Iron and nitrate in a coastal upwelling ecosystem: How can we assess their effects on primary production over large time and space scales?

Kenneth S. Johnson
Moss Landing Marine Laboratories

Thursday, September 24, 1998
3:00 p.m.—Pacific Forum

Low iron concentrations limit primary production rates and biomass accumulation in a variety of open-ocean environments, including the equatorial Pacific, the sub-Arctic Pacific and the Southern Ocean. Recent work by Hutchins and Bruland (Nature 393, 561-564, 1998) demonstrates that iron limitation also occurs in some upwelling ecosystems off central California. However, the extent of iron limitation in the coastal zone is not well understood due to a paucity of measurements, and we cannot easily assess its importance on broad spatial and temporal scales. Upwelling may occur by several processes and not all of these processes will impact iron distributions equally. At one extreme, waters that upwell from the inner shelf may have an excess of iron relative to nitrate, and iron is unlikely to limit biological processes in this environment. At the other extreme, open-ocean upwelling processes, similar to those occurring along the equator, bring iron-depleted and nitrate-rich water to the surface. This leads to an iron- regulated environment. Each of these environments may be found near the continental margin.

In the first half of the seminar, I will review the work we have done to assess the processes that regulate iron and nitrate concentrations in the central California region. I will then discuss the work we are doing to develop the moored sensors for iron (as well as other metals) and nitrate needed to determine the spatial and temporal scales over which each of these chemicals may regulate primary production.

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