis‐respiration cycles, coastal upwelling, and other influences ([94], [95], [96]). Often times, other stressors such as ocean warming and/or low bottom water dissolved oxygen can co-occur. Dissolved oxygen concentrations at or below 5 mg/liter are considered problematic for marine life. Although concentrations between 3-5 mg/liter may not be low enough to directly cause death in many marine animals, research focused on marine species has identified other negative impacts such as reduced metabolism, feeding, growth, and reproduction at these levels. Lower hypoxic concentrations of dissolved oxygen (< 3 mg/liter) have been directly associated with mortalities in some organisms in other coastal regions around the world.
Any one stressor may not itself be an issue due to the resiliency of many coastal species to fluctuating natural environmental conditions. However, when more than one stressor occurs simultaneously, an organism may become unable to fully withstand changes. The impacts of multiple stressors occurring simultaneously on organism health are much less well known.
The spatio-temporal variability of OA in the NES is still poorly described, and as of yet, there are no analyses that have determined times or locations where multiple environmental stressors are co-located. The purpose of developing data products for OA and other stressors (e.g., warmer than average temperatures, lower than average dissolved oxygen concentrations, lower than average food availability) for the State of the Ecosystems reports is to determine locations and times where potential stressors overlap in space and time to assist in identifying potentially vulnerable species habitats.
67.2 Key Results and Visualizations
The products developed here include: static and animated maps of seasonal surface and bottom pH and aragonite saturation state (omega) on the U.S. Northeast Shelf; maps of locations where species sensitivity levels for aragonite saturation state were reached in bottom or surface waters within the species habitat depth range (winter 2012-2025, spring 2006-2025, summer 2004-2025, fall 2013-2024); and maps of bottom temperature anomalies, bottom aragonite saturation state, and surface chlorophyll anomalies for the summers of 2023-2025.
Seasonal surface and bottom pH and aragonite saturation state on the U.S. Northeast Shelf
Using available quality-controlled vessel- and glider-based datasets accompanying observations on the U.S. Northeast Shelf, maps of seasonal surface and bottom pH and aragonite saturation state (ΩArag) were generated for the U.S. Northeast Shelf (Figs. 1-4). For bottom observations (Figs. 1-2): winter= 2012-2025, spring= 2014-2025, summer= 2007-2025, fall= 2013-2024. For surface observations (Figs. 3-4): winter=2012-2025, spring= 2006-2025, summer= 2004-2025, fall= 2013-2024. These were also combined to create animations of pH and omega over time in the region (bottom pH: access here; surface pH: access here; bottom omega: access here; surface omega: access here).
The animations depict high variability in time and space and increases in sampling frequency and spatial coverage over time. Spatially, the lowest surface and bottom pH and aragonite saturation state have occurred primarily in the western Gulf of Maine, western Long Island Sound, nearshore to mid-shelf waters of the Mid-Atlantic Bight off the coast of New Jersey and New York, and, for bottom aragonite saturation state, in waters > 1000 meters. Seasonally in the U.S. Northeast Shelf, the lowest bottom pH and aragonite saturation state levels, covering the most spatial area, have occurred primarily in the summer, followed by fall. Observed bottom aragonite saturation states in the Mid-Atlantic were low during summer 2023 and lowest in summer 2024, relative to historical observations between 2007-2022; however, summer 2025 bottom aragonite saturation state values rebounded to pre-2023 levels.
The lowest observed values of surface pH in the Mid-Atlantic were observed in the summer, and the lowest surface aragonite saturation state levels occurred in western Long Island Sound and nearshore New Jersey coastal waters during the summer. Shelf waters off southern New England exhibited low surface and bottom pH and aragonite saturation state during winter 2024. In the Gulf of Maine, the lowest observed values of surface pH and aragonite saturation state occurred in the winter and fall seasons.
Figure 1. Seasonal bottom pH on the U.S. Northeast Shelf plotted from available quality-controlled vessel- and glider-based datasets. Winter (Dec, Jan, Feb) data availability: 2012-2025; Spring (Mar, Apr, May) data availability: 2014-2025; Summer (Jun, Jul, Aug) data availability: 2007-2025; and Fall (Sept, Oct, Nov) data availability: 2013-2024.
Figure 2. Seasonal bottom aragonite saturation state (omega) on the U.S. Northeast Shelf plotted from available quality-controlled vessel- and glider-based datasets. Winter (Dec, Jan, Feb) data availability: 2012-2025; Spring (Mar, Apr, May) data availability: 2014-2025; Summer (Jun, Jul, Aug) data availability: 2007-2025; and Fall (Sept, Oct, Nov) data availability: 2013-2024.
Figure 3. Seasonal surface pH on the U.S. Northeast Shelf plotted from available quality-controlled vessel- and glider-based datasets. Winter (Dec, Jan, Feb) data availability: 2012-2025; Spring (Mar, Apr, May) data availability: 2006-2025; Summer (Jun, Jul, Aug) data availability: 2004-2025; and Fall (Sept, Oct, Nov) data availability: 2013-2024.
Figure 4. Seasonal surface aragonite saturation state (omega) on the U.S. Northeast Shelf plotted from available quality-controlled vessel- and glider-based datasets. Winter (Dec, Jan, Feb) data availability: 2012-2025; Spring (Mar, Apr, May) data availability: 2006-2025; Summer (Jun, Jul, Aug) data availability: 2004-2025; and Fall (Sept, Oct, Nov) data availability: 2013-2024.Locations and times where species sensitivity levels for aragonite saturation state were reached
Ocean acidification risks vary among species and include reduced survival, growth, reproduction, and productivity, where high OA risk indicates potential negative effects to species. Using the bottom water aragonite saturation state data (or surface for pteropods and longfin squid), maps depicting locations where bottom aragonite saturation state reached lab-derived sensitivity levels within the habitat depth range of designated target species in the Mid-Atlantic Bight (MAB) and Gulf of Maine (GoM) were developed for winter (Dec, Jan, Feb; MAB: 2012-2025, GoM: 2012-2024), spring (Mar, Apr, May; MAB: 2006-2025, GoM: 2006-2022), summer (Jun, Jul, Aug; MAB: 2007-2025, GoM: 2004-2024), and fall (Sept, Oct, Nov; MAB and GoM: 2013-2024) seasons. The target species selected for the Mid-Atlantic were Atlantic sea scallop (Placopecten magellanicus) and longfin squid (Doryteuthis pealeii), and the target species selected for the Gulf of Maine were Atlantic cod (Gadus morhua) and American lobster (Homarus americanus). Maps developed for pteropods (Limacina spp.) included both the Mid-Atlantic and Gulf of Maine. Note there are no data yet available for fall 2025 for Mid-Atlantic species. Additionally, maps for Atlantic cod, American lobster, and pteropods in the Gulf of Maine do not include any 2025 data because data from this year for the Gulf of Maine are not available. However, the maps for the individual years between 2004-2024 and the combined map for this same time period are available for these species and described below. Bottom and surface water data collected during 2025 (with the exception of the fall season) were incorporated to update this product for the Mid-Atlantic species, Atlantic sea scallop, longfin squid and pteropods (described below). Plots for each target species can be accessed here.
Aragonite saturation state was at or below the sensitivity levels for both Atlantic sea scallop (Placopecten magellanicus; Fig. 5) and longfin squid (Doryteuthis pealeii; Fig. 6) most frequently during the summer (2007-2025) and fall (only 2023, 2024), although these levels were reached for both species also during winter 2024 and, for Atlantic sea scallop in spring 2024 and 2025. Higher OA risk conditions were observed for Atlantic sea scallop and longfin squid in Long Island Sound and the nearshore and mid shelf regions of the New Jersey shelf during summers of 2016, 2018, 2019, 2023, 2024, and, for longfin squid only, 2025. The spatial range of potentially unfavorable habitat for Atlantic sea scallops and longfin squid in the summer-time was highest in 2023 and 2024 compared to the observed past years and the most recent summer 2025.
Figure 5. Locations where bottom aragonite saturation state (omega) in the habitat depth range were at or below the laboratory-derived sensitivity level for Atlantic sea scallop during different seasons for the time periods 2007-2022 (dark cyan), 2023 only (magenta), 2024 only (cyan), and 2025 only (orange). There are no data included for fall 2025. Gray circles indicate locations where bottom omega values were above the species-specific sensitivity values within the habitat depth range.
Figure 6. Locations where bottom aragonite saturation state (omega) in the habitat depth range were at or below the laboratory-derived sensitivity level for longfin squid during different seasons for the time periods 2007-2022 (dark cyan), 2023 only (magenta), 2024 only (cyan), and 2025 only (orange). There are no data included for fall 2025. Gray circles indicate locations where bottom omega values were above the species-specific sensitivity values within the habitat depth range.Higher OA risk conditions observed for Atlantic cod (Gadus morhua; Fig. 7) and American lobster (Homarus americanus; Fig. 8) were more spotty compared to those observed for Atlantic sea scallop and longfin squid in the Mid-Atlantic. However, when observed, they occurred most frequently during the summer, although these levels were also reached at a few locations for both species during fall and winter. The locations this typically occurred within their habitat depth ranges were in western Gulf of Maine and off the coast of eastern Maine. These areas include Stellwagen Bank, slope waters south of Penobscot and Blue Hill Bays, and Wilkinson Basin and additionally, for Atlantic cod, slope waters south of Maquoit Bay and in waters of Jeffreys Ledge and Jordan Basin. Unlike in the Mid-Atlantic, higher OA risk conditions in the Gulf of Maine for Atlantic cod and American lobster occurred in 2023 compared to 2024.
Figure 7. Locations where bottom aragonite saturation state (omega) in the habitat depth range were at or below the laboratory-derived sensitivity level for Atlantic cod during different seasons for the time periods 2007-2022 (dark cyan), 2023 only (magenta), and 2024 (cyan). There are no data included for spring 2023-2024 and for 2025. Gray circles indicate locations where bottom omega values were above the species-specific sensitivity values within the habitat depth range.
Figure 8. Locations where bottom aragonite saturation state (omega) in the habitat depth range were at or below the laboratory-derived sensitivity level for American lobster during different seasons for the time periods 2007-2022 (dark cyan), 2023 only (magenta), and 2024 (cyan). There are no data included for spring 2023-2024 and for 2025. Gray circles indicate locations where bottom omega values were above the species-specific sensitivity values within the habitat depth range.OA risk conditions observed for pteropods (Limacina spp.) exhibited spatiotemporal differences whereby the spatial range of potentially unfavorable habitat was highest in the Gulf of Maine in 2023 (primarily winter and fall; Fig. 9) highest in the Mid-Atlantic in 2024 (primarily offshore in winter and nearshore in summer; Fig. 10). This could be related to the large volume of Labrador slope water, with lower temperatures and omega values, that flowed from the north, through Gulf of Maine in late 2023/early 2024, and then through the Mid-Atlantic Bight in 2024. In 2025, sensitivity levels of pteropods were reached during spring and summer in the Mid-Atlantic, although we are lacking observations for fall 2025 in the Mid-Atlantic and for all of 2025 in the Gulf of Maine.
Figure 9. Locations in the Gulf of Maine where surface aragonite saturation state (omega) in the habitat depth range were at or below the laboratory-derived sensitivity level for pteropods during different seasons for the time periods 2004-2022 (dark cyan), 2023 only (magenta), 2024 (cyan), and 2025 (orange). There are no data included for spring 2023-2024 and for 2025. Gray circles indicate locations where bottom omega values were above the species-specific sensitivity values within the habitat depth range.
Figure 10. Locations in the Mid-Atlantic Bight where surface aragonite saturation state (omega) in the habitat depth range were at or below the laboratory-derived sensitivity level for pteropods during different seasons for the time periods 2007-2022 (dark cyan), 2023 only (magenta), 2024 (cyan), and 2025 (orange). There are no data included for fall 2025. Gray circles indicate locations where bottom omega values were above the species-specific sensitivity values within the habitat depth range.Interannual variability between summers of 2023, 2024, 2025
The progression from summer 2023 to summer 2025 in the Mid-Atlantic and Gulf of Maine is characterized by a switch from a warmer bottom temperature anomaly in 2023 to colder bottom temperature anomalies in 2024 and 2025 (Fig. 11, row A). In the Mid-Atlantic, the greater (colder) anomalies were concentrated inshore in 2024 but on the shelf break in 2025. In the Mid-Atlantic, the cooling between the summers of 2023 and 2024 corresponded with decreasing summer bottom aragonite saturation state values (Fig. 11, row B). The bottom aragonite saturation state observed on the Mid-Atlantic coastal shelf during summer 2024 were the lowest values recorded when compared to all of the years sampled (since 2007 and in 2025). However, bottom aragonite saturation state values were generally higher in the Mid-Atlantic in summer 2025. With low quantity of recent data in the Gulf of Maine (none for 2025), it is difficult to make the interannual comparison. At the same time, there was little difference in chlorophyll-a anomalies between the 3 years in the Mid-Atlantic compared to the Gulf of Maine which saw a large swing from positive to negative chlorophyll-a anomaly between 2023 and 2024 (Fig. 11, row C). If we loosely define chlorophyll climatology as a proxy for food availability, this suggests that for the Mid-Atlantic, there is little to no change in that lower-on-average availability over time, even during periods of low aragonite saturation state.
Figure 11. Bottom temperature anomaly (row A), bottom aragonite saturation state (row B), and surface chlorophyll anomaly (row C) for the summers of 2023 (left column), 2024 (middle column), and 2025 (right column). The bottom temperature anomaly for each summer-year was calculated as the difference from the summer mean 2012-2025. Bottom temperature data courtesy of the E.U. Copernicus Marine Service Information (CMEMS): Global Ocean Physics Reanalysis (GLORYS12V1) dataset. The bottom aragonite saturation state values were those observed from available vessel- and glider-based efforts during the summers of 2023, 2024, and 2025. The surface chlorophyll-a anomaly for each summer-year was calculated as the difference from the summer mean 2012-2025. Satellite-derived surface chlorophyll-a data courtesy of the NASA Ocean Biology Distributed Active Archive Center (OB.DAAC).67.3 Indicator statistics
Spatial scale: Seasonal surface and bottom pH and aragonite saturation state on the U.S. Northeast Shelf: full U.S. Northeast Shelf; Locations and times where species sensitivity levels for aragonite saturation state were reached: Mid-Atlantic Bight (Atlantic sea scallop, longfin squid) and Gulf of Maine (Atlantic cod, American lobster); Interannual variability between summers of 2023, 2024, 2025: full U.S. Northeast Shelf
Temporal scale: Seasonal surface and bottom pH and aragonite saturation state on the U.S. Northeast Shelf: winter (Dec, Jan, Feb)= 2012-2025, spring (Mar, Apr, May)= 2006-2025, summer (Jun, Jul, Aug)= 2004-2025, fall (Sep, Oct, Nov)= 2013-2024 (for surface observations) and winter= 2012-2025, spring= 2014-2025, summer= 2007-2025, fall= 2013-2024 (for bottom observations); Locations and times where species sensitivity levels for aragonite saturation state were reached for target species in the Mid-Atlantic Bight (MAB) and Gulf of Maine (GoM): winter= 2012-2024, spring= 2014-2025, summer= 2007-2025, fall= 2013-2024 (for the MAB) and winter= 2012-2024, spring= 2014-2022, summer= 2012-2024, fall= 2013-2023 (for the GoM); Interannual variability between summers of 2023, 2024, 2025: summer (June-August) 2023, 2024, 2025 and summer-year anomalies for bottom temperature and surface chlorophyll-a were calculated as the difference from the summer mean 2012-2025.
Synthesis Theme:
67.4 Implications
While the sparsity of carbonate chemistry data at annual and seasonal scales to date limits our ability to calculate anomalies from a long-term mean or to determine exposure frequency and duration of unfavorable conditions to marine species, locations where recurring low levels of seawater pH and aragonite saturation state can be identified from the available data. These areas include the western Gulf of Maine, western Long Island Sound, and nearshore to mid-shelf waters of the Mid-Atlantic Bight off the coast of New Jersey and New York. This information helps to identify potential vulnerable species habitats and can guide future targeted observations aimed at determining stressor exposure frequency and duration and the co-occurrence of additional environmental stressors.
The updated datasets this year provided the seasonal evaluation of available carbonate chemistry data to be included in this report, and they highlighted the lowest and most spatially extensive pH and aragonite saturation state levels occurred in bottom water and primarily in the summer season, followed by fall. The addition of the three most recent years of data to the bottom water products in the Mid-Atlantic (2007-2025) revealed that the lowest observed aragonite saturation state values were recorded during summer 2024, and the lower aragonite saturation state values during the summers of 2023 and 2024 increased the spatial and temporal range of potentially unfavorable habitat for local species compared to prior years (2007-2022). However, summer 2025 aragonite saturation state values were higher compared to 2023 and 2024. Events such as those observed in 2023 and 2024 may prevent the ability to sustain normal populations of marine organisms are concerning, not only for the ocean ecosystem but also for the local economy and commercial and recreational fishing industries. Understanding the factors that cause these events will aid in projecting the severity and duration of these events under ongoing climate change and provide important support for guiding policy and management options and identifying priorities for science and monitoring.
67.5 Get the data
Point of contact: Grace Saba (saba@marine.rutgers.edu); Lori Garzio (lgarzio@marine.rutgers.edu)
ecodata name: No dataset
Variable definitions
- depth_interpolated meters 2) temperature degrees Celsius 3) chlorophyll_a µg L-1
- oxygen_concentration_shifted_mgL mg L-1 5) pH_shifted 6) aragonite_saturation_state
No Data
Indicator Category: