The oceans play a key role in balancing the climate and are a reservoir of biodiversity, but the warning signs are multiplying. A recent study reveals that in just 40 years, the waters around Bermuda have undergone unprecedented transformations, impacting their physical and chemical properties. These changes, which reflect global trends, threaten the stability of marine ecosystems and highlight the critical environmental challenges we face in preserving this vital balance.
By Laurie Henry
Cover photo : © Bermuda Institute of Ocean Sciences
The chemistry and biology of the oceans are undergoing profound changes as a result of climate change and human activities. These changes, although often invisible, are redefining marine ecosystems and threatening global food chains. A study published in the journal Frontiers in Marine Science, led by researchers from the Bermuda Institute of Ocean Sciences (BIOS) and Arizona State University, analysed forty years of data collected in the subtropical Atlantic.
This research reveals a marked increase in the temperature, acidity and salinity of the waters around Bermuda. This volcanic archipelago is the only landmass in the heart of the Sargasso Sea, bounded by ocean currents that form a subtropical gyre but have no coastline. By documenting the changes in the region, the study sheds light on global environmental challenges and provides a basis for understanding the future dynamics of the oceans, which are essential to the survival of many species, demonstrating the urgent need for action.
Rising temperatures and salinity
The data were collected at the Bermuda Atlantic Time-series Study (BATS) measuring point and at Hydrostation S, located 80 km and 25 km south-east of Bermuda respectively. Since 1983, monthly campaigns have measured physical, chemical and biological parameters at various depths down to 4,500 metres. CTD sensors have recorded temperature, salinity and dissolved oxygen concentration, while samples have been analysed for alkalinity, dissolved carbon and pH. These data were then corrected to eliminate seasonal variations and identify significant long-term trends.

Map showing the location of the island of Bermuda and two ocean time series sites (Hydrostation S, 1954 to present; and the Bermuda Atlantic Time Series, BATS, 1988 to present) © Bates et al., 2023
The data show an average increase of 1°C in the surface temperatures of the Atlantic Ocean around Bermuda since 1983, an average annual warming of 0.024°C. This rate, although gradual, has accelerated over the last four years, reflecting the global intensification of global warming. This rise in temperature is leading to increased stratification of ocean waters, altering their ability to mix deep and shallow layers. This has an impact on the biological balance, particularly by disrupting the life cycles of phytoplankton, the cornerstone of the marine food chain. This imbalance in turn threatens the equilibrium of larger organisms, such as fish and marine mammals.
At the same time, the salinity of surface waters has increased by 0.136 parts per thousand in 40 years, reflecting an intensification of the hydrological cycle. This phenomenon is explained by more intense evaporation due to high temperatures, combined with changes in regional precipitation. This rise in salinity is disrupting ocean currents, which regulate the global climate. As Nicholas Bates, lead author of the study, points out, ‘ these changes are emblematic of the climate upheavals underway, affecting natural processes such as thermohaline circulation, which plays a crucial role in the redistribution of heat between the polar and equatorial regions ’.
Water acidification, a threat to marine ecosystems
Since 1983, the pH of the surface waters of the Sargasso has fallen by 0.018 per decade, an overall acidification of 30% in forty years. This phenomenon is the result of increased absorption of atmospheric CO₂, which has risen by 77 µatm to levels never before recorded. This chemical variation reduces the saturation of essential minerals, such as aragonite and calcite, with respective decreases of 0.35 and 0.55 over the same period. These minerals are crucial for calcifying organisms such as corals, molluscs and coccolithophores, whose structures are now subject to dissolution. Nicholas Bates points out that ‘ current values of surface water chemistry exceed the seasonal ranges of the 1980s, indicating a drastic change in ecosystem dynamics’.

Data related to ocean acidification: (A) Changes in surface pH over time; (B) Changes in aragonite saturation over time © Bates et al., 2023
The consequences of this acidification go beyond the Bermuda region alone, as it also affects regions such as the Pacific and the North Atlantic, according to researchers at the Bermuda Institute of Ocean Sciences (BIOS).
The study also reveals that levels of dissolved inorganic carbon (DIC) have increased by 12.9 µmol.kg-¹ per decade, a key indicator of global chemical change. These disturbances directly affect food chains and disrupt marine ecosystems, from lower trophic levels to large predators. The impact on the oceans’ ability to act as a carbon sink is worrying, as the continued increase in atmospheric CO₂ limits the effectiveness of this carbon sink role and exacerbates the climate and ecological crises.
The decline in dissolved oxygen, another worrying consequence
The concentration of dissolved oxygen (DO) in the surface waters of the Sargasso has fallen by 6% since the 1980s, an average drop of 3.1 µmol-kg-¹ per decade. This deoxygenation is directly linked to the warming of the waters, measured at +1°C over 40 years, which reduces the solubility of oxygen in water. In addition, the resulting oceanic stratification limits mixing between surface waters and deep waters, preventing oxygen penetration to the lower zones and amplifying this deficit. Rod Johnson, co-author of the study, points out that ‘ this loss of oxygen is not only affecting marine organisms today, but is also an indicator of future climate upheavals ’.

Seawater properties, adjusted for seasonal variations, measured at the BATS site (1988-2023) and combined with earlier Hydrostation S data (1983-1988). (A) Surface temperature anomalies (B) Surface salinity anomalies (C) Surface dissolved oxygen anomalies © Bates et al., 2023
The effects of this deoxygenation are manifold: a reduction in viable habitats for marine species sensitive to hypoxia, disruption of reproduction cycles, and altered migrations of large marine predators. The data also show an increase in areas of low oxygen levels, which are essential for regulating ecosystems. In the long term, this trend could compromise entire ecosystems, particularly in regions where dissolved oxygen levels are close to the critical threshold for the survival of many species. The researchers warn that these changes are spreading to other ocean regions, illustrating a global crisis in marine ecosystems exacerbated by climate change.
The study points out that the BATS records are among the few to last more than 20 years, but that in recent decades ocean observations have been made in other locations to provide a better picture of the challenges facing the oceans.
‘ There will be an urgent need to combine, compare and synthesise ocean carbon and biogeochemical data, both for the highest quality analyses of climate trend data and for synthesis of local, regional and basin data,’ says the report.
Source : Bates NR and Johnson RJ (2023) “Forty years of ocean acidification observations (1983–2023) in the Sargasso Sea at the Bermuda Atlantic Time-series Study site”. Front. Mar. Sci. 10:1289931. doi: 10.3389/fmars.2023.1289931