The Pacific Small Island Developing States (Pacific SIDS) are at the forefront of climate change impacts, yet these effects remain understudied and therefore poorly understood. Ocean acidification (OA) research in Samoa is in its infancy, with a new OA project established to monitor seawater carbon chemistry in coastal ecosystems. The study measured seawater carbon chemistry—including pH, dissolved carbon dioxide and total alkalinity—within the marine protected area (MPA) of Savaia, Lefaga, on Upolu Island, Samoa. Sampling was conducted over twelve months at four locations within the MPA (Site 1, Site 2, Site 3, Site 4), focusing on spatio-temporal variability in pH, temperature and total alkalinity. Measurements were obtained using water samples and an iSAMI pH sensor. The mean pH recorded was 8.06 ± 0.07, while the mean total alkalinity was 1955.1 ± 25 µmol∙kg −1 , which is lower than the typical value of 2305 µmol∙kg −1 observed in tropical environments. This decline in total alkalinity and the corresponding low pH could be a potential source of acidification to downstream coastal ecosystems with potential implications for coral reefs, biodiversity and fishery livelihoods.
Whistler, W.A. (1992) Vegetation of Samoa and Tonga. Pacific Science , 46, 159-178.
Government of Samoa (2020) Samoa Ocean Strategy. 2020-2030, Integrated Management for a Healthy and. Abundant Future of Samoa’s Ocean. 48 p.
Richmond, B.M. (1992) Coastal Geology of Upolu, Western Samoa. In: Keating, B.H. and Bolton, B.R., Eds., Geology and Offshore Mineral Resources of the Central Pacific Basin , Springer, 101-125. https://doi.org/10.1007/978-1-4612-2896-7_8
Green, A.L. (1996) Status of the Coral Reefs of the Samoan Archipelago. Department of Marine and Wildlife Resources Report Series, Government of American Samoa.
McAdoo, B.G., Ah-Leong, J.S., Bell, L., Ifopo, P., Ward, J., Lovell, E., et al . (2011) Coral Reefs as Buffers during the 2009 South Pacific Tsunami, Upolu Island, Samoa. Earth-Science Reviews , 107, 147-155. https://doi.org/10.1016/j.earscirev.2010.11.005
Graham, N.A.J. and Nash, K.L. (2013) The Importance of Structural Complexity in Coral Reef Ecosystems. Coral Reefs , 32, 315-326. https://doi.org/10.1007/s00338-012-0984-y
Burke, L. and Spalding, M. (2022) Shoreline Protection by the World’s Coral Reefs: Mapping the Benefits to People, Assets, and Infrastructure. Marine Policy , 146, Article ID: 105311. https://doi.org/10.1016/j.marpol.2022.105311
Fezzi, C., Ford, D.J. and Oleson, K.L.L. (2023) The Economic Value of Coral Reefs: Climate Change Impacts and Spatial Targeting of Restoration Measures. Ecological Economics , 203, Article ID: 107628. https://doi.org/10.1016/j.ecolecon.2022.107628
Edwards, A., Guest, J. and Humanes, A. (2024) Rehabilitating Coral Reefs in the Anthropocene. Current Biology , 34, R399-R406. https://doi.org/10.1016/j.cub.2023.12.054
Zann, L.P. and Bell, L. (1991) The Effects of the Crown-of-Thorns Starfish (Alamea) on Samoan Reefs. FAO (No. 8). UNDP SAM/89/002 Field Report.
Ziegler, M., Quéré, G., Ghiglione, J., Iwankow, G., Barbe, V., Boissin, E., et al . (2018) Status of Coral Reefs of Upolu (Independent State of Samoa) in the South West Pacific and Recommendations to Promote Resilience and Recovery of Coastal Ecosystems. Marine Pollution Bulletin , 129, 392-398. https://doi.org/10.1016/j.marpolbul.2018.02.044
Doney, S.C., Mahowald, N., Lima, I., Feely, R.A., Mackenzie, F.T., Lamarque, J., et al . (2007) Impact of Anthropogenic Atmospheric Nitrogen and Sulfur Deposition on Ocean Acidification and the Inorganic Carbon System. Proceedings of the National Academy of Sciences of the United States of America , 104, 14580-14585. https://doi.org/10.1073/pnas.0702218104
Feely, R.A., Sabine, C.L., Lee, K., Berelson, W., Kleypas, J., Fabry, V.J., et al . (2004) Impact of Anthropogenic CO 2 on the CaCo 3 System in the Oceans. Science , 305, 362-366. https://doi.org/10.1126/science.1097329
Kleypas, J. and Yates, K. (2009) Coral Reefs and Ocean Acidification. Oceanography , 22, 108-117. https://doi.org/10.5670/oceanog.2009.101
Hofmann, G.E., Barry, J.P., Edmunds, P.J., Gates, R.D., Hutchins, D.A., Klinger, T., et al . (2010) The Effect of Ocean Acidification on Calcifying Organisms in Marine Ecosystems: An Organism-to-Ecosystem Perspective. Annual Review of Ecology , Evolution , and Systematics , 41, 127-147. https://doi.org/10.1146/annurev.ecolsys.110308.120227
Morse, J.W., Arvidson, R.S. and Lüttge, A. (2007) Calcium Carbonate Formation and Dissolution. Chemical Reviews , 107, 342-381. https://doi.org/10.1021/cr050358j
Hall-Spencer, J.M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S.M., et al . (2008) Volcanic Carbon Dioxide Vents Show Ecosystem Effects of Ocean Acidification. Nature , 454, 96-99. https://doi.org/10.1038/nature07051
Andersson, A.J. and Gledhill, D. (2013) Ocean Acidification and Coral Reefs: Effects on Breakdown, Dissolution, and Net Ecosystem Calcification. Annual Review of Marine Science , 5, 321-348. https://doi.org/10.1146/annurev-marine-121211-172241
Doney, S.C., Busch, D.S., Cooley, S.R. and Kroeker, K.J. (2020) The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities. Annual Review of Environment and Resources , 45, 83-112. https://doi.org/10.1146/annurev-environ-012320-083019
Sulpis, O., Jeansson, E., Dinauer, A., Lauvset, S.K. and Middelburg, J.J. (2021) Calcium Carbonate Dissolution Patterns in the Ocean. Nature Geoscience , 14, 423-428. https://doi.org/10.1038/s41561-021-00743-y
Gattuso, J.P., Frankignoulle, M. and Wollast, R. (1998) Carbon and Carbonate Metabolism in Coastal Aquatic Ecosystems. Annual Review of Ecology and Systematics , 29, 405-434. https://doi.org/10.1146/annurev.ecolsys.29.1.405
Kleypas, J.A.K.A. (2019) Climate Change and Tropical Marine Ecosystems: A Review with an Emphasis on Coral Reefs. UNED Research Journal , 11, S24-S35. https://doi.org/10.22458/urj.v11i1.2317
Anthony, K.R.N., Kline, D.I., Diaz-Pulido, G., Dove, S. and Hoegh-Guldberg, O. (2008) Ocean Acidification Causes Bleaching and Productivity Loss in Coral Reef Builders. Proceedings of the National Academy of Sciences of the United States of America , 105, 17442-17446. https://doi.org/10.1073/pnas.0804478105
Cohen, A. and Holcomb, M. (2009) Why Corals Care about Ocean Acidification: Uncovering the Mechanism. Oceanography , 22, 118-127. https://doi.org/10.5670/oceanog.2009.102
Cornwall, C.E., Comeau, S., Kornder, N.A., Perry, C.T., van Hooidonk, R., DeCarlo, T.M., et al . (2021) Global Declines in Coral Reef Calcium Carbonate Production under Ocean Acidification and Warming. Proceedings of the National Academy of Sciences of the United States of America , 118, e2015265118. https://doi.org/10.1073/pnas.2015265118
Amosa, P. (2015) The Dissolution Kinetics of Calcium Carbonate in Seawater: Development and Application of an Automated pH-Stat Method for the Measurement of Dissolution Rates. Ph.D. Thesis, University of Otago.
Hepburn, C.D., Pritchard, D.W., Cornwall, C.E., McLeod, R.J., Beardall, J., Raven, J.A., et al . (2011) Diversity of Carbon Use Strategies in a Kelp Forest Community: Implications for a High CO2 Ocean. Global Change Biology , 17, 2488-2497. https://doi.org/10.1111/j.1365-2486.2011.02411.x
Ho, M., McBroom, J., Bergstrom, E. and Diaz-Pulido, G. (2021) Physiological Responses to Temperature and Ocean Acidification in Tropical Fleshy Macroalgae with Varying Affinities for Inorganic Carbon. ICES Journal of Marine Science , 78, 89-100. https://doi.org/10.1093/icesjms/fsaa195
Taise, A., Krieger, E., Bury, S.J. and Cornwall, C.E. (2025) Physiological Responses of Caulerpa spp. (with Different Dissolved Inorganic Carbon Physiologies) to Ocean Acidification. New Zealand Journal of Botany , 63, 108-132. https://doi.org/10.1080/0028825x.2023.2289432
Ministry of Natural Resources and Environment (2022) Annual Report for the Financial Year 2021/2022. Government of Samoa. http://www.mnre.gov.ws/
Cornwall, C.E. and Eddy, T.D. (2015) Effects of Near-Future Ocean Acidification, Fishing, and Marine Protection on a Temperate Coastal Ecosystem. Conservation Biology , 29, 207-215. https://doi.org/10.1111/cobi.12394
Naidu, R., Hallock, P., Erez, J. and Maata, M. (2017) Response of Marginopora vertebralis (Foraminifera) from Laucala Bay, Fiji, to Changing Ocean pH. In: Leal Filho, W., Ed., Climate Change Management , Springer, 137-150. https://doi.org/10.1007/978-3-319-50094-2_8
Law, C.S., Bell, J.J., Bostock, H.C., Cornwall, C.E., Cummings, V.J., Currie, K., et al . (2018) Ocean Acidification in New Zealand Waters: Trends and Impacts. New Zealand Journal of Marine and Freshwater Research , 52, 155-195. https://doi.org/10.1080/00288330.2017.1374983
Krieger, E.C., Nelson, W.A., Grand, J., Le Ru, E.C., Bury, S.J., Cossais, A., et al . (2023) The Role of Irradiance in Controlling Coralline Algal Calcification. Limnology and Oceanography , 68, 1269-1284. https://doi.org/10.1002/lno.12345
Albright, R., Takeshita, Y., Koweek, D.A., Ninokawa, A., Wolfe, K., Rivlin, T., et al . (2018) Carbon Dioxide Addition to Coral Reef Waters Suppresses Net Community Calcification. Nature , 555, 516-519. https://doi.org/10.1038/nature25968
Ministry of Natural Resources and Environment (MNRE) (2023) Samoa State of the Environment Report 2023. Secretariat of the Pacific Regional Environment Programme (SPREP). https://library.sprep.org/sites/default/files/2024-09/Samoa-SOE-2023.pdf
Ministry of Finance (2021) Pathway for the Development of Samoa: FY2021/22-FY2025/26. Government of Samoa. https://mwcsd.gov.ws/wp-content/uploads/2024/09/Pathway-for-the-Development-of-Samoa.pdf
Goodwin, I.D. and Grossman, E.E. (2003) Middle to Late Holocene Coastal Evolution along the South Coast of Upolu Island, Samoa. Marine Geology , 202, 1-16. https://doi.org/10.1016/s0025-3227(03)00284-6
Dickson, A.G., Sabine, C.L. and Christian, J.R. (2007) Guide to Best Practices for Ocean CO 2 Measurements. North Pacific Marine Science Organization.
Riebesell, U., Fabry, V.J., Hansson, L. and Gattuso, J.P. (2011) Guide to Best Practices for Ocean Acidification Research and Data Reporting. Office for Official Publications of the European Communities.
Lewis, E. and Wallace, D. (1998) Program developed for CO 2 system calculations.
R Core Team (2021) Team. R: A Language and Environment for Statistical Computing.
Mehrbach, C., Culberson, C.H., Hawley, J.E. and Pytkowicx, R.M. (1973) Measurement of the Apparent Dissociation Constants of Carbonic Acid in Seawater at Atmospheric Pressure. Limnology and Oceanography , 18, 897-907. https://doi.org/10.4319/lo.1973.18.6.0897
Schmalz, R.F. and Swanson, F.J. (1969) Diurnal Variations in the Carbonate Saturation of Seawater. Journal of Sedimentary Research , 39, 255-267. https://doi.org/10.1306/74d71c2b-2b21-11d7-8648000102c1865d
Provoost, P., van Heuven, S., Soetaert, K., Laane, R.W.P.M. and Middelburg, J.J. (2010) Seasonal and Long-Term Changes in pH in the Dutch Coastal Zone. Biogeosciences , 7, 3869-3878. https://doi.org/10.5194/bg-7-3869-2010
Muth, A.F., Kelley, A.L. and Dunton, K.H. (2022) High-Frequency pH Time Series Reveals Pronounced Seasonality in Arctic Coastal Waters. Limnology and Oceanography , 67, 1429-1442. https://doi.org/10.1002/lno.12080
Maberly, S.C. (1996) Diel, Episodic and Seasonal Changes in pH and Concentrations of Inorganic Carbon in a Productive Lake. Freshwater Biology , 35, 579-598. https://doi.org/10.1111/j.1365-2427.1996.tb01770.x
Ono, T., Muraoka, D., Hayashi, M., Yorifuji, M., Dazai, A., Omoto, S., Tanaka T., Okamura, T., Onitsuka, G. andWakita, M. (2023) Short-Term Variation in pH in Seawaters around Coastal Areas of Japan: Characteristics and Forcings. Biogeosciences , 21, 177-199.
Guo, X., Yao, Z., Gao, Y., Luo, Y., Xu, Y. and Zhai, W. (2021) Seasonal Variability and Future Projection of Ocean Acidification on the East China Sea Shelf off the Changjiang Estuary. Frontiers in Marine Science , 8, Article 770034. https://doi.org/10.3389/fmars.2021.770034
Xiao, J., Wang, B., Qiu, X., Yang, M. and Liu, C. (2021) Interaction between Carbon Cycling and Phytoplankton Community Succession in Hydropower Reservoirs: Evidence from Stable Carbon Isotope Analysis. Science of the Total Environment , 774, Article ID: 145141. https://doi.org/10.1016/j.scitotenv.2021.145141
Middelboe, A. and Hansen, P. (2007) High pH in Shallow-Water Macroalgal Habitats. Marine Ecology Progress Series , 338, 107-117. https://doi.org/10.3354/meps338107
Frieder, C.A., Nam, S.H., Martz, T.R. and Levin, L.A. (2012) High Temporal and Spatial Variability of Dissolved Oxygen and pH in a Nearshore California Kelp Forest. Biogeosciences , 9, 3917-3930. https://doi.org/10.5194/bg-9-3917-2012
Aktan, Y. (2011) Large-Scale Patterns in Summer Surface Water Phytoplankton (Except Picophytoplankton) in the Eastern Mediterranean. Estuarine , Coastal and Shelf Science , 91, 551-558. https://doi.org/10.1016/j.ecss.2010.12.010
Semesi, I.S., Kangwe, J. and Björk, M. (2009) Alterations in Seawater pH and CO 2 Affect Calcification and Photosynthesis in the Tropical Coralline Alga, Hydrolithon sp. (Rhodophyta). Estuarine , Coastal and Shelf Science , 84, 337-341. https://doi.org/10.1016/j.ecss.2009.03.038
Kawahata, H., Suzuki, A. and Goto, K. (1997) Coral Reef Ecosystems as a Source of Atmospheric CO 2 : Evidence from pCO 2 Measurements of Surface Waters. Coral Reefs , 16, 261-266. https://doi.org/10.1007/s003380050082
Suzuki, A., Kawahata, H., Ayukai, T. and Goto, K. (2001) The Oceanic CO 2 System and Carbon Budget in the Great Barrier Reef, Australia. Geophysical Research Letters , 28, 1243-1246. https://doi.org/10.1029/2000gl011875
Kerrison, P., Hall-Spencer, J.M., Suggett, D.J., Hepburn, L.J. and Steinke, M. (2011) Assessment of pH Variability at a Coastal CO 2 Vent for Ocean Acidification Studies. Estuarine , Coastal and Shelf Science , 94, 129-137. https://doi.org/10.1016/j.ecss.2011.05.025
Millero, F.J. (1986) The pH of Estuarine Waters. Limnology and Oceanography , 31, 839-847. https://doi.org/10.4319/lo.1986.31.4.0839
Miranda, R., Rios-Touma, B., Falconí-López, A., Pino-del-Carpio, A., Gaspar, S., Ortega, H., et al . (2022) Evaluating the Influence of Environmental Variables on Fish Assemblages along Tropical Andes: Considerations from Ecology to Conservation. Hydrobiologia , 849, 4569-4585. https://doi.org/10.1007/s10750-021-04726-3
Mosley, L.M., Peake, B.M. and Hunter, K.A. (2010) Modelling of pH and Inorganic Carbon Speciation in Estuaries Using the Composition of the River and Seawater End Members. Environmental Modelling & Software , 25, 1658-1663. https://doi.org/10.1016/j.envsoft.2010.06.014
Carstensen, J. and Duarte, C.M. (2019) Drivers of pH Variability in Coastal Ecosystems. Environmental Science & Technology , 53, 4020-4029. https://doi.org/10.1021/acs.est.8b03655
Balch, W.M., Gordon, H.R., Bowler, B.C., Drapeau, D.T. and Booth, E.S. (2005) Calcium Carbonate Measurements in the Surface Global Ocean Based on Moderate‐Resolution Imaging Spectroradiometer Data. Journal of Geophysical Research : Oceans , 110, C07001. https://doi.org/10.1029/2004jc002560
Lee, Y.W., Park, M.O., Kim, S.G., Kim, T.H., Oh, Y.H., Lee, S.H. and Joung, D.J. (2024) Long-Term Variations of pH in Coastal Waters along the Korean Peninsula. Biogeosciences , 22, 675-690.
He, Q. and Silliman, B.R. (2019) Climate Change, Human Impacts, and Coastal Ecosystems in the Anthropocene. Current Biology , 29, R1021-R1035. https://doi.org/10.1016/j.cub.2019.08.042
Robbins, L.L. and Lisle, J.T. (2018) Regional Acidification Trends in Florida Shellfish Estuaries: A 20+ Year Look at pH, Oxygen, Temperature, and Salinity. Estuaries and Coasts , 41, 1268-1281. https://doi.org/10.1007/s12237-017-0353-8
Ministry of Natural Resources and Environment (2018) A’ana Alofi III Community Integrated Management Plan. MNRE.
Marion, G.M., Millero, F.J., Camões, M.F., Spitzer, P., Feistel, R. and Chen, C.A. (2011) pH of Seawater. Marine Chemistry , 126, 89-96. https://doi.org/10.1016/j.marchem.2011.04.002
Lee, K., Tong, L.T., Millero, F.J., Sabine, C.L., Dickson, A.G., Goyet, C., et al . (2006) Global Relationships of Total Alkalinity with Salinity and Temperature in Surface Waters of the World’s Oceans. Geophysical Research Letters , 33, L19605. https://doi.org/10.1029/2006gl027207
Jiang, Z., Tyrrell, T., Hydes, D.J., Dai, M. and Hartman, S.E. (2014) Variability of Alkalinity and the Alkalinity-Salinity Relationship in the Tropical and Subtropical Surface Ocean. Global Biogeochemical Cycles , 28, 729-742. https://doi.org/10.1002/2013gb004678
Takahashi, T., Sutherland, S.C., Chipman, D.W., Goddard, J.G., Ho, C., Newberger, T., et al . (2014) Climatological Distributions of pH, pCO 2 , Total CO 2 , Alkalinity, and CaCO 3 Saturation in the Global Surface Ocean, and Temporal Changes at Selected Locations. Marine Chemistry , 164, 95-125. https://doi.org/10.1016/j.marchem.2014.06.004