Before any commercial application of ocean fertilization is considered, it is essential that adequate attention be given to potential unintended consequences, some of which may be deleterious to the marine environment or its users (sensu London convention and protocol) either in the short term (1-10 years) or in the longer term (centuries). Here, we can only provide a brief description of these potential side effects, but subsequently they must be explored in detail so that any benefits of sequestration can be balanced against any potential damage. There will be significant uncertainties in the scientific assessment of several of these side effects. Nevertheless, it will be necessary to estimate probabilities so that a cost-benefit-risk analysis can be carried out in a rational and well-informed manner. We identify and briefly discuss seven areas of potential side effect that will require specific attention in the future although we cannot discount the possibility that others will occur.
Defined as the detrimental response of an ecosystem to excess macronutrients, eutrophication is a coastal phenomenon of worldwide concern (Diaz et al. 2004; UNEP 2004). The key features of eutrophication of relevance here include reductions in oxygen levels, changes in phytoplankton species including development of harmful algal blooms (HABs) and a lowering of biological diversity. It is important to note that the degree to which eutrophication might occur in artificially fertilized areas of the open ocean is debatable on account of differences in circulation patterns, nutrient supply mechanisms and biological communities compared with coastal seas.
Responses of marine organisms to low oxygen are almost entirely negative (Diaz 2001; Levin et al. 2001; Cowie 2005; Domenici et al. 2007). While physiological adaptation can occur, extended exposure (more than 60 days) to anoxia leads to total mortality (Knoll et al. 2007). The likelihood of such prolonged exposure will depend on how well different parts of the deep sea are ventilated. Closer to the continental margins, artificially enhanced POC fluxes may combine with the already higher productive shelf systems to increase the risk of low-oxygen conditions in bottom waters. Such changes potentially reduce the capacity of the system to support commercial fisheries. Prolonged (more than 1 year) anoxia promotes burial of organic carbon into the long-term geological record (Hedges & Keil 1995) and may be a means to sequester carbon but the degree of success will again depend on circulation patterns and/or proximity to the higher productive shelf-ocean margin systems. However, the promotion of bottom water anoxia as a sequestration strategy has to be judged against its serious detrimental effects on marine life. Furthermore, purposefully lowering the oxygen content of waters increases the risk of enhanced release of N2O, a greenhouse gas more potent than CO2, negating any potential benefit from fertilization (Fuhrman & Capone 1991; Jin & Gruber 2003). In more extreme situations, 'sulphur eruptions' can occur and the so-called 'black tides' of H2S-laden water can cause extensive and prolonged mortality for almost all marine organisms (Weeks et al. 2002). Interestingly, susceptibility of organisms to hypoxia is also tied to temperature ranges (i.e. a thermal envelope that varies from species to species; Portner et al. 2005) suggesting the possibility of identifying, by latitude, higher and lower risk regions for fertilization.
The changes in nutrient input ratios (N: P: Si) can alter phytoplankton community composition. Enrichment of N relative to Si has been accompanied by shifts in species dominance from diatoms to dinoflagellates (see Cloern 2001) whereas the changes in N: P ratios (below Redfield) may have promoted 'nuisance' phy-toplankton species such as Phaeocystis sp. (Riegman et al. 1992). Eutrophication also causes HABs (e.g. Chrysochromulina polylepis), which in productive fishery regions has serious economic impacts (Underdal et al. 1989) and can lead to human fatalities through the consumption of contaminated shellfish (Hallegraeff 1993). Hence, tampering with natural oceanic nutrient ratios through fertilization may promote phytoplankton, which are harmful to marine life and human health.
In oligotrophic oceanic regions, artificially enhanced POC fluxes may have a positive effect on the benthic biomass (Section 8.3.7). However, closer to the productive continental shelves, increase in productivity due to eutrophication may reduce diversity in the benthos. Consequently, ocean fertilization strategies need to consider ecosystem characteristics (e.g. biological community structure) and both proximity to shallower shelf environments and circulation patterns, which can transport organic matter horizontally over large (more than 100 km) distances.
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