Johnson and his coauthors found that the amount of oxygen being produced near the surface through photosynthesis was directly proportional to the amount of nitrate that was being consumed in deeper water.
Based on the decline in nitrate concentrations at depth, the researchers estimated how much algal growth could have taken place during the year. They found that their estimates of algal growth were very similar to algal growth rates measured during the University of Hawaii's oceanographic cruises in that part of the Pacific.
Because there is not enough sunlight for algae to grow below 100 meters, the researchers conclude that algae growing near the surface somehow obtain nitrate from deeper water, and use this nitrate to grow and reproduce. But exactly how the algae obtain these deep nutrients is still unclear.
One possible mechanism is ocean eddies. Satellite and drifter data suggest that slow, swirling eddies occasionally form hundreds of meters below the surface of the Pacific. The ISUS data demonstrate that some of these eddies can carry nitrate up to about 70 meters below the ocean surface. Yet these pulses of nitrate do not appear to reach the upper 50 meters of the water column, where most of the algae grow.
Johnson and his coauthors speculate that dormant microalgae may inhabit the waters below 100 meters. Open-ocean eddies occasionally carry these algae upward, to depths of perhaps 70 meters. At this point, the algae may consume any available nitrate and then migrate farther up into the sunlit surface waters.
Johnson suggests that testing this hypothesis will provide an interesting challenge for marine biologists. Scientists already know that some algae can swim, using tiny, whip-like flagella. Ot
|Contact: Kim Fulton-Bennett|
Monterey Bay Aquarium Research Institute