Tuesday, November 11, 2008

Cold freshwater plumes deliver nutrients to the reef

There are special places in the salty ocean off the Islands, where you can swim through plumes of cold, fresh water.

Early residents in arid areas would carry gourds to such areas, and collect drinking water, amazingly, in the ocean.

(Image: Bird's-eye perspective view of the submarine groundwater discharge exiting from Kaloko-Honokohau National Historical Park and Honokoko Harbor in West Hawai'i. The waters are made “visible” using advanced thermal infrared techniques from low flying aircraft. The huge volumes of groundwaters exiting West Hawaii are plumes of cold nutrient-rich waters that float on top of normal seawater. The flow rates and concentrations of the major nutrients (white inset) of the plume are determined by the scientist’s oceanographic studies, which are then incorporated into the large-scale surface temperature maps. The inset of the Kona coast shows the position of only the largest groundwater plumes throughout the region. Credit: Craig Glenn/ SOEST/ University of Hawai'i.)

Scientists are now learning much more about these oceanic freshwater oases, using advanced imaging techniques.

A recent paper in the journal Geophysical Research Letters reviews some of the techniques and the results. The paper, “Aerial infrared imaging reveals large nutrient-rich groundwater inputs to the ocean,” was written by Adam Johnson, Craig Glenn and Paul Lucey of the University of Hawai'i's School of Ocean and Earth Science and Technology, and William Burnett and Richard Peterson of Florida State University's Department of Oceanography.

They used low-altitude infrared photography to identify places where water temperature was different. The fresh water that flows out of the island aquifers is much colder than the ambient ocean temperature, and shows up in aerial infrared images as cool plumes that emerge from the island and eventually disperse in the warmer salty sea. Since fresh water tends to float on top of denser salty water, these sites are easy to distinguish from the air using temperature sensing equipment.

The researchers found that this is more than just a fresh water addition to the ocean. It is also an injection of nutrients into the nutrient-deprived Hawaiian ocean—which helps support marine life along the coast. In addition to using infrared techinques to check the water temperature, the authors tested the nutrient contents of water from water samples from the coastal ocean as well as from fresh and brackish wells near the shore.

Groundwater tends to have high levels of nitrogen and phosphorous, which can act as fertilizers to the nearshore marine habitats.

In areas without regular surface rivers or streams, like the dry kona coasts of the islands, the impact is even greater than where there are rivers.

“The input of nutrients to coastal environments via (submarine groundwater discharge) is disproportionately large due to its elevated nutrient load,” the authors write.

And the impact is also changing with the changing face of the landscape.

As human development of the coast expands, more and more nutrients make their way into the groundwater—and eventually into the nearshore waters. The increased nutrient load comes from things like fertilizers and septic systems.

The researchers worked in the dry Kona coast of the Big Island, where groundwater “is the only significant source of freshwater to the coastal ocean.” They found more than 30 major plumes of fresh water into the coastal ocean.

One of the classic views is at the Honokohau Small Boat Harbor. The inner part of the harbor is dominated with cold fresh water, which grows warmer and more brackish as it moves seaward. The propellers of the many boats that operate there pull warm water out of the depths and drag is to the surface, and these warm trails are visible on aerial infrared pictures.

©2008 Jan W. TenBruggencate

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