March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
May 22, 2001: Leg 5; Day 1
Ken Johnson writes: The Western Flyer is underway rounding Kaena Point on Oahu, the last land we (Ken Johnson, Ginger Elrod, Steve Fitzwater and Josh Plant of MBARI and Sara Tanner of Moss Landing Marine Laboratories) will see until our arrival in Moss Landing on June 2 (sooner if the engineers turn on the secret 5th engine). Our objective on this leg of the Hawaii Expedition is a study of the effects on phytoplankton from dust that falls on the surface ocean. Phytoplankton require iron to grow (it is used to make chlorophyll, reduce nitrate to ammonia and transfer electrons during photosynthesis). Low concentrations of iron in the deep sea, relative to nitrate, prevent vertical transport processes from supplying enough iron to support full utilization of upwelled macronutrients (nitrate, phosphate, silicate) by phytoplankton in the open ocean. Deep water supplies only ~70% of the iron required by phytoplankton on a global average. External inputs of iron, primarily from the atmosphere, must occur if macronutrients are to be fully utilized, which allows phytoplankton to grow to their full potential.
The major dust input to the central Pacific comes during spring when the steppes of Mongolia dry out. Massive storms sweep up this material and clouds of dust cross the Pacific to the US. When this material settles on the ocean 2 to 5% of the iron and aluminum in the dust dissolve into the water where the phytoplankton rapidly consume it and grow. These clouds of dust aerosol can be tracked by the SeaWIFS satellite. A particularly large Asian dust cloud passed across the Pacific, all the way across the US, and reached the Atlantic during mid-April of this year. You can seen a very nice animation of the April event, on the Aerosol page at the Naval Research Laboratories in Monterey.
We measured the concentration of iron and aluminum on Leg 1 of the Hawaii Expedition in late March, when the WESTERN FLYER crossed from Moss Landing to Hawaii. On this return trip, we’ll make similar measurements to understand more about the processes that control the transport of metals from the atmosphere to the ocean. Changes in aluminum, which the phytoplankton don’t use, will tell us how much dust has reached the ocean surface. Changes in iron concentration will tell us about how the phytoplankton respond to the dust input.
An international science program called Surface Ocean/Lower Atmosphere Study (SOLAS) is being designed to study these types of interactions. Our MBARI SOLAS study will generate data that will help implement these large international programs. To learn more about iron and aluminum in seawater visit the MBARI Periodic Table of Elements in the Ocean.