SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

9-13 October 2017

BIOMIXING IN OCEAN MOTION


NOTE: This document is a repeat of that appearing as a Supplementary Information File for last week.


The traditional explanation for ocean circulation and mixing focuses almost exclusively on physical processes. That is, ocean waters move in response to (1) the coupling between wind and water (e.g., wind-driven surface currents, Ekman transport, upwelling/downwelling), and (2) small differences in water density arising from small differences in temperature and salinity (i.e., thermohaline circulation). Marine organisms are typically portrayed as passive participants in these physical processes such as when upwelling transports life-supporting nutrients into the photic zone. Recent research findings, however, are making a convincing case that marine organisms contribute significantly to motion in the ocean.

Claims that biomixing of ocean waters could be significant prompted ocean scientist Eric Kunze and his colleagues from the University of Victoria, British Columbia, to take a close look at conditions in Saanich Inlet, a fjord along the coast of Vancouver Island. The waters of Saanich Inlet are stratified (stable) and turbulence (mixing) is about as low as in deep ocean water. In 2005, the team of scientists investigated the water's abundant krill population and their possible role in biomixing within the fjord.

Krill are shrimplike organisms, about 1 to 2 cm in length, that engage in vertical migration to avoid predation. The photic zone is where food is most plentiful but it is also a dangerous place for krill and other marine organisms because predators can easily see them. Krill avoid this threat by vertical migration. Each day at dusk, they come to the surface zone to feed on phytoplankton. As daylight comes, they return to the relative safety of darker, deep waters. These migrations, typically over a distance of about 100 m (330 ft), require expenditure of an enormous amount of energy.

The Canadian scientists lowered an instrument package in the path of the vertically migrating krill and found that during the 10 to 15 minute period when the krill passed by, turbulence increased by a factor of 2000 to 20,000. (Averaged over a full day, turbulence increased 100-fold.) Based on this finding, it appears reasonable to assume that schools of krill in the Southern Ocean surrounding Antarctica would stir water upward during their daily vertical migrations and thereby replenish nutrients in surface waters.

Calculations by other scientists indicate that turbulent mixing of waters by krill and other schooling marine organisms such as anchovies and blue whales may involve an amount of energy equivalent to half that required to mix deep, cold water to the ocean surface. Biomixing may have important implications for global climate, influencing the exchange of gases between the ocean and atmosphere. In fact, some scientists speculate that a decline in the stocks of big fish and whales could reduce biomixing to levels that may influence climate change.


Return to RealTime Ocean Portal
Prepared by J.M. Moran, Ph.D., and Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2017, The American Meteorological Society.