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Stanford Engineers find that certain types of underwater waves may harm deep-water organisms

Voluminous underwater waves known as “internal bores" can create major decreases in dissolved oxygen in water near the shore.

Monterey Bay is often smooth as a mill pond, but this surface tranquility cloaks a dynamic environment where physical oceanography and marine biology intersect in fascinating ways. Teasing out the individual forces and their effects is challenging, but Stanford Civil and Environmental Engineering Department Chair Stephen Monismith and his team have made impressive headway.

They discovered that voluminous underwater waves known as “internal bores” can have a dramatic impact on Monterey Bay’s marine productivity, potentially affecting the abundance and distribution of a variety of organisms. The research implies similar impacts for other coastal zones where such bores may occur.

Their findings were published in a recent issue of the Journal of Geophysical Research, with former Stanford PhD student Ryan Walter (now an assistant professor of physics at Cal Poly San Luis Obispo) as the lead author and co-authors Monismith, C. Brock Woodson, an assistant professor of engineering at the University of Georgia, and Paul Leary, a graduate student at Stanford’s Hopkins Marine Station.

It has long been know that upwelling – the wind-driven transport of deep, cold water to inshore areas – stimulates marine productivity along the California coast. This water is rich in nutrients that are consumed by phytoplankton, which in turn nourish zooplankton that are then eaten by anchovies, krill, sardines and juvenile rockfish. These species support, either directly or indirectly, everything from salmon to seabirds to marine mammals.

But upwelled water is often low in dissolved oxygen, which fish and marine invertebrates – including crabs, abalone, clams and octopi – need to survive. Over relatively long periods of time, upwelling can reduce oxygen levels in nearshore areas, though such conditions are seldom severe enough to kill local organisms.

Sometimes, though, Monterey Bay’s nearshore water registers alarmingly low dissolved oxygen levels – so low it can harm bottom-dwelling organisms such as abalone. This seems linked to intense but relatively short periods of oxygen-deficient water that sometimes occur during periods of low upwelling. And that’s counterintuitive. Since low oxygen levels are associated with strong upwelling, why should such events spike during upwelling lulls? Enter the internal bore.

Bores are abrupt increases in volume that can form along a fluid’s surface. Tidal bores – where the leading edge of an incoming tide forms a wave as it travels up a river against a current – are the most familiar variety.

“But bores aren’t confined to the ocean’s surface,” said Monismith, Obayashi Professor in the School of Engineering. “They can also form along the marine thermocline – the division line between relatively warm, mixed water and denser, colder, more stable water.”

Monismith said thermocline depth varies more in temperate than tropical waters and moves about with some frequency in the seas off Monterey Bay. He noted that the region’s thermocline is relatively close to the surface during strong winds and upwelling, allowing cold, oxygen-deprived water to “pool up” in nearshore areas for long periods; usually, though, enough oxygen remains to sustain life. But when the wind slackens and upwelling weakens, the thermocline deepens. This allows strong internal bores to push large volumes of water that are very low in dissolved oxygen toward the shore.

“We observed that individual bores can create major drops in dissolved oxygen (in nearshore waters) in very short periods of time,” he said.

underwater flux tower containing oceanic instruments

A Stanford Engineering team used used flux towers like this one to record dissolved oxygen levels, temperatures, layer stratification, currents and turbulence in Monterey Bay’s water column. (Photo courtesy of Stephen Monismith)

Fish may be able to avoid these dead zones created by internal bores, Monismith said, but bottom-dwelling organisms such as abalone, sea anemones, sea urchins and crabs probably have a harder time of it. In simple terms, they can’t get out of the way quick enough and may perish en masse unless they have other ways to cope. These responses are areas of ongoing research at Hopkins and elsewhere aimed at clarifying the impact internal bores have on the ecosystem as a whole.

Further, the bores could depress the reproductive ability of bottom-dwelling creatures. Abalone, for example, propagate by releasing ova and sperm into the water, which conjoin into zygotes. These eventually form free-swimming larvae that later settle into bottom-dwelling, hard-shelled mollusks.

“The problem is that abalone are already at low densities,” Monismith said. “And the lower the density, the less chance you have for successful spawning – things get tough when individual animals are too far apart.” Strong internal bores, he continued, could either smother the slow-moving larvae with water low in dissolved oxygen or increase water velocities to a degree that make it difficult for sperm and ova to hook up.

Internal bores have been known for many years, and they have been the subject of considerable research. But the laboratory work generally used highly simplified models, and most of the field work focused on relatively deep continental shelf waters. The impact of internal bores propagating from the continental shelf to the shallow-water nearshore environment has remained largely a mystery.

One thing, however, is clear: While internal bores are a globally-distributed phenomenon, their strengths and configurations vary widely.

“You need strong (water column) stratification at the right depths for internal bores to form,” Monismith said, noting that Monterey Bay seems to be a sweet spot for them. He observed that Monterey Canyon – the giant complex of subterranean gorges that lies just offshore – could be a key factor in internal bore formation.

Conducting field research on Monterey Bay’s internal bores isn’t easy. That’s because it’s underwater research. The seas are cold and often turbid, the currents can be strong, and creatures such as great white sharks are poking around. Monismith and crew had to put in a lot dive time installing oceanographic instruments including a flux tower, an odd-looking contraption that was essential to their experiments.

“We had anyone in our department who had a dive certification putting on a wet suit and getting in the water,” Monismith said. “It was a challenging project.”

Flux towers are frames designed to support various sensors and recording mechanisms, and they generally are used by land-based researchers to measure turbulent fluxes of greenhouse gases such as carbon dioxide or methane. Monismith’s team used the towers to record dissolved oxygen levels, temperatures, layer stratification, currents and turbulence in the bay’s water column.

Along with identifying the role internal bores play in moving water low in dissolved oxygen toward the shore, the researchers noticed other things, most notably the effect the bores seem to have on the bay’s signature kelp forests.

“When an internal bore comes through, it really creates turbulence in the kelp, mixing up detritus and plankton and changing temperatures,” Monismith said. “You see a lot of baby rockfish darting around gobbling up bits of this and that. So that might be a positive effect for marine productivity. There’s room for a lot more research there.”

For Monismith, the project seems to confirm the old saw that still waters run deep. Events that may register only subliminally on the human senses can have a profound effect on large-scale geophysical processes.

“Sometimes when you’re on the surface, you see these funny little foam lines moving around,” he said. “We’ve come to take those as a pretty good sign of an internal bore. That’s what is so fascinating about this work – we really didn’t have a sense of what was going on until we looked.” The research was supported by the National Science Foundation and Stanford’s Woods Institute for the Environment.

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