Search

An in situ iron-enrichment experiment near the Galapagos Islands was performed in October 1993. Here we report the theoretical and practical considerations of creating such a patch of iron-enriched surface water, as well as the strategies employed for the detection of the patch and the biological and chemical signals which developed, in an area dominated by advective processes. Physical and chemical models were used to predict the speciation, solubility, and the final concentration of iron in surface waters injected with acidic iron sulfate. A trial injection off the California coast in which 800 L of a 0.5 M FeSO4 were introduced into the ship’s wake over a 1.5 km2 area, was used to test these predictions. Iron concentrations were determined continually onboard during the initial experiment as the ship steamed in transects through the enriched patch. The results indicate excellent spatial agreement with model predictions and final concentrations that were consistent with the chemical model. However, the use of a Cartesian coordinate system during injection resulted in an extremely compressed, heterogeneous patch. Results from this preliminary experiment were then applied towards the development and implementation of the first open ocean iron enrichment experiment (IronEx I) near the Galapagos Islands in October 1993. The development and results of these methodologies are presented. In the IronEx I equatorial experiment, a Lagrangian coordinate system was established using a drogued buoy (equipped with GPS and packet radio) and the iron-enriched area (64 km2 containing 443 kg of Fe) was tagged with the inert chemical tracer sulfurhexafluoride (SF6). This strategy resulted in a fairly rectangular, homogeneous enriched patch initially detectable by both Fe and SF6 determination. Shipboard analysis and airborne observations confirmed good spatial agreement between the Lagrangian drifter and the biological and chemical signatures in the patch. Biological and chemical sampling of the enriched area showed an increase in chlorophyll, primary production, biomass and photosynthetic energy conversion efficiency relative to waters outside the patch, supporting the hypothesis that iron limits phytoplankton growth and biomass in a ‘bottom up’ manner in this area. The ability to create a coherent patch and track it over time led to this first open-ocean test of the iron hypothesis.

The idea that iron might limit phytoplankton growth in large regions of the ocean has been tested by enriching an area of 64 km2 in the open equatorial Pacific Ocean with iron. This resulted in a doubling of plant biomass, a threefold increase in chlorophyll and a fourfold increase in plant production. Similar increases were found in a chlorophyll-rich plume down-stream of the Galapagos Islands, which was naturally enriched in iron. These findings indicate that iron limitation can control rates of phytoplankton productivity and biomass in the ocean.

The springtime stratospheric ozone (O3) layer over the Antarctic is thinning by as much as 50 percent, resulting in increased midultraviolet (UVB) radiation reaching the surface of the Southern Ocean. There is concern that phytoplankton communities confined to near-surface waters of the marginal ice zone will be harmed by increased UVB irradiance penetrating the ocean surface, thereby altering the dynamics of Antarctic marine ecosystems. Results from a 6-week cruise (Icecolors) in the marginal ice zone of the Bellingshausen Sea in austral spring of 1990 indicated that as the O3 layer thinned: (i) sea surface- and depth-dependent ratios of UVB irradiance (280 to 320 nanometers) to total irradiance (280 to 700 nanometers) increased and (ii) UVB inhibition of photosynthesis increased. These and other Icecolors findings suggest that O3-dependent shifts of in-water spectral irradiances alter the balance of spectrally dependent phytoplankton processes, including photoinhibition, photoreactivation, photoprotection, and photosynthesis. A minimum 6 to 12 percent reduction in primary production associated with O3 depletion was estimated for the duration of the cruise.