The giant subsurface waves the T-team are studying are triggered thousands of kilometers away. After beaming through the Southern Ocean, the waves break against the continental slope, mixing the deep ocean. But, like bath-time with a hyperactive toddler and an especially slippery rubber ducky, these waves occasionally slosh up and over the edge of the tub. In the relatively shallow waters of the adjacent continental shelf of Tasmania, a whole new set of phenomena takes place, one that ultimately influences both the growth of sea life and the carbon dioxide content of the atmosphere.
The ocean’s food web begins with the phytoplankton. These tiny, autotrophic organisms harvest energy from sunlight to produce sugars through the process of photosynthesis. The energy harvested from the sun by the uncountable swarms of natural solar cells is transferred on to the zooplankton, the tiny, heterotrophic grazers of the ocean. After chowing down on the phytoplankton, the zooplankton in turn become food for small fish, and small fish for bigger, and on and on all the way up to your tuna sandwich.
Photosynthesis is one of the great marvels of nature: the delicate, complex biochemical process ultimately responsible for 99% of life on the planet. The machinery that harvests energy from photons zipping past, called chlorophyll, as well as the precursors necessary for fixing inorganic carbon dioxide into organic carbohydrates, must be synthesized from compounds acquired from the environment. Since the demand is high in the sunlit surface ocean, those necessary nutrients are always in short supply. The rate of nutrient supply thus controls the productivity of the phytoplankton, and indirectly influences the ocean’s food web and it’s ability to take up atmospheric carbon dioxide. But what controls the supply of nutrients? That’s where the giant subsurface waves, and the T-Shelf project, come in.
In much the same way that turbulence in the ocean’s abyss mixes cold water with warm, controlling the ocean’s ability to transport heat, mixing at the boundary between the sunlit surface waters and the deeper, dark waters below controls the supply of nutrients necessary for photosynthesis. This happens because the vast, deep zones of the ocean are nutrient reservoirs, created by the biological recycling of organic material that rains down from above. These nutrients can be brought into the sunlit surface waters through a number of physical mechanisms. Recently, we have come to appreciate that an important, and poorly studied, pathway is through mixing and transport driven by breaking internal waves.
The T-Shelf program aims to understand the fate of internal tide breakers as they slosh onto the flat continental shelf. At less than 150 m, the depth of the shelf means that much of the water above receives adequate sunlight for photosynthesis. The phytoplankton there are limited by the supply of nutrients, and we think that much of the nutrient supply comes from the transport and mixing associated with these undersea breakers.
We have deployed a series of moorings to measure the breakers as they cross the continental shelf break and the continental shelf. These moorings are in many ways like the much longer and deeper moorings that make up the T-TIDE mooring array. But they differ in two significant aspects: first, the moorings carry instruments to measure the quantity of phytoplankton and the amount of sediment in the water. Second, we are measuring phenomena occurring on small scales relative to the deep array, and have instruments that measure the currents, temperature, salinity, phytoplankton, sediments, turbulence, and nutrients with very fine vertical resolution. The combination of the T-TIDE, T-Beam, and T-Shelf data will allow us to discriminate between mixing and transport from remotely generated internal waves, tracked by T-TIDE and T-Beam from south of New Zealand, and internal waves generated at the Tasman shelf break itself.
We ultimately hope to unravel the complicated puzzle of the relationship between breaking internal waves, nutrient supply, and the biological character of the local ocean offshore Tasmania. These same processes are likely to be responsible for driving the food web in many places in the ocean, and are yet another important, fundamental process in the wonderful, interconnected planet we call home.
Drew Lucas and Nicole Jones, Revelle
The Tasman Tidal Dissipation Experiment / Supported by the National Science Foundation
The Tasman Shelf Flux Experiment / Supported by the Australian Research Council and the University of Western Australia