Modification of global iron balance

It has been suggested that we should not alter the global balance of this essential trace element as it will become scarce elsewhere. The alternative view has been expressed that addition of iron to specific regions should be considered as pollution. We demonstrate below that neither of these concerns is justified.

Iron is supplied to the surface ocean via the atmospheric transport of dust and its deposition, as well as by the upwelling, entrainment or mixing of deeper waters that are relatively rich in iron and other nutrients (Watson 1997). These sources supply new iron to the euphotic zone (i.e. not acquired via recycling). Rivers and continental margin sediments are also a significant source of iron to coastal waters (Tappin 2002; Laes et al. 2003). However, uptake by coastal phytoplankton and sedimentation of the fluvial inputs are likely to render this iron supply inaccessible to oceanic phytoplankton. In the oceanic euphotic zone, iron is also recycled from living matter to sustain regenerated biological production. The currently projected iron additions by the ocean iron fertilization (OIF) industries are estimated as 10 000 tonnes per year, which is less than 0.1 per cent of the amount delivered to the ocean by dust (15.5 x 106 tonnes yr-1) or rivers (650 x 106 tonnes yr-1). Consequently, the projected OIF activities will not significantly upset the global oceanic balance of iron.

Iron is a highly reactive element, and is subject to very rapid removal through inorganic precipitation and scavenging processes, in addition to biological uptake. Iron added to the ocean by natural or anthropogenic processes will consequently be rapidly removed from the ocean surface waters. A consequence of this strong removal mechanism is that oceanic surface waters are depleted of iron, with increasing concentrations with depth (Measures et al. 2008). Iron added through OIF will hence be rapidly removed from the surface waters (1-5 months; Sarthou et al. 2003), and continual additions would be required to replenish iron concentrations.

8.3.5 Generation of other climate-relevant gases (greenhouse/cloud forming)

It is important to consider ocean fertilization in the context of radiative forcing and not simply in terms of the carbon cycle. Carbon dioxide is only partially responsible for greenhouse warming and although this gas is intimately linked to the biological production of the oceans, others are also controlled to a large extent by the biological and chemical processes taking place in the oceans. Some of these processes increase radiative forcing while others cause a reduction and, in Table 8.2, we provide an overview of the processes involved in the budgets of these various gases and the factors that are likely to be affected by ocean fertilization.

Fluxes are potentially large particularly if the anticipated decrease in oxygen concentration is sufficient to generate larger quantities of methane and nitrous oxide. The interactions are complex and not well constrained with potentially a number of both positive and negative feedbacks. The critical research now needed is to determine and model the production rates of these gasses in response to fertilization and hence to determine the influence on greenhouse forcing.

8.3.6 Change to pelagic ecosystem structure

While the purpose of ocean fertilization is to enhance carbon sequestration, one probable consequence is a change to the structure and function of the biological communities especially in the euphotic zone. These changes may affect fisheries directly or indirectly or they may alter the details of the export process such as by modifying the characteristics of the settling particles produced by the euphotic zone communities (chemical composition, sinking rates and palatability for sub-euphotic zone communities, etc.).

Approximately 1.3 billion people depend on fisheries for a major part of their sustenance and economic welfare so it is appropriate we should consider ecosystem changes that might be a consequence of ocean fertilization and which might affect

Table 8.2 Gases and aerosols affecting the radiative balance of the Earth, their current effects, the fluxes to and from the ocean and the ways in which ocean fertilization are likely to alter their influence.

Ocean to Radiative atmosphere forcing supply rate (Wm-2) (molyr-1)

Factors causing increase or decrease

References

Methane 0.5 Halocarbons 0.3

Ozone

Nitrous oxide

Aerosols (direct)

(albedo)

8.0 x 1012

Greater than 1 x 1011 (summation of various compounds)

1.2 1011

6.9 1011

Increased sequestration and carbon export will reduce forcing but not well constrained Anoxia increases production

Enhanced production due to phytoplankton metabolic processes. Bromo and chloro compounds increase forcing. Iodine compounds may lead to increases in aerosols and albedo enhancing cooling (cf. DMS) Reduction in stratospheric ozone due to increased halocarbons will reduce its negative effect on global warming. Conversely depletion of tropospheric ozone will reduce its radiative forcing Increase forcing due to biological production by phytoplankton Any increase in sea salt input will increase aerosol production

IPCC (2001)

Houweling et al. (2000)

Harper (2000), Quack & Wallace (2003)and Smythe-Wright et al. (2006)

Solomon et al. (1994), Dvortsov etal. (1999) and Vogt etal. (1999)

Jin & Gruber

IPCC (2001)

such human communities either negatively or positively. Although most fisheries are on the continental shelves and the ocean fertilization schemes we discuss are in oceanic areas, it has been claimed by enthusiasts of ocean fertilization that such schemes will inevitably enhance both carbon sequestration and fisheries yield. Computational models have to date been extremely poor at predicting community structure and in spite of the massive efforts over the past century at providing accurate predictions of fish yield, the uncertainties are usually very large even in relatively well-constrained coastal environments. This hope of double benefit seems optimistic.

There are various examples where environmental change appears to have caused alterations in community structure. For example, it has been suggested that jellyfish replace bony fish in some ecosystems in response to climate change (Mills 2001; Purcell et al. 2007). Elsewhere, for example, the salp Salpa thompsoni appears to be replacing Antarctic krill in the Southern Ocean (Atkinson et al. 2004). Similarly, the decline in the cod population of the North Sea is thought to be due largely to subtle changes in the timing of the zooplankton communities that are the staple diet of juvenile cod (Beaugrand et al. 2003). This latter case is a classic example of the match-mismatch hypothesis whereby the food for larval growth and hence adult recruitment is required at precisely the correct time (Cushing 1975). Similarly, changes in global environmental indicators such as the North Atlantic Oscillation (NAO) or the El Nino Southern Oscillation (ENSO) have been shown to elicit ecosystem changes albeit ones that are hard to predict (Stenseth et al. 2002).

In addition to the direct effects on fisheries, indirect impacts such as by the promotion of HABs should not be ignored. As described above, HABs sometimes occur in response to coastal eutrophication and although unlikely to become a feature of fertilization of the open ocean they provide examples of major community changes that are demonstrably difficult to predict with confidence (Cloern 2001). The possibility that ocean fertilization will elicit comparable effects cannot be ruled out although we think it unlikely.

As mentioned above, ecosystem changes in response to ocean fertilization may also affect the nature of the export process. The biological pump is mediated by the members of the euphotic zone community and changes to that community will necessarily change the nature of the settling particles in terms of their morphology (e.g. marine snow aggregates versus faecal pellets as the principal vehicles for flux) or the chemical composition of the particles affecting, for instance, the Redfield ratio of these particles and the balance between the production of POC and PIC.

It is widely accepted that changes in nutrient input ratios (N: P: Si) affect phytoplankton community composition (Arrigo 2005). For example, long-term regime shifts in species dominance from diatoms to dinoflagellates in the North Sea are thought to be a reflection of nitrogen enrichment relative to silicon (see Cloern 2001) whereas changes in N: P ratios (below Redfield) may have promoted undesirable phytoplankton species such as Phaeocystis sp. in northwest European coastal waters (Riegman et al. 1992).

Similarly during the natural fertilization CROZEX project, iron fertilization had the somewhat unexpected result of increasing the abundance, diameter and biomass of the colonial forms of Phaeocystis antarctica which proved both unpalatable to mesozooplankton and were inefficiently exported (Lucas et al. 2007).

Our conclusion is that ocean fertilization is likely to change pelagic ecosystem structure and function. This may have a direct effect on fisheries and will certainly modify the details of the biological pump. The types of change will depend heavily on the proposed method of fertilization but a clear conclusion about either of these is not possible until the large-scale fieldwork and associated modelling has been completed.

8.3.7 Change to benthic ecosystem structure

Approximately 0.4 Gt of carbon is deposited on the abyssal seafloor each year, the end member of the biological pump (Jahnke 1996). Of this, approximately 96 per cent dissolves or is remineralized each year to DIC and hence influences air-sea CO2 exchange on a timescale of a few centuries (Tyson 1995). The remaining 4 per cent is buried and incorporated into the geological sediment and hence removed from atmospheric interaction for many millions of years. The processes that determine the proportion of the sedimented material that is buried are largely driven by the benthic biota and it is therefore of importance to determine potential effects on this community. With this in mind, it will be possible to estimate the effects of ocean fertilization on sequestration on the centennial timescale agreed upon by the IPCC and on the much longer timescales of geological significance. From the strict perspective of the 100-year timescale we are considering here, the effects of changes to the benthic communities can probably be ignored.

The abundance, biomass and diversity of the deep-sea benthos are intimately linked to inputs of organic matter from the euphotic zone (Gage & Tyler 1991). In general, there is a decrease in benthic biomass and abundance with decreasing organic carbon flux (Figure 8.5a) (Rowe 1983; Rex et al. 2006). Diversity generally increases from regions of low to moderate productivity, and then declines towards regions of higher productivity (Figure 8.5b). The response of the benthos to increases in organic carbon inputs will therefore depend on where it sits on this continuum. In the characteristically low productivity oligotrophic gyres where ocean fertilization has been suggested, it is likely that enhanced POC fluxes to the seafloor would result in increased biomass and abundance (Glover et al. 2002; Hughes et al. 2007) and enhanced diversity (Levin et al. 2001). This change in the assemblages may influence ecosystem functioning (Sokolova 2000; Danovaro et al. 2008). However, the relationship between POC fluxes and benthic response is not simple; for example, recent changes in megafaunal species dominance in the abyssal North Atlantic (Billett et al. 2001) appear to be related to changes in the

oligotrophia eutrophic

POC llux

Figure 8.5 (a) The relationship between estimated POC flux and wet weight biomass and abundance of the deep-sea macrobenthos in the western North Atlantic (adapted from Johnson et al. 2007). (b) Schematic showing the pattern of diversity change with POC flux (adapted from Levin et al. 2001).

oligotrophia eutrophic

POC llux

Figure 8.5 (a) The relationship between estimated POC flux and wet weight biomass and abundance of the deep-sea macrobenthos in the western North Atlantic (adapted from Johnson et al. 2007). (b) Schematic showing the pattern of diversity change with POC flux (adapted from Levin et al. 2001).

composition of the organic matter (Wigham et al. 2003), and not simply to changes in total export flux (Lampitt et al. 2001).

Benthic ecosystems are in a complex state of dynamic equilibrium. While this equilibrium may be altered by enhanced fluxes (e.g. seasonal phytodetritus; Beaulieu 2002), after the period of fertilization has ceased, the system may revert to the earlier equilibrium. It is not clear what will happen to the carbon that was contained in the increased biomass; some of this may be incorporated into the geological record although the majority will be released into the water column by remineralization.

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