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Images: Between the Beach and the Deep Blue Sea

The gateway between the beach and the deep sea
Anything traveling from the shoreline to the ocean and vice versa—water, fish larvae, sand, pollutants—must go through the shallow inner shelf, which connects the beach to the deep ocean. There’s a whole lot of physics going on there. Take sand, for instance. As waves and tides moveback and forth, they can stir up sand and sediments in the shallow inner shelf region. That can lift particles into the water, where they can be transported horizontally by currents. How far and in which direction the sand will travel depends on several factors, including how high they are lifted, which can change with the season.

Warm water is less dense than cold water and fresher water is less dense than saltier water. In winter, frigid air cools water on the surface, making it denser. It sinks, mixing all the water below it on the way down. Winter storm winds also do lots of mixing so all of the water is the same temperature from top to bottom. With the ocean comprised of one big, thick, cold layer, sand and mud can mix throughout the whole water depth, all the way to the surface, and can potentially be carried a longer distance horizontally.

In summer, warm air and sunshine heat up the surface water, creating a layer of warmer, less dense water that stays above colder, denser water below. This layering makes it harder for water from the surface and bottom to mix; sand and mud can be trapped in the lower layer of water. Recent research has shown that this capping effect has significant implications and should not be overlooked when scientists try to predict the movement of materials through the inner shelf regions.

(Illustration by Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)

On a cold winter day, MIT/ WHOI graduate student Rachel Horwitz boarded the WHOI coastal research vessel Tioga to retrieve instruments that had been placed off Martha's Vineyard to study the dynamics of the shallow inner shelf, a gateway region that connects the beach to deeper parts of the ocean. (Photo by Rachel Horwitz, MIT/WHOI Joint Program)

Tioga is the 60-foot coastal research vessel of Woods Hole Oceanographic Institution. (Photo by Craig Marquette, Woods Hole Oceanographic Institution)

WHOI physical oceanographer Steve Lentz and research engineer Craig Marquette help retrieve the buoy atop a mooring that had been deployed in the ocean for six months. Insturments attached to mooring lines take measurements of water temperature, salinity, and pressure, particles in the water, and current speed and direction. (Photo by Rachel Horwitz, MIT/WHOI Joint Program)

Graduate student Rachel Horwitz extracts samples of sand and mud sccoped up from the seafloor with a grab sampler as part of her research examining the forces that govern how sand is transported on the inner shelf. (Photo by Anthony Kirincich, Woods Hole Oceanographic Institution)

WHOI physical oceanographer Steve Lentz holds on to an instrument on a mooring cable. Both are covered with marine life, which typically exploits the cables as a substrate to live and grow on. (Photo by Rachel Horwitz, MIT/WHOI Joint Program)