The Block Island Times

The Science of Our Waters: Oceanographic fieldwork on Block Island

Swimming through Temperature Stratification
By Susan Willis | Nov 08, 2013

Have you ever jumped in the water at one of the island’s beautiful beaches in summer, and gone far enough out so that you can’t touch the bottom? Have you ever dived down to pick up a shell or a hermit crab, and felt a shocking burst of cold water as you reached the bottom? If so, you’ve gone through a surface thermocline.

A ‘thermocline’ refers to the region in a water body between warm and cold layers. “Thermo” refers to temperature, and “cline” means change. Strong, noticeable thermoclines form in the water off Block Island during the summer, when the sun is out for many hours each day, heating the surface. The sun’s rays only reach down so far (that’s why it gets darker as you dive down), so their warming effect is less strong in deeper water. At the same time, summer breezes mix the surface layer, creating a warm “cap” on top of the cooler water below. This happens out in the open ocean, too, but most of this heat stratification (structure that’s not well-mixed) is destroyed by wind blowing across the surface.

Some water profiles were measured with a CTD, an oceanographic instrument that measures salt concentration (salinity), temperature, and depth at State Beach on Aug. 31, 2013. While walking perpendicular to the beach, out into the water about 40 feet, the CTD was dropped into the water to measure temperature and salinity with depth and distance from shore.

Why do we care?

Besides remembering to hold your breath tightly when you dive in the water on a summer’s day, there are many reasons why thermoclines are important. From a biological perspective, thermoclines are important for food and habitat for coastal species. Many plankton, which form a huge source of food for fish, like to live in the thermocline region, where they can get light (from the sun) and food (from nutrient-rich waters below). Warm surface layers, and the reduced vertical mixing caused by the temperature cap, allow egg sacks of some species to develop in a comfortable environment — these are those strange jelly blobs that appear in late summer in long lines near the shore.

From a physical and environmental perspective, thermoclines change the behavior of water mixing by essentially dampening any movement. This means that pollutants dumped near the shore (wastewater, storm runoff, drain runoff, fertilizers, etc.) don’t mix out as quickly when a thermocline is present. This means that, in some cases, pollutants are trapped near shore in high concentrations, where they are harmful to humans or marine species.

Susan Willis is a life-long summer resident and a PhD Candidate in Environmental Engineering at the University of California, Berkeley. The data in this article is from a field project on the island in August 2013, made possible through the support of Dr. Mark Stacey of UC Berkeley. You can reach Susan at

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