Basin sill depth and episodic flushing influence bathypelagic nekton communities and goose-beaked whale foraging in the Southern California Bight

Imagine the deep ocean not as a flat, empty blue void, but as a giant, three-dimensional city with distinct neighborhoods, each with its own weather, traffic patterns, and residents. In this underwater metropolis, the "buildings" are the massive underwater basins—huge bowl-shaped depressions in the seafloor—separated by underwater mountain ridges called "sills." Just like a city with a low bridge might keep heavy trucks out of a specific district, these sills control which water masses can flow into the basins. Some basins are like open-air plazas where fresh, oxygen-rich water flows in freely, while others are like sealed basements where the air gets stale and oxygen-starved.

Living in these deep neighborhoods are the "mid-trophic" residents: squishy squids, tiny crustaceans, and small fish that don't live on the surface but don't live on the bottom either. They are the middle managers of the ocean food web. Hovering above them, like a highly skilled delivery driver looking for a specific package, is the goose-beaked whale. These whales are deep-diving predators that must find these scattered, patchy groups of prey to survive. The big question scientists have been asking is: what makes these underwater neighborhoods different from one another, and how do the whales know exactly where to find their dinner in the dark? This study dives into that mystery, exploring how the shape of the ocean floor and the occasional "flushing" of fresh water change the lives of these deep-sea creatures.

---

The Deep-Sea Neighborhoods and the Whale's GPS

In the Southern California Bight, a stretch of ocean off the coast of California, researchers set up a trio of underwater listening stations to solve a deep-sea mystery. They wanted to know how the shape of the ocean floor and the movement of water affect the tiny creatures that live 1,000 meters down, and how those changes impact the goose-beaked whales (Ziphius cavirostris) that hunt them.

Think of the three study sites as three different houses in a neighborhood.

• Site W is the "open-concept" house. It's the deepest and has a wide, deep doorway (a deep sill) that lets fresh, oxygen-rich water from the open ocean flow in easily.
• Site E is the "moderate" house. It has a medium-sized doorway. Sometimes fresh water flows in, but sometimes it gets a bit stuffy.
• Site H is the "basement" house. It has a very shallow doorway that blocks most of the fresh water. As a result, the water here gets very low on oxygen, creating a "hypoxic" (oxygen-starved) environment.

The scientists spent years listening to the ocean with high-tech gear. They used sound waves to "see" the tiny squids and fish (nekton) swimming around, like a bat using echolocation. They also used hydrophones to listen for the clicking sounds of the goose-beaked whales, which they use to hunt. To understand the water itself, they measured temperature, saltiness, and oxygen levels. They even took water samples to grab tiny bits of DNA (eDNA) shed by the creatures, which helped them identify exactly which families of squids were living where.

The "Flush" That Changed Everything

One of the most exciting discoveries was how these underwater neighborhoods react to "flushing events." Imagine a sudden storm that pushes a wave of fresh, cold, oxygen-rich water over the shallow doorway of a house and into the basement. In the ocean, this happens when surface winds push water away, allowing denser, colder water from deeper in the ocean to spill over the sills and flood the basins.

The researchers found that these flushing events are rare but powerful. At Site E, when a flushing event happened in October 2017, it brought in a surge of fresh Upper Circumpolar Deep Water (UCDW). Almost immediately, the "traffic" of prey animals increased. The sound waves bouncing off the squids and fish got stronger, indicating a bigger crowd. And guess what? The goose-beaked whales showed up right after. The whales' acoustic presence (their clicking) went up, suggesting they smelled the fresh water, found a buffet of prey, and came to eat.

However, this didn't happen at Site H. Because its doorway was too shallow, no flushing events occurred there. The water stayed stale and oxygen-poor. The prey community there was different, dominated by squids that could survive the low oxygen, but the whales didn't seem to flock there in the same way. This suggests that the "freshness" of the water is a key signal for the whales.

The Whale's Seasonal Calendar

The study also revealed that the whales have a strict seasonal schedule. They are most active in the late fall and winter, which is when the prey is most abundant and diverse. During the late summer (August and September), the prey population drops, and the whales seem to leave or stop hunting in that area.

It turns out the whales are very picky eaters. They don't just want any prey; they want the right kind of prey in the right conditions. The data showed that when the water density and oxygen levels hit a "sweet spot," the prey animals gather in larger groups. The whales, being smart hunters, track these changes. When the water gets too warm or too low in oxygen, the prey scatters or changes, and the whales move on.

The Big Picture: A Mosaic of Habitats

The main takeaway from this research is that the deep ocean is not a uniform, boring soup. It is a complex mosaic of different habitats. The shape of the seafloor (the depth of the sills) acts like a gatekeeper, deciding which water masses get in and which stay out. This gatekeeping determines whether a basin is a bustling, oxygen-rich city full of diverse prey or a quiet, oxygen-starved basement with a specialized, hardy community.

The study suggests that these physical barriers and the occasional "flushing" of fresh water are the architects of the deep-sea food web. They create the conditions that allow certain prey to thrive, which in turn dictates where the hungry whales go. While the researchers couldn't pinpoint every single cause-and-effect (the ocean is too complex for that), the evidence strongly suggests that if you want to find the goose-beaked whales, you need to look for the basins that are getting fresh, oxygenated water.

In short, the deep ocean is a dynamic place where the shape of the floor and the rhythm of the tides build the stage, and the whales are the actors waiting for the perfect scene to begin their hunt.