Before gaining widespread public attention for being associated with geoengineering (and reckless entrepreneurship), iron (Fe) fertilization had mainly been a topic of intense interest among a small group of oceanographers seeking to understand how iron availability regulates primary production in areas of the ocean with high nutrient levels but low biomass. Iron is an essential micronutrient that phytoplankton require for photosynthesis, so it is believed that adding it to these so-called high nutrient, low chlorophyll (HNLC) regions can stimulate the development of large blooms. These large blooms would in turn draw down significant amounts of carbon dioxide and, when the phytoplankton die and sink to the ocean floor (or deep enough to prevent the CO 2 from reaching the surface and outgassing), sequester it for centuries or longer.

Thirteen artificial iron fertilization experiments later—and 20 years since IronEx I, the first such experiment, was successfully completed—and researchers now have a much better grasp of the processes at play during iron fertilization. Still, they have yet to fully resolve the range of factors that can influence the amount of CO 2 that is sequestered.

Some new findings now indicate that the structure of the plankton community may play a pivotal role in controlling how much organic carbon can be exported to the deep ocean. The authors and participants in LOHAFEX, a fertilization experiment recently carried out in a silicon (Si)-depleted area of the Southern Ocean, report a complete lack of fertilization-induced export of organic carbon in the absence of a robust diatom population. The results were published online earlier this week in the journal Global Biogeochemical Cycles.

Why diatoms matter

In spring, massive phytoplankton blooms fueled by nutrient-rich upwelled waters consist of multiple phases, during which a rotating cast of species sequentially dominates primary production. Diatom populations are usually among the first to explode because of their superlative ability to fix CO 2 and grow quickly. Yet they are also among the first to crash when nutrient concentrations, particularly of silicon (which they require to build their frustules, or shells), are eventually depleted. Their heavy frustules and relatively large cell size enable them to rapidly sink after they die, making them a very effective conduit for sequestering CO 2 in the deep ocean for timescales of centuries or more.

Indeed, the results of a previous experiment carried out in 2004, which was described in Nature last year, showed that a significant proportion of the biomass from a large, fertilization-induced diatom bloom had reached the ocean floor. Like the most recent fertilization experiment, the 2004 one was conducted in the Southern Ocean within a clockwise-rotating eddy core, a rotating column of seawater that brings upwelled waters to the surface. So why the major difference in their findings? It all comes down to silicon, or rather a lack of it.

No silicon, no diatoms, no export?

At the outset of their experiment, the researchers dumped several tons of iron sulfate (FeSO 4 )—equivalent to about 2 nanomolar of dissolved iron—in sulfur hexafluoride (SF 6 )-labeled seawater over an area spanning 300 square km. (Gaseous sulfur hexafluoride is commonly used to trace water masses.) Eighteen days later, they added the same amount of FeSO 4 again to sustain the bloom. They monitored this fertilized patch and another unfertilized patch in the same eddy that served as a control over a period of 39 days by monitoring the SF 6 concentration, the concentration of chlorophyll a, and the photosynthetic quantum efficiency of phytoplankton.

To assess net community production, they continually measured the seawater O 2 concentration to argon (Ar) concentration ratio. They reasoned that since Ar is an inert gas influenced only by physical factors, whereas O 2 is affected by both biological and physical factors, any changes in the seawater O 2 :Ar ratio would have to be the result of biological activity. The measurements were corrected to account for dilution with unfertilized waters, natural outgassing, and other processes that might bias the ratios one way or the other.

Measurements of thorium-234 were performed to track sinking particles resulting from biological activity and decomposition through the water column, and sediment traps were deployed inside and outside of the fertilized patch at 200 and 450 m for several days at a time to catch different-sized particles (and, yes, fecal pellets). While most of their operations went smoothly, the authors did note a few setbacks that may have slightly marred some of their results: the SF 6 outgassed in about two weeks, complicating their ability to track the fertilized patch, and they were only able to measure net community production for 30 days.

Overall, the authors found that fertilization almost doubled primary productivity and chlorophyll a concentrations within the patch and provided a measurable boost to photosynthetic quantum efficiency. Crucially, because these were low-silicon waters, diatoms never comprised more than 10 percent of the phytoplankton biomass, with small-celled flagellates making up the rest. Grazing pressure by zooplankton was very high; the researchers estimate that copepods gobbled up the equivalent of over a third of the resulting net primary productivity from the added iron. Net community production greatly increased within the patch and was consistently higher within it than outside.

The results from their thorium-234 and sediment trap measurements indicated no net fertilization-induced export, either within the upper 450 m of the water column or below 1000 m. Instead, the export flux varied as much during the first six days of the experiment as it did during the entirety of it, and there were no significant differences between the particle flux captured by in-patch versus out-patch traps.

The amount of sinking particulate matter from the surface declined significantly between the surface and 100 m and then between 200 and 450 m, presumably as a result of both bacterial and zooplankton activity—some of which may feed on the detritus. Moreover, they showed no increase in the export of the type of large-sized particles that would be needed to effectively sequester organic carbon in the deep ocean.

While the main takeaway from this study seems to be that only an iron fertilization experiment carried out in silicon-rich waters can effectively lead to long-term CO 2 sequestration, the authors note that since iron fertilization has been known to lower the silicon-to-carbon ratio of exported material—and thus sequester more organic carbon for the same quantity of silicon—their findings do not necessarily mean that all low-silicon waters would yield the same outcome.

They also add the important caveat that they had a very difficult time disentangling the effects of the small diatom population from that of the high zooplankton grazing pressure on the relatively low particle export. As is common with these studies, the more variables and parameters you have to consider, the more difficult it becomes to arrive at a definite conclusion. What is clear is that another round of studies is needed to better ascertain the effects that zooplankton community composition and activity exert on particle export and, by extension, CO 2 sequestration. Who's up for another 13 fertilization experiments?

Global Biogeochemical Cycles, 2013. DOI: 10.1002/gbc.20077 (About DOIs).

Nature, 2012. DOI: 10.1038/nature11229

Listing image by Thomas Steuer, Alfred Wegener Institute