Nutrients Cause Aquatic Acidification NOW, Not Emissions
At the United Nations Ocean Conference this month (June 2025) a scientific declaration was made as to the causes of “ocean acidification”. Experts stated that
“Ocean acidification is primarily caused by the increase in CO2 levels in the atmosphere, which leads to higher levels of CO2 being absorbed by the oceans. Based on research, some other causes of ocean acidification are the burning of fossil fuels, industrial processes, and deforestation. Actions such as burning fossil fuels, deforestation, and industrial processes also increase the concentration of hydrogen ions in the water, decreasing pH and making the water more acidic.”
Of course, emissions and overconsumption must be decreased as soon as possible by moving quickly to renewable energy, choosing more sustainable diets, and becoming climate activists. These are global priorities for every global citizen and family.
However, our community of aquatic scientists have scientific and social responsibilities to understand aquatic acidification as much as possible and propose direct actions to assist in the recovery of aquatic ecosystems from accelerated anthropogenic damage. There is a need to understand IPCC projections of climate impacts on aquatic systems and aquatic systems contributing to greenhouse gas emissions in addition to carbon emissions from the burning of fossil fuels, industrial processes, and deforestation (Jackson 2022).
The acid conditions now being experienced in coastal/freshwater ecosystems are not expected to occur in the open ocean within the next hundred years - if ever - given IPCC projections of anthropogenic atmospheric carbon dioxide increase from 472 to 567 ppm CO2 (IPCC Data Distribution Centre).
Ocean acidification is a terrifying concept. But the most urgent priority is aquatic ecosystem acidification from nutrients especially nitrogen which has been called “the forgotten element of climate change”.
Greenhouse gas emissions from unsustainable burning must decrease; but we are now in a major aquatic acidification crisis due to the accelerating discharges of nutrients (N to estuarine/coastal systems, P to freshwaters) causing severe hyper-eutrophication and a cascade of adverse impacts to ecosystem functions and critical habitats. Aquatic acidification is clearly linked to anoxia, hypoxia, intense algal blooms, and eutrophication.
Wastewater discharges aquatic ecosystems from coastal urbanization and unsustainable agriculture control the Earth’s coastal and freshwater nitrogen and phosphorus cycles in the Anthropocene, not geochemistry. The Millennium Ecosystem Assessment predicted an increase in global sewage emissions of 12.0–15.5 Tg N and 2.4–3.1 TgP/year by 2050. These have already been exceeded. Over 60% percent of the planet’s population lack sewage connections and use open defecation, pit latrines, or septic tanks (WHO/UNICEF 2017). While expensive technology capable of removing 90% of sewage nitrogen exists even in rich countries such as the USA, less than 1% of its sewage is treated with the best technologies due to high capital and operating costs (Sobota et al. 2013).
Lu et al. (2024) found that as a city grows energy use becomes more efficient and greenhouse gas emissions decrease, but that water use increases exponentially; thus, wastewater discharges grow at a faster rate than a growing urban population. Tuholske et al. (2021) showed that 58% of coral reefs and 88% of seagrass beds of the 135,000 watersheds mapped globally were exposed to wastewater nitrogen inputs.
Greenhouse Gas Emissions from Aquatic Systems
Many (most?) freshwater and coastal ecosystems are “sewage-fueled” ecosystems and net heterotrophic (Duarte and Prairie 2005). Aquatic ecosystems are a major contributor to greenhouse gas emissions due to nutrient pollution.
Almost 30 years ago, Frankignoulle et al. (1998) found an area of European estuaries emitted ~3000 MT carbon dioxide per day to the atmosphere. Carbon dioxide emissions from the Scheldt estuary (Belgium/Netherlands) were two-thirds due to heterotrophy and one-third due to ventilation. Jamaica Bay off New York City is a hypereutrophic estuary that receives ~90% of its nitrogen from four wastewater treatment plants that discharge ~ 1 billion liters of effluent daily. Dissolved carbon dioxide concentrations in bottom waters in summer exceed 2000 μatm (the atmosphere is ~437 ppm now) (Wallace et al. 2014).
Tranvik et al. (2009) estimated global freshwater carbon dioxide emissions of ~1.4 PgC (carbon dioxide equivalents) per year, about the same as carbon emissions caused by deforestation (~1.6 Pg).
Short note here. 1Pg is 1 Pentagram = 100,000 million metric tons (MMT). A big number. Let's translate. A coal fired power plant releases ~4-6 MMT/year (Carbon Emissions: Coal Plants). So, freshwaters globally release the amount of carbon dioxide of ~25,000 coal fired power plants each year.
Freshwater ecosystems are very important components of the balance of global greenhouse gas emissions (Bastviken et al. 2025)! These must be incorporated into any future revision of global greenhouse gas fluxes. Considering the surprising magnitude of these, Bastviken et al. (2025) stated that “the terrestrial greenhouse gas sink may be smaller than currently believed”.
“Solutions” to accelerating urban wastewater discharges to coastal/freshwater systems are dominated by proposals by policy-makers, regulators, sanitary engineers and scientists for the massive expansion of capital-intensive sewer systems, the requiring installations of “alternative septic systems, or dilution “offshore” in the hope of restoring the water quality of valuable aquatic ecosystems. Such calls require large and unprecedented government expenditures and residential tax increases.
But alternative community-based, lower cost integrated restorative aquaculture with nutrient diversion models are underway (Wald 2022; Zweig et al. 2025).
References
Bastviken, D. et al. 2010. Freshwater methane emissions offset the continental carbon sink. Science 331, https://doi.org/10.1126/science.119680
Duarte, C.M. and Y.T. Prairie. 2005. Prevalence of heterotrophy and atmospheric CO2 emissions from aquatic ecosystems. Ecosystems 8: 862–870, https://doi.org/10.1007/s10021-005-0177-4
Frankignoulle, M. et al. 1998. Carbon dioxide emission from European estuaries. Science 282, https://doi.org/10.1126/science.282.5388.434
Jackson, L. 2022. Ocean acidification isn't just a carbon story; it's also about nitrogen. Responsible Food Advocate, Global Seafood Alliance, April 11, 2022
Lu, M. et al. 2024. Worldwide scaling of waste generation in urban systems. Nature Cities, 1, https://doi.org/10.1038/s44284-023-00021-5
Sobota, D. et al. 2013. Reactive nitrogen inputs to US lands and waterways: how certain are we about sources and fluxes? Front Ecol Environ 11: 82–90.
Tranvik, L. et al. 2009. Lakes and reservoirs as regulators of carbon cycling and climate. Limnol. Oceanogr., 54(6, part 2), 2009, 2298–2314.
Tuholske, C. et al. 2021. Mapping global inputs and impacts from human sewage in coastal ecosystems. PLoS ONE 16(11): e0258898. https://doi.org/10.1371/journal.pone.0258898
Wald, C. 2022. The urine revolution: how recycling pee could help to save the world. Nature 6502, 202. https://doi.org/10.1038/d41586-022-00338-6
Wallace, R. et al. 2014. Coastal ocean acidification: The other eutrophication problem. Estuarine, Coastal and Shelf Science 148 (2014) 1e13
WHO/UNICEF. 2017 WHO/UNICEF Joint Monitoring Project. 2017. https://washdata.org/data/downloads#WLD
Zweig, R.D. et al. 2025. Restorative aquaculture as part of an integrated solution to the accelerating nitrogen crisis damaging coastal marine ecosystems. Presented at the World Aquaculture Society, New Orleans, LA, USA. Available at: https://oceanfoods.org/pdf/zweig-et-al.-2025.pdf
