In the spring of 2015, California sea lions began washing ashore along Southern California beaches in alarming numbers, many of them seizing, disoriented, or already dead. The culprit was domoic acid, a potent neurotoxin produced by blooms of the diatom Pseudo-nitzschia that had exploded in abnormally warm coastal waters. That toxic bloom, the largest ever recorded on the U.S. West Coast according to research published in the Proceedings of the National Academy of Sciences, was a warning. A decade later, the forces behind it have only intensified.
Marine life along the Southwest coast now faces a compounding problem: ocean temperatures that stay elevated even through winter, paired with atmospheric rivers that slam the coastline with record rainfall. Together, these twin pressures are reshaping food webs, poisoning marine mammals, and pushing fish populations into unfamiliar waters. NOAA data and peer-reviewed research show the effects go well beyond what either extreme produces on its own.
A coast that won’t cool down
NOAA tracks marine heatwaves using a standardized method developed by Hobday and colleagues, applied to the agency’s quarter-degree Optimum Interpolation Sea Surface Temperature dataset (OISST). When ocean surface temperatures exceed the 90th percentile of a 30-year baseline for a given location and season, the water is officially in heatwave territory. By that measure, the coastal ocean off the U.S. West Coast has spent an unusual amount of recent time above that threshold. NOAA’s most recent reporting available as of spring 2026 shows elevated temperatures persisting through the winter of 2025-2026, a period that historically offered marine life a thermal reprieve, according to NOAA’s marine heatwave monitoring.
The biological consequences are not hypothetical. During the 2014 to 2016 marine heatwave known as “the Blob,” larval fish communities off Southern California shifted in ways not seen in the previous 65 years of continuous monitoring, according to research published in Global Change Biology and archived by NOAA. Warm-water species surged while cooler-water species declined. Because larval abundance determines future adult populations, those shifts signaled something more lasting than a temporary reshuffling. Entire fisheries may now be on a different trajectory than the one they followed through the late 20th century.
The 2015 domoic acid crisis drove that point home with brutal clarity. Researchers linked the unprecedented toxic bloom directly to the Blob’s anomalously warm conditions, finding that the combination of a regional heatwave and favorable nutrient supply created ideal growing conditions for Pseudo-nitzschia. NOAA Fisheries identified upwelling nutrients and localized warming as the physical drivers.
“We were seeing animals come in with seizures that wouldn’t stop,” said Shawn Johnson, director of veterinary science at The Marine Mammal Center in Sausalito, California, describing the 2015 strandings in interviews at the time. Wildlife rehabilitation centers along the California coast reported sea lions and dolphins arriving with severe neurological damage consistent with domoic acid poisoning. The state’s Dungeness crab fishery was delayed for months as a precaution, costing the industry tens of millions of dollars.
By late 2024, NOAA’s National Environmental Satellite, Data, and Information Service flagged a new marine heatwave off the West Coast with characteristics reminiscent of the Blob. As of spring 2026, NOAA has not published a formal determination on whether that heatwave has fully dissipated or continues in a weakened state; satellite SST data and buoy records suggest warm anomalies have persisted in some subregions, but comprehensive ecosystem status reports covering early 2026 have not yet been released. Expected consequences of the warming have included further food-web disruption and northward or deeper migration of commercially important fish species. Fisheries managers began incorporating those projections into stock assessments and seasonal planning, anticipating that catch locations and timing would continue to shift.
When the rain meets the heat
While the ocean has been running hot, the atmosphere has been delivering water in increasingly violent bursts. In February 2024, a powerful atmospheric river struck Southern California, triggering extreme rainfall and flooding across the region. A peer-reviewed study published in npj Climate and Atmospheric Science attributed the event to converging climate drivers: El Niño, the Madden-Julian Oscillation, and a shifted North Pacific jet stream that together supercharged the atmospheric river’s intensity.
Events like that one send enormous volumes of freshwater, sediment, fertilizer runoff, and sewage into nearshore waters within hours. The influx alters salinity, smothers benthic habitats with sediment, and delivers a pulse of land-based nutrients into an environment already primed for trouble. For marine organisms already coping with heat stress, the added disruption to water chemistry can be a tipping point.
“What keeps me up at night is the combination,” said Clarissa Anderson, executive director of the Southern California Coastal Ocean Observing System, in a 2024 briefing on harmful algal bloom forecasting. “We have models that handle warm water and models that handle runoff, but we don’t yet have a model that handles both hitting at the same time.”
The concern is straightforward: nutrient-rich winter runoff flowing into abnormally warm coastal waters could fuel algal blooms of a scale and toxicity beyond what either stressor has historically produced alone. Heavy rains wash nitrogen and phosphorus into the ocean, while warm surface temperatures accelerate algal growth and strengthen water-column stratification, trapping those nutrients near the surface where blooms thrive. The mechanism is well understood in principle, but no peer-reviewed study has yet confirmed this compounding effect in the Southwest specifically. Existing bloom forecasts are calibrated to past heatwave-only events or to runoff spikes that occurred in cooler years, leaving a gap in predictive capability.
Recovery is not guaranteed
One of the more unsettling findings from the Blob era is that ecosystems did not simply snap back when temperatures returned to normal. Long-term larval fish data show that communities off Southern California did not immediately revert to their pre-heatwave composition, suggesting that a single intense marine heatwave can leave a lasting imprint on species dominance and reproductive success.
Scientists do not yet know whether repeated cycles of warm anomalies and extreme rainfall will push vulnerable species past thresholds they cannot recover from, or whether more adaptable generalist species will fill the ecological roles left behind. That uncertainty has real consequences for fisheries management, seabird colony protection, and marine mammal conservation planning along the entire Southwest coast.
The timeline of the current West Coast warming adds another layer of difficulty. NOAA has described the warm anomaly as persisting through the winter of 2025-2026, but as of April 2026, formal ecosystem status reports have not yet specified whether conditions have intensified, stabilized, or begun to ease. Satellite-based sea surface temperature products and buoy networks provide near-real-time data, but the comprehensive syntheses that managers rely on lag behind those raw measurements by weeks or months.
What forecasting tools can and cannot see
NOAA and its partners operate the California Harmful Algae Risk Mapping system (C-HARM), which combines ocean circulation modeling, satellite ocean-color data, and ecological algorithms to predict domoic acid risk in near-real time. The system represents an operational commitment to tracking bloom danger as conditions shift, though its predictions are trained on historical events and may not fully capture novel scenarios created by simultaneous marine heatwaves and extreme runoff.
The most reliable conclusions from the current body of research stay close to the documented record: marine heatwaves along the Southwest coast are becoming more frequent and persistent; these events have already reshaped larval fish communities and fueled toxic algal blooms with direct harm to marine mammals and fisheries; and powerful atmospheric rivers are delivering larger pulses of freshwater and nutrients to the coastal ocean. What remains more speculative is exactly how these forces will interact in the years ahead, which species will benefit or decline, and whether entirely new categories of harmful blooms will emerge from the overlap.
Monitoring programs that link river discharge, ocean temperature, and biological response in real time will be essential for closing those gaps. Until integrated datasets arrive, fisheries managers and conservation agencies are making decisions under partial knowledge, guided by the clearest signal the data can offer: the Southwest’s coastal ocean is changing faster than the tools built to track it.
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*This article was researched with the help of AI, with human editors creating the final content.
