The study, published in Frontiers in Marine Science, draws on water-quality records collected between 2001 and 2024 at 13 fixed monitoring stations along Kuwait’s coastline, six inside Kuwait Bay and seven distributed along the open Gulf coast to the south. Chlorophyll-a, the pigment that gives phytoplankton their green color, is widely used as a proxy for phytoplankton biomass and as a practical indicator of the trophic status of coastal waters. Because phytoplankton respond rapidly to changes in nutrient availability, light, temperature and water-column stability, long-term chlorophyll-a records are among the most effective tools for detecting eutrophication and distinguishing gradual ecological trends from abrupt shifts in baseline conditions.

The numbers are striking. Between the pre-construction period of 2001–2012 and the construction years of 2013–2017, mean annual chlorophyll-a increased 2.9-fold at the Bay sites, rising from 1.29 to 3.7 milligrams per cubic meter, and 2.7-fold at the Gulf sites, rising from 1.16 to 3.08 milligrams per cubic meter. Annual means peaked in 2016, reaching 7.6 milligrams per cubic meter inside the Bay and 5.0 in the adjacent Gulf waters. Concentrations during 2021–2024 declined from those construction-era highs, but at 2.31 and 1.92 milligrams per cubic meter respectively they remained well above the pre-shift baseline. The system, in other words, did not simply return to normal once the construction cranes left.

Statistically, the pattern is better described as an abrupt regime shift than a steady climb. The researchers applied the non-parametric Mann–Kendall test and Sen’s slope estimator to annual mean chlorophyll-a in each region. Over the full 2001–2024 record, chlorophyll-a increased by 0.061 milligrams per cubic meter per year at the Bay sites and 0.051 at the Gulf sites, but the monotonic trend was statistically significant only inside Kuwait Bay, where the Mann–Kendall tau of 0.38 carried a p-value of 0.025. The Pettitt change-point test, a non-parametric method designed to detect a single abrupt shift in the mean of a time series, pinpointed a significant change at the 2012–2013 boundary in both regions. Crucially, when the pre- and post-change segments were analyzed separately, neither showed a significant internal trend. The positive full-series slopes therefore reflected a step-like transition to a new, higher baseline rather than continuous year-by-year accumulation.

What makes the finding genuinely puzzling is that the usual suspect, nutrient enrichment, does not line up with the timing of the shift. Multiple regression models fitted to log-transformed chlorophyll-a and seven co-measured environmental variables, including water temperature, salinity, dissolved oxygen, nitrate, nitrite, phosphate and total suspended solids, explained only 9.5 percent of chlorophyll-a variability at the Bay sites and 11.9 percent at the Gulf sites. Exploratory Pettitt tests applied to each environmental variable found no significant shift at the 2012–2013 chlorophyll-a boundary after correction for multiple testing. The abrupt ecological change, the authors conclude, cannot be explained by a corresponding increase in concurrent dissolved nutrient concentrations.

This nutrient–biomass decoupling is less paradoxical than it first appears, and it echoes patterns documented in coastal systems worldwide. Chlorophyll-a reflects accumulated phytoplankton biomass, whereas a water sample measures only the dissolved nutrient pool remaining after transport, biological uptake and internal recycling. During a shelf-sea spring bloom in the Celtic Sea, for example, rising chlorophyll-a was accompanied by rapid nitrate drawdown over days, illustrating how phytoplankton growth can deplete the very nutrients that fuel it. Monthly sampling may simply miss such short-lived dynamics. Internal nutrient recycling can also sustain production without producing elevated nitrate or phosphate readings, and longer water residence time can allow biomass to accumulate before it is exported from a coastal system. The negative association between nitrate and chlorophyll-a retained in the Gulf-site mixed-effects analysis is consistent with biological drawdown during bloom periods, and the positive dissolved-oxygen association likely reflects photosynthetic oxygen production by the phytoplankton themselves rather than oxygen driving biomass.

The temporal correspondence with the causeway is where the story becomes genuinely provocative. The Sheikh Jaber Al-Ahmad Causeway, a 36-kilometer crossing of Kuwait Bay built largely between 2013 and 2018 and opened in 2019, is among the longest marine crossings in the world. Independent field observations and hydrodynamic modelling show that the structure and associated land reclamation measurably altered the bay’s circulation. Surface-current speeds within 5 kilometers of the causeway declined by roughly 16 percent in summer and 12 percent in spring, an effect attributed to the densely arranged piers acting as a physical barrier to water exchange. Subsequent modelling found that the bay-wide average water residence time increased by 1.29 days, or 3.49 percent, while local effects ranged from a 66-day reduction to a 56-day increase depending on location. Complete renewal of Kuwait Bay, which already renews slowly, was estimated to require 150 to 320 days. Chlorophyll-a reached its highest recorded levels in 2015–2016, during the most intensive phase of construction.

The authors are careful to state that temporal correspondence does not establish causation, and they did not identify an independently documented abrupt regional hydrographic or climatic event at the 2012–2013 boundary. Longer-term hydrographic change provides relevant context but not a discrete trigger: the estimated freshwater contribution at a northern Gulf station declined from 25.6–42.5 percent in 1997 to 0.8–4.6 percent by 2012–2013, accompanied by a salinity increase of roughly six units between 2000 and 2013, a progressive decadal trend rather than a sudden event. Comparable engineering studies support the proposed mechanism, however. In Doha Bay, Qatar, land reclamation increased mean water residence time by 3–6 days between 2000 and 2020, tripling in some areas, while at Korea’s 33.9-kilometer Saemangeum seawall chlorophyll-a was consistently higher inside the enclosure and particularly elevated when sluice-gate operation restricted water exchange. Restricted exchange, these studies suggest, can promote phytoplankton biomass accumulation by extending the time nutrients and cells are retained within a semi-enclosed system.

The spatial pattern adds another layer of intrigue. The shift was detected not only inside Kuwait Bay but also at Gulf stations distributed along roughly 80 kilometers of open coastline, some within 14 to 27 kilometers of the Bay, indicating a coastal-scale response rather than a purely local one. This divergence from the wider Gulf is notable: satellite ocean-color analyses indicate that chlorophyll-a across the Gulf as a whole declined by approximately 16 percent between 2003 and 2021, with interannual variability governed largely by climate oscillations such as the Indian Ocean Dipole and the El Niño–Southern Oscillation. Kuwait’s coastal waters bucked that regional decline, pointing to local and subregional processes, including chronic nutrient inputs from urban and industrial sources and changes in circulation and retention, acting alongside broader climatic forcing. Because the Bay and adjacent Gulf waters form a hydrodynamically connected system, changes affecting circulation and biological retention need not remain confined within the Bay.

The implications reach well beyond Kuwait. The study demonstrates that routine water-quality monitoring can detect abrupt ecological change but cannot, on its own, disentangle the relative contributions of nutrient loading, biological processing and hydrodynamic modification. The authors argue that chlorophyll-a time series should be paired with phytoplankton community observations, continuous high-frequency measurements of chlorophyll-a, dissolved oxygen and temperature, event-based nutrient sampling, and direct measurements of currents and water exchange supported by hydrodynamic modelling. For future coastal engineering projects, monitoring should begin before construction and continue through and after it, with appropriate comparison sites, so that project-associated effects can be separated from broader regional variability. In one of the world’s most environmentally extreme seas, where summer temperatures reach 37 degrees Celsius and salinity exceeds 40 practical salinity units, the message is clear: when humans reshape the physics of a coastal sea, the biology may answer with an abrupt, persistent and difficult-to-reverse shift that no nutrient measurement alone could have predicted.

Subject of Research: Marine Science

Article Title: Long-term chlorophyll-a dynamics reveal abrupt baseline shifts in coastal waters of the northwestern Arabian Gulf

Article References: Abdulhussain, A. H., Alosaimi, W., & Ben-Hasan, A. (2026). Long-term chlorophyll-a dynamics reveal abrupt baseline shifts in coastal waters of the northwestern Arabian Gulf. Frontiers in Marine Science, 13, Article 1949281. https://doi.org/10.3389/fmars.2026.1949281

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1949281

Keywords: chlorophyll-a, Kuwait Bay, Arabian Gulf, phytoplankton, causeway, coastal engineering, regime shift, marine monitoring, Long-term, dynamics, reveal, abrupt