Ecosystem structure

Feedback on biogeo-chemical cycles (elemental stoichiometry C: N:P, DOC exudation)

Tolerance thresholds likely vary between species and phyla, but still await quantification for most organisms. Due to differential sensitivities among and within organisms, a continuum of impacts on ecosystems is more likely than the existence of a well-defined threshold beyond which CO2 cannot be tolerated. Many species may be able to tolerate transient CO2 fluctuations, but may not be able to settle and thrive in areas where CO2 levels remain permanently elevated. At concentrations that do not cause acute mortality, limited tolerance may include reduced capacities of higher functions, that is added CO2 could reduce the capacity of growth and reproduction, or hamper resistance to infection (Burnett, 1997).

It could also reduce the capacity to attack or escape predation, which would have consequences for the organism's food supply and thus overall fitness with consequences for the rest of the ecosystem.

Complex organisms like animals proved to be more sensitive to changing environmental conditions like temperature extremes than are simpler, especially unicellular, organisms (Portner, 2002). It is not known whether animals are also more sensitive to extremes in CO2. CO2 affects many physiological mechanisms that are also affected by temperature and hypoxia (Figure 6.26). Challenges presented by added CO2 could lower long-term resistance to temperature extremes and thus narrow zoogeographical distribution ranges of affected species (Reynaud et al, 2003, Portner et al., 2005).

At the ecosystem level, few studies carried out in surface oceans report that species may benefit under elevated CO2 levels. Riebesell (2004) summarized observations in surface ocean mesocosms under glacial (190 ppm) and increased CO2 concentrations (790 ppm). High CO2 concentrations caused higher net community production of phytoplankton. Diatoms dominated under glacial and elevated CO2 conditions, whereas Emiliania huxleyi dominated under present CO2 conditions. This example illustrates how species that are less sensitive to added CO2 could become dominant in a high CO2 environment, in this case due to stimulation of photosynthesis in resource limited phytoplankton species (Riebesell 2004). These conclusions have limited applicability to the deep sea, where animals and bacteria dominate. In animals, most processes are expected to be depressed by high CO2 and low pH levels (Table 6.4).

6.7.4 Biological consequences for water column release scenarios

Overall, extrapolation from knowledge mostly available for surface oceans indicates that acute CO2 effects (e.g., narcosis, mortality) will only occur in areas where pCO2 plumes reach significantly above 5000 ppm of atmospheric pressure (in the most sensitive squid) or above 13,000 or 40,000 ppm for juvenile or adult fish, respectively. Such effects are thus expected at CO2 increases with ApH < -1.0 for squid. According to the example presented in Figure 6.12, a towed pipe could avoid pH changes of this magnitude, however a fixed pipe without design optimization would produce a volume of several km3 with this pH change for an injection rate of 100 kg s-1. Depending on the scale of injection such immediate effects may thus be chosen to be confined to a small region of the ocean (Figures 6.13 and 6.14).

Available knowledge of CO2 effects and underlying mechanisms indicate that effects on marine fauna and their ecosystems will likely set in during long-term exposure to pCO2 of more than 400 to 500 ppm or associated moderate pH changes (by about 0.1-0.3 units), primarily in marine invertebrates (Portner et al. 2005) and, possibly, unicellular organisms. For injection at a rate of 0.37 GtCO2 yr-1 for 100 years (Figure 6.14), such critical pH shifts would occur in less than 1% of the total ocean volume by the end of this period. However, baseline pH shifts by 0.2 to 0.4 pH-units expected from the WRE550 stabilization scenario already reach that magnitude of change. Additional long-term repeated large-scale global injection of 10% of the CO2 originating from 18,000 GtCO2 fossil fuel would cause an extension of these pH shifts from the surface ocean to significantly larger (deeper) fractions of the ocean by 2100 to 2300 (Figure 6.15). Finally, large-scale ocean disposal of all of the CO2 would lead to pH decreases of more than 0.3 and associated long-term effects in most of the ocean. Expected effects will include a reduction in the productivity of calcifying organisms leading to higher ratios of non-calcifiers over calcifiers (Portner et al., 2005).

Reduced capacities for growth, productivity, behaviours, and reduced lifespan imply a reduction in population densities and productivities of some species, if not reduced biodiversity. Food chain length and composition may be reduced associated with reduced food availability for high trophic levels. This may diminish resources for local or global fisheries. The suggested scenarios of functional depression also include a CO2 induced reduction in tolerance to thermal extremes, which may go hand in hand with reduced distribution ranges as well as enhanced geographical distribution shifts. All of these expectations result from extrapolations of current physiological and ecological knowledge and require verification in experimental field studies. The capacity of ecosystems to compensate or adjust to such CO2 induced shifts is also unknown. Continued research efforts could identify critical mechanisms and address the potential for adaptation on evolutionary time scales.

6.7.5 Biological consequences associated with CO2 lakes

Strategies that release liquid CO2 close to the sea floor will be affecting two ecosystems: the ecosystem living on the sea floor, and deep-sea ecosystem living in the overlying water. Storage as a topographically confined 'CO2 lake' would limit immediate large-scale effects of CO2 addition, but result in the mortality of most organisms under the lake that are not able to flee and of organisms that wander into the lake. CO2 will dissolve from the lake into the bottom water, and this will disperse around the lake, with effects similar to direct release of CO2 into the overlying water. According to the scenarios depicted in Figures 6.11 and 6.12 for CO2 releases near the sea floor, pH reductions expected in the near field are well within the scope of those expected to exert significant effect on marine biota, depending on the length of exposure.

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