Natural Iron Enrichment in Ocean Fronts

Ken Johnson, Francisco Chavez (MBARI)

Ocean ecosystems evolve in a manner that eludes prediction. We cannot reliably forecast the succession of phytoplankton ecosystems throughout the course of a year or from year to year. In particular, the occurrence of events such as harmful algal blooms are quite unpredictable. Without such capability, it is not possible to unequivocally assess impacts of processes such as climate change, eutrophication or introduced species on coastal ecosystems.

Recent work with trace metals has demonstrated that very small changes in the abundance of these elements can have a dramatic effect on marine ecosystems. The classic example is IronEx II, where the addition of 400 kg of iron to equatorial Pacific waters led to production of at least 1,000,000 kg of phytoplankton carbon. These results have led many oceanographers to the conclusion that ecosystems models based which include only macro-nutrient (nitrate, ammonia, phosphate and silicate) chemical components will not achieve the predictive skill needed to understand marine ecosystems. Such models are missing essential trace nutrient components that play a critical role in regulating ecosystem rates, structure and biomass.

Analytical developments have made it feasible to measure trace metal concentrations in seawater with much greater resolution and much less complicated instrumentation than ever before. Iron measurements from moorings will be feasible in the near future and other metals will rapidly follow as the basic, modular hardware needed is developed. This will allow us to study the ecosystem in a manner not possible before, and to have a realistic chance to assess the importance of metals in controlling ecosystems from the "bottom-up."

Working Hypotheses
H1: Resuspension of particulate iron is a major source of bioavailable iron. Benthic observatories and moored iron sensors are required to assess this process. Biological sensors will monitor ecosystem response.

H2: Not all upwelling processes will have a large of resuspended iron component. This phenomenon will lead to different ecosystem responses. Continuous measurements of iron and nitrate as well as high-resolution measurements of phytoplankton species or phytoplankton classes are required.

H3: Frontal boundaries are the major loci of vertical transport. Moorings do not have sufficient spatial density to monitor frontal boundaries. A major AUV component will be required to obtain the needed spatial resolution.

H4: Geographically fixed physical processes  lead to predictable variation in key midwater and benthic community properties. Such properties include sediment community oxygen consumption. An example of a geographically fixed physical process is an upwelling center fixed in one coastal location.

The initial MUSE experiment in the Monterey Bay region was designed to resolve the relative importance of dissolved iron and  particulate iron sources. It also provided high-resolution information on iron source regions needed to plan a moored observing system.

Data Index Aircraft AUV CODAR
Drifters Moorings Satellites Ships