Abundance & Distribution Discovery

Time series data show that the dinner rush on the Mid-Atlantic Ridge ecosystem can involve a daily upward migration of up to 500m for a diverse compliment of fauna including zooplankton, cephalopods, and fish.

In 2004, researchers from the Mid Atlantic Ridge Ecosystem (MAR-ECO) project of the Census of Marine Life deployed a self-contained echo sounder in 1000 m of water in the middle of the Northern Atlantic Ocean. This "lander" was left on the sea bottom for one year, during which it continuously recorded echo soundings from within the water column. In 2005 when the lander was recovered, its data revealed one of the most spectacular daily vertical migrations ever to be recorded in the ocean.

The data show that every night at dusk, a multitude of organisms migrate hundreds of meters vertically in the water column up to the surface waters from their daytime depths to feed, returning to the deeper waters at daybreak. Changes in light trigger this mass migration of a vast range of organisms, including zooplankton and fish. Even the smallest organisms (mm-sized) can travel this vertical journey of up to 500 m. Researchers suggest that this is a defensive behavior that serves to help these organisms avoid predators while still affording them the opportunity to take advantage of the rich food sources in the surface waters. The majority of these animals tend to stay in deeper waters, only traveling to the surface waters to feed on phytoplankton during the night when they can avoid detection.

This link to available light is so strong that it follows seasonal changes, happening earlier during the winter and later during the summer, and can even be affected by such conditions as cloud cover and the phase of the moon. Because the echo sounding equipment, in this case, was placed on the seabed, as opposed the being suspended at the surface as in previous studies, the quality of the observations were higher than any prior work. This innovation in methodology produced data of such sufficient resolution that researchers were able to pinpoint the link between changes in light and the triggering of the migration. Such information could provide further insight into the ecology of open ocean communities, specifically how organic material and energy flows between the surface waters and the deep ocean.



What: Zooplankton, fish, and other organisms can travel hundreds of meters vertically every morning and evening in order to access surface food sources while avoiding predation.
Who: MAR-ECO Scientists --> Odd Aksel Bergstad, Olav Rune Godo, and Ruben Patel
When: 2005
Where: Mid-Atlantic Ridge, North East Atlantic Ocean
How: Using a upward looking echo sounder built into a "lander" positioned on the seabed at 1000m water depth
Reference: These findings have not yet been published.

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