Warming Seas Shrink the Gulf of California’s Most Productive Waters, 22-Year Satellite Record Shows

The Gulf of California, the narrow sea between the Baja California peninsula and mainland Mexico that sustains nearly 70 percent of the country’s fisheries, lost a substantial share of its phytoplankton productivity during a prolonged warm episode, according to a new 22-year analysis of satellite data. Researchers led by Eduardo González-Rodríguez of CICESE’s La Paz unit examined net primary productivity (NPP) across the gulf from 2003 to 2024 and found that a pronounced negative anomaly between 2013 and 2019 reduced total production by roughly 9 percent relative to the long-term mean of 124 teragrams of carbon per year. The decline coincided with sustained ocean warming, repeated marine heatwaves, and the strong 2015–2016 El Niño, and was followed by a partial recovery between 2020 and 2024.

NPP, defined as gross primary productivity minus respiration, measures the carbon fixed by phytoplankton after accounting for their own metabolic losses. Because phytoplankton form the base of the marine food web, tracking NPP is essential for gauging how much energy is available to fish, squid, and other commercially harvested species. In situ measurements in the Gulf of California have long been scarce, particularly in winter, so the team relied on the satellite-based Vertically Generalised Production Model, which estimates carbon fixation through the sunlit euphotic zone using chlorophyll a, sea surface temperature, and light. Monthly fields at 4-kilometer resolution from Aqua-MODIS were combined with reanalysis products for subsurface variables such as mixed-layer depth, nitrate, phosphate, and salinity, which lack consistent long-term observational coverage in the region.

To organize the gulf’s complex spatial patterns, the researchers applied the k-means++ clustering algorithm to the monthly productivity data, selecting three clusters as the most parsimonious and ecologically interpretable solution using elbow and silhouette diagnostics. The resulting zones were a low-productivity zone (LPZ) with values below 900 milligrams of carbon per square meter per day, a mid-productivity zone (MPZ) between 900 and 1700, and a high-productivity zone (HPZ) above 1700. A discriminant analysis using ten environmental variables, with NPP deliberately excluded to avoid circularity, confirmed that the three classes correspond to distinct hydrographic regimes, with a low overall misclassification rate of 8.7 percent. Importantly, the zones represent dynamic productivity conditions rather than fixed geographic regions, so their spatial extent could expand and contract month to month and year to year.

The resulting map of the gulf’s productivity engine is strikingly asymmetric. The HPZ, anchored along the continental coasts of Nayarit, Sinaloa, and Sonora, around the Midriff Archipelago, and at the Colorado River mouth, occupied only about 30 percent of the gulf’s area yet contributed 61.4 percent of total NPP over the study period. The LPZ, by contrast, covered roughly 36 percent of the basin, mostly in the oligotrophic southern entrance, but supplied just 16.9 percent of the carbon. The MPZ accounted for the remaining 21.6 percent. This imbalance reflects the physical machinery of the gulf: strong along-gulf north-westerly winds from December to May drive coastal upwelling, wind-stress curl, and tidal mixing that inject nutrient-rich subsurface water into the euphotic zone, while weak southeasterly summer winds fail to break down the intense thermal stratification that develops from June to November.

Seasonality is correspondingly extreme. The cold season, from December to May, delivered 68 percent of annual NPP, with production peaking in March at 14.9 teragrams of carbon per month. The warm season, from June to November, contributed only 32 percent, bottoming out in August at 4.4 teragrams. During winter, high-productivity waters covered on average 44.3 percent of the gulf, while in summer the low-productivity zone expanded to 68.5 percent, pushing northward toward 28 degrees N. The seasonal cycle ranged overall from 642 to 3487 milligrams of carbon per square meter per day, a span that underscores how dramatically the gulf’s productive habitat swings between wind-driven winter mixing and summer stratification.

The interannual story is where the warming signal emerges most clearly. Sea surface temperature anomalies were predominantly negative from 2003 to 2013, then turned persistently positive from 2014 onward. During 2014 to 2016, the gulf logged 118, 95, and 51 marine heatwave days respectively, together accounting for 50.2 percent of all heatwave days in the entire 2003–2024 record. These years overlapped with the North Pacific Blob, a strong El Niño, and a positive phase of the Pacific Decadal Oscillation. The consequences for productivity were severe: the 2014–2016 interval recorded the lowest mean NPP of the study, about 101 teragrams of carbon per year, with 2016 marking the absolute minimum at 85 teragrams. The low-productivity zone swelled to nearly half the gulf and advanced north of 26 degrees N, while cold-season high-productivity coverage fell to as little as 22.9 percent.

The mechanism, the authors argue, was a non-linear interaction of several stressors. Sea surface temperature anomalies exceeding 2 degrees Celsius intensified thermal stratification, quantified by a density-based stratification index, and restricted the upward flux of nutrients into the euphotic zone. At the same time, the prevailing north-westerly winds weakened dramatically, dropping to about 8 meters per second against a climatological mean above 20, which crippled coastal upwelling. Warm tropical surface water intruded from the mouth of the gulf, reinforcing stratification and suppressing mesoscale activity in the south. Nutrient records showed a significant decline in phosphate alongside nitrate reductions, suggesting nitrogen–phosphorus co-limitation during the extreme warm event. The high-productivity zone retreated to the upper gulf, the mid-productivity zone shifted northward, and oligotrophic conditions reached unprecedented extents.

Statistical modeling revealed that the environmental controls on productivity differ fundamentally between zones. The team fitted generalized linear mixed models with a Gamma error distribution and log link, using mixed-layer depth, nitrate, phosphate, along-gulf wind and current components, the stratification index, and sea surface salinity as fixed effects, with month as a random intercept. Nitrate and stratification emerged as the dominant predictors overall, but with opposite signs depending on the zone. In the low-productivity zone, nitrate was negatively associated with NPP and the stratification index positively so, whereas in the mid- and high-productivity zones nitrate was strongly positive and stratification strongly negative. Phosphate mattered significantly only in the high-productivity zone, and the along-gulf wind component was positive in the low-productivity zone but negative in the high-productivity zone. These reversals highlight that a single basin-wide rule cannot describe the gulf: nutrient-driven, mixing-dependent waters respond to warming in ways that stratified, oligotrophic waters do not.

Climate indices told a subtler story. Neither the Pacific Decadal Oscillation nor the North Pacific Gyre Oscillation was significantly correlated with the spatial coverage of any productivity zone, but the multivariate ENSO index showed a weak yet statistically significant negative correlation with high-productivity coverage. La Niña years such as 2008, 2011, 2012, and 2021, with cooler seas, stronger winds, deeper mixed layers, and richer nutrient supplies, saw the high-productivity zone contribute more than 70 percent of total NPP while occupying only 35 to 44 percent of the area. The recovery from 2020 to 2024 fits this pattern: 2024 ranked fourth in the record at 152 teragrams of carbon per year, even though 2023 was the warmest oceanic year on record globally and remained slightly below the long-term mean at 111 teragrams. The authors suggest this resilience may be linked to the negative phase of the Pacific Decadal Oscillation that began in late 2019.

The longer-term implications are sobering. The study identified declining NPP trends of 50 to 100 milligrams of carbon per square meter per day per year in the upper gulf, La Paz Bay, and off the coast of Nayarit, consistent with global analyses linking ocean warming of 0.1 to 0.2 degrees Celsius per decade to falling productivity. The Gulf of California is warming faster still, at rates between 0.38 and 0.57 degrees per decade. Because anomalously warm conditions shrink the high-productivity zone and favor the expansion of oligotrophic waters, continued warming and enhanced stratification are expected to extend low-productivity conditions further into the basin, with potential consequences for the fisheries and biodiversity that depend on the gulf’s extraordinary productivity. The authors caution that further research is needed to pin down the drivers of the low-productivity expansion observed over the past decade and its downstream effects on ecosystem structure, but the 22-year baseline they have established provides a clear benchmark for monitoring what happens next.

Subject of Research: Satellite-based analysis of net primary productivity patterns, drivers, and warming-related decline in the Gulf of California from 2003 to 2024

Article Title: Net primary productivity in the Gulf of California: spatio-temporal patterns and drivers in a warming climate (2003–2024)

Article References: González-Rodríguez, E., Cervantes-Duarte, R., Acevedo-Acosta, J. D., & Gómez-Gutiérrez, J. (2026). Net primary productivity in the Gulf of California: spatio-temporal patterns and drivers in a warming climate (2003–2024). Ocean Science, 22(5), 3385-3401. https://doi.org/10.5194/os-22-3385-2026

Image Credits: AI Generated

DOI: 10.5194/os-22-3385-2026

Keywords: Gulf of California, net primary productivity, phytoplankton, marine heatwaves, El Niño, ocean warming, stratification, upwelling, satellite remote sensing, k-means clustering, nutrients, fisheries