Less than two decades ago it became clear that the low abundance of iron in certain remote areas of the surface ocean represented an important limitation to phytoplankton growth. Over most of the temperate and tropical latitudes of the world's oceans, the main factor controlling phytoplank-ton growth rates is thought to be the availability of the nutrients nitrate and phosphate. The concentrations of both of these nutrients are extremely low in such waters, having been efficiently consumed by plankton. indeed, as originally postulated by Arthur Redfield, the molar ratio of nitrate to phosphate in the global ocean is remarkably constant at
Table 2 Annual flux of dust delivered to different ocean basins Region Dust flux (Tg yr~1)
Pacific Pacific N. Atlantic S. Atlantic N. Indian S. Indian Global
480 39 220 24 100 44 910
about 15:1, almost exactly the same as the requirements of phytoplankton for these elements. This constant ratio is probably maintained by a balance between phosphate availability and the more biochemical alternative of nitrogen fixation that is available to nitrogen-fixing plankton.
However, in certain areas of the ocean, these nutrients remain at high residual concentrations, suggesting that another factor has come into play as a limitation on growth. These regions, which are characterized by high nutrient but low chlorophyll (HNLC), became strikingly obvious once both detailed surface maps of nutrients became available (starting with the pioneering GEOSCS Program in the 1960s and later through the programs JGOFS and WOCE) (e.g., Figure 1) and also the ability to map surface water chlorophyll concentrations using satellites such as the Coastal Zone Color Scanner (Figure 2). A comparison of these figures indicates that relatively low chlorophyll concentrations are found in regions of high nutrients, especially the Southern Ocean HNLC region.
The late John Martin, of Moss Landing Marine Laboratory in California, first made the suggestion that a lack of iron inhibited phytoplankton growth in these HNLC waters. He did this using incubation experiments in which seawater samples were inoculated with small additions (1-2 nM) of iron. Several days after inoculation, considerable increases in chlorophyll and plankton growth rate were observed compared to controls. A key to the success of these experiments was the ability to collect and handle seawater samples under scrupulously clean conditions that minimized the influence of dust contamination introduced by the experimenter. From these results, Martin speculated that iron was the growth-limiting factor in HNLC waters. He also went on to claim that periods of enhanced growth during glacial times might have been a result of enhanced dust input during more arid glacial climates. Periodic inputs of such dust are recorded in the polar ice core record, and seem to correlate well with periods of low atmospheric CO2, consistent with enhanced plankton growth.
In spite of these convincing arguments, there were many skeptics. A major criticism centered on the artificiality of the small bottle incubation experiments. Grazing is also an important controlling factor on phy-toplankton populations, and small bottles would not contain a sufficient population of the larger grazers. This criticism was settled by several mesoscale iron-enrichment experiments initiated in the mid-1990s. In these, a large area typically 8 x 8 km2 was fertilized with several tonnes of iron (as FeSO4) along with an inert tracer SF6 to mark the patch of iron-fertilized water. The first two experiments, IronEx I and II, took place in the equatorial Pacific Ocean, which is mildly HNLC. However, in 2002 a group of NZ and British scientists conducted the Southern Ocean Iron Enrichment Experiment (SOIREE) in the HNLC waters of the Southern Ocean south of Tasmania.
In these experiments a dramatic increase in chlorophyll as a result of a phytoplankton bloom was observed several days after the initial infusion of iron. This was accompanied by a decrease in the CO2 equilibrium partial pressure in the water, indicating biological uptake of CO2 by plankton. More detailed examination showed that the main beneficiaries of the added iron, and thus the main source of the new chlorophyll, were large pennate diatoms such as Fragilieria kergulensis. These are not the dominant organisms under normal, low-Fe conditions. All other things being equal, the best strategy for surviving under limited iron conditions is to have as small a cell
Figure 2 Satellite map showing the annual mean chlorophyll concentration of ocean surface waters (blue indicates low values; red indicates high values). Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE.
as possible. The SOIREE iron-induced bloom was particularly intense, and evidence was still visible in chlorophyll satellite images up to 55 days after the initial iron infusion.
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