One of the more challenging tasks for scientists can be to parse the full range of effects born of large events. Sometimes it’s easy to pin an outcome to its cause. Last summer, for example, when temperatures soared to triple-digit highs for three days throughout the Pacific Northwest, attributing the widespread baking of intertidal life to this so-called heat dome was fairly straightforward.

Sometimes, though, it is perhaps less straightforward. Take the marine heatwave now known colloquially as "the blob." Beginning in the late fall of 2013, and persisting until 2016, a large patch of the northeast Pacific Ocean began to warm (or, more precisely, failed to cool). At its greatest extent, this warm patch encompassed millions of square miles of ocean, from Alaska and British Columbia south to California. Sea surface temperatures at their peak were almost 3 ºC above long-term averages.

At the time, the blob was the largest and longest-lasting marine heatwave in recorded history. (It would be displaced as the largest marine heatwave a few years later, in 2019.) In the years since it ended, many papers have come out exploring its impacts—physical, chemical, ecological—on the outer coast of North America. The known consequences for marine life in particular were spectacular and severe: tropical creatures showed up off the North American coast, marine birds and whales and sea lions starved and died by the thousands, and so on.

Not much had been written about the blob’s impacts on the Salish Sea, however. That hadn’t stopped people within the Salish Sea environment from speculating about the blob’s effects here. “People were blaming everything on higher water temperatures from the blob,” says Tarang Khangaonkar, the head of the Salish Sea Modeling Center at the University of Washington and a researcher with the Pacific Northwest National Laboratory. “I thought, How can you say that? It could have been a lot of things.” [Editor's note: The Salish Sea Modeling Center is housed at our affiliate organization the Center for Urban Waters.]

“Each of the blob years were marked by much higher-than-average river inputs,” Khangaonkar says. All that extra freshwater flowing out pulled more oceanwater in. So for extended period, a strong circulation regime was bringing in nutrients from the Pacific.

Khangaonkar had a way to test some of those speculations: the Salish Sea Model. As a representation of the interior marine ecosystem, the model’s digitized spaces stretch from the tip of Vancouver Island to central Oregon, and extend west out into the Pacific about to the edge of the continental shelf. But its main focus is the major waterways that make up the Salish Sea: the Strait of Juan de Fuca, the Strait of Georgia, and Puget Sound. “Over the years,” Khangaonkar says, “the model has grown to be a diagnostic tool that allows us to look at things we might not otherwise be able to test.”

What this meant for Khangaonkar and his colleagues at the University of Washington and the Washington Department of Ecology was that they could parse the blob and the Salish Sea into a suite of main effects, include those effects or remove them in turn, and see how the model responded. If it was able to reproduce observed conditions, they would perhaps have an idea of the physical mechanisms behind them.

In a recent paper in Frontiers in Marine Science, Khangaonkar presented the model’s results. What he and his colleagues found was that their model did not support some of the speculations about the blob’s effects on the Salish Sea. One phenomenon they selected for scrutiny was zooplankton. During the blob years, first phytoplankton biomass and then zooplankton biomass increased substantially — up to 14% and 18%, respectively. Zooplankton are near the base of the food web, so organisms that feed on them, such as herring, benefited, their populations rising as well. “It seemed pretty straightforward, a clear temperature effect,” Khangaonkar says.

So they ran the model, and with the blob’s conditions the model did indeed reproduce the increased zooplankton growth. Next, though, Khangaonkar and his colleagues did a sensitivity test, removing the blob’s warming effect and running the model again. That was when things got a little less clear. Even in the absence of heightened blob-like water temperatures, zooplankton abundance still increased. “That was something we definitely didn’t expect,” Khangaonkar says.

Maybe something else was at work, they thought. If a temperature increase alone could not account for rising zooplankton densities, perhaps it affected a different process — such as the way water circulates through the Salish Sea — and it was changes to that process that explained why zooplankton proliferated.

The circulation of freshwater and seawater in the Salish Sea is of course complex, but the basic dynamics are like so: despite all the rivers that flow into the Salish Sea — the Fraser, the Skagit, the Snohomish, others — most of its water comes from the Pacific Ocean. Since saltwater is denser than freshwater, water from the Pacific tend to flow in along the bottom, while lighter, fresher water flows out on the top. The Pacific typically supplies the bulk of the nutrients that drive the Salish Sea’s rich biological activity, and because the system flushes fairly regularly, in general the water quality is good.

During the blob, circulation rates had increased by 8%, with what Khangaonkar called “an immediate physiological reaction,” or boost in biological productivity. When he ran the model, the effect was reproduced, and he and his colleagues thought they had found their smoking gun: higher circulation rates, driven by influxes of warm blob water, increased the number of nutrients, which boosted zooplankton abundance. But, again, when they removed the blob’s heat forcing effect, the exchange rates between the Pacific the Salish Sea were still high; the blob wasn’t the cause. “So we were kind of left where we started,” Khangaonkar says.

So what, in the end, was the reason for changes to zooplankton and the food web if not the blob? According to the Salish Sea Model, it was the rivers. “Each of the blob years were marked by much higher-than-average river inputs,” Khangaonkar says. All that extra freshwater flowing out pulled more oceanwater in. So for extended period, a strong circulation regime was bringing in nutrients from the Pacific. Also, Khangaonkar notes, increased freshwater flow also means higher terrestrial nutrient loads, from pollution or fertilizer, which could stimulate phytoplankton growth. During blob years, then, there were more nutrients in Salish Sea, but they were coming from freshwater sources. “And that’s what the models are telling us,” Khangaonkar says. “There was no speculation, no guesswork. This is what numbers say.”

At the same time, Khangaonkar also preaches prudence, even caution. “We cannot claim to have solved the problem,” he says. The Salish Sea Model may have provided a provocative explanation for the effects of the blob — even Khangaonkar’s co-authors were a little surprised at the model’s results — but it is not the only model for the workings of the Salish Sea. Other models, once they have crunched the numbers, may suggest different hypotheses, point to different mechanisms, different outcomes. And as marine heatwaves like the blob are predicted to become both more frequent and severe — the blob itself was displaced in 2019 as the largest marine heatwave in the northeast Pacific — all of these models will be necessary to uncover the consequences of this brave new marine world.