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while increasing the volume of water experiencing a lesser ApH. Further examples indicating the spatial extent of ocean chemistry change from added CO2 are represented in Figures 6.11, 6.12, 6.13, 6.14, and 6.15.

On evolutionary time scales most extant animal life has adapted to, on average, low ambient CO2 levels. Accordingly, extant animal life may rely on these low pCO2 values and it is unclear to what extent species would be able to adapt to permanently elevated CO2 levels. Exposure to high CO2 levels and extremely acidic water can cause acute mortality, but more limited shifts in CO2, pH, and carbonate levels can be tolerated at least temporarily. Studies of shallow water organisms have identified a variety of physiological mechanisms by which changes in the chemical environment can affect fauna. These mechanisms should also apply to organisms living in the deep ocean. However, knowing physiological mechanisms alone does not enable full assessment of impacts at ecosystem levels. Long-term effects, for intervals greater than the duration of the reproduction cycle or the lifespan of an individual, may be overlooked, yet may still drastically change an ecosystem.

Species living in the open ocean are exposed to low and relatively constant CO2 levels, and thus may be sensitive to CO2 exposure. In contrast, species dwelling in marine sediments, especially in the intertidal zone, are regularly exposed to CO2 fluctuations and thus may be better adapted to high and variable CO2 concentrations. Physiological mechanisms associated with CO2 adaptation have been studied mostly in these organisms. They respond to elevated CO2 concentrations by transiently diminishing energy turnover. However, such responses are likely become detrimental during long-term exposure, as reduced metabolism involves a reduction in physical activity, growth, and reproduction. Overall, marine invertebrates appear more sensitive than fish (Portner et al., 2005).

CO2 effects have been studied primarily in fish and invertebrates from shallow waters, although some of these cover wide depth ranges down to below 2000 m or are adapted to cold temperatures (e.g., Langenbuch and Portner, 2003, 2004). Some in situ biological experiments used CO2 in the deep ocean (See Box 6.6).

6.7.2 Physiological effects of CO2

6.7.2.1 Effects of CO2 on cold-blooded water breathing animals

Hypercapnia is the condition attained when an organism (or part thereof) is surrounded by high concentrations of CO2. Under these conditions, CO2 enters the organisms by diffusion across body and especially respiratory surfaces and equilibrates with all body compartments. This internal accumulation of CO2 will be responsible for most of the effects observed in animals (reviewed by Portner and Reipschlager, 1996, Seibel and Walsh, 2001, Ishimatsu et al., 2004, 2005; Portner et al., 2004, 2005). Respiratory distress, narcosis, and mortality are the most obvious short-term effects at high CO2 concentrations, but lower concentrations may have important effects on longer time scales. The CO2 level to which an organism has acclimated may affect its acute critical CO2 thresholds, however, the capacity to acclimate has not been investigated to date.

6.7.2.2 Effects of CO2 versus pH

Typically, tolerance limits to CO2 have been characterized by changes in ocean pH or pCO2 (see Shirayama, 1995; Auerbach et al., 1997). However, changes in molecular CO2, carbonate, and bicarbonate concentrations in ambient water and body fluids may each have specific effects on marine organisms (Portner and Reipschlager, 1996). In water breathers like fish or invertebrates CO2 entry causes immediate disturbances in acid-base status, which need to be compensated for by ion exchange mechanisms. The acute effect of CO2 accumulation is more severe than that of the reduction in pH or carbonate-ion concentrations. For example, fish larvae are more sensitive to low pH and high CO2 than low pH and low CO2 (achieved by addition of HCl with pCO2 levels kept low by aeration; Ishimatsu et al, 2004).

CO2 added to sea water will change the hydrogen ion concentration (pH). This change in hydrogen ion concentration may affect marine life through mechanisms that do not directly involve CO2. Studies of effects of lowered pH (without concomitant CO2 accumulation) on aquatic organisms have a

Box 6.6 In-situ observations of the response of deep-sea biota to added CO2.

In-situ experiments concerning the sensitivity of deep and shallow-living marine biota to elevated carbon dioxide levels have been limited in scope. Significant CO2 effects have been observed in experiments, consistent with the mechanisms of CO2 action reported in Section 6.7.2. Some animals avoid CO2 plumes, others do not.

Studies evaluating the behaviour and survival of deep-sea animals exposed to liquid CO2 or to CO2-rich sea water have been performed on the continental slope and rise off California. Experiments in which about 20-70 kg of liquid CO2 were released in small corrals on the sea floor at 3600 m depth were used to measure the response of animals that came in contact with liquid CO2, and to the dissolution plume emanating from CO2 pools (Barry et al., 2004). Larger bottom-living animals collected from the sea floor were held in cages and placed at distances of 1-50 m from CO2 pools. In addition, organisms living in the sediment were collected at a range of distances from CO2 pools, both before CO2 release and 1-3 months later.

The response of animals to direct contact with liquid CO2 varied among species. Sea cucumbers (holothurians like Scotoplanes sp.) and brittle stars (ophiuroids, unidentified species) died immediately after contact with liquid CO2 (Barry et al., 2005). A few individuals (<5 individuals) of deep-sea fish (grenadiers, Coryphaenoides armatus) that approached CO2 pools and made contact with the fluid turned immediately and swam out of view. Other deep-sea experiments (Tamburri et al. 2000) evaluating the behavioural response of animals to a saturated CO2 / sea water solution have shown that some scavenger species (deep-sea hagfish) will not avoid acidic, CO2-rich seawater if chemical cues from decaying bait are also present. In fact, hagfish would maintain contact with the CO2-rich / bait-scented plume long enough to be apparently 'narcotized' by the CO2.

Survival rates of abyssal animals exposed to CO2 dissolution plumes in these experiments varied with the range of pH perturbation and the distance from the CO2 source. Abyssal animals held in cages or inhabiting sediments that were near (<1 m) CO2 pools, and which were exposed episodically to large pH reduction (1-1.5 pH units) experienced high rates of mortality (>80%). Animals affected included small (meio-)fauna (flagellates, amoebae, nematodes; Barry et al., 2004) and larger (macro and mega-)fauna (Ampeliscid amphipod species, invertebrates like holothurians, echinoids, and fish like macrourids). Other fish like eelpout (zoarcids), however, all survived month-long exposure to episodic pH shifts of about -1.0 pH units. Animals held further (3-10 m) from CO2 pools were exposed to mild episodic pH reductions (about 0.1 - 0.2 pH units) exhibited mortality rates were (about 2050%) higher than at control sites (Barry et al., 2005).

It is unknown whether mortality was caused primarily by short-term exposure to large pH / CO2 shifts or by chronic, milder pH perturbations. Tidal variation in current direction resulted in a highly variable exposure to pH perturbations with the most intense exposure to dissolution plumes when the current was flowing directly towards the study animals. During other tidal periods there was often no pH reduction, increasing the difficulty of interpreting these experiments.

Three controlled in-situ experiments were carried out at 2000 m in the Kumano Trough using a specially designed chamber (Figure 6.24; Ishida et al. 2005) to address the impact of 5,000 and 20,000 ppm rises in pCO2 (with resulting pHs of 6.8 and 6.3) on the abundance and diversity of bacteria and of small animals (nano- and meiobenthos). Significant impacts of elevated pCO2 on meiobenthic organisms could not be found except for one case where the abundance of foraminifera decreased significantly within 3 days at 20,000 ppm. The abundance of nanobenthos decreased significantly in most cases, whereas the abundance of bacteria increased at 20,000 ppm (Figure 6.25).

In-situ studies of short-term effects of elevated CO2 concentrations on deep-sea megafauna have been conducted using CO2 released naturally from the Loihi Seamount (Hawaii) at depths of 1200 to 1300 m (Vetter and Smith, 2005). A submersible was used to manipulate baited traps and bait parcels in Loihi's CO2 plume to explore the effects of elevated CO2 on typical deep-sea scavengers. Vent-specialist shrimp were attracted to the bait and proved to be pre-adapted to the high CO2 levels found close to volcanic vents. Free swimming, amphipods, synaphobranchid eels, and hexanchid sharks avoided open bait parcels placed in the CO2 plumes

Figure 6.24 Experimental chamber going to the sea floor (Ishida et al. 2004). The bottom part houses a chamber that penetrates into the sediment. The top part houses electronics, pumps, valves, and water bags, that are used to control the CO2 concentration inside the chamber, and to sample sea water in the chamber at designated times. At the time of recovery, the bottom of the chamber is closed, weights are released, and the system returns to the surface of the ocean using buoyancy provided by the glass bulbs (yellow structures around the top).

Figure 6.24 Experimental chamber going to the sea floor (Ishida et al. 2004). The bottom part houses a chamber that penetrates into the sediment. The top part houses electronics, pumps, valves, and water bags, that are used to control the CO2 concentration inside the chamber, and to sample sea water in the chamber at designated times. At the time of recovery, the bottom of the chamber is closed, weights are released, and the system returns to the surface of the ocean using buoyancy provided by the glass bulbs (yellow structures around the top).

Figure 6.25 Preliminary investigations into the change of bacteria, nanobenthos and meiobenthos abundance after exposure to 20,000 and 5,000 ppm CO2 for 77 to 375 hr during three experiments carried out at 2,000 m depth in Nankai Trough, north-western Pacific. Error bars represent one standard deviation (Ishida et al. 2005).

Figure 6.25 Preliminary investigations into the change of bacteria, nanobenthos and meiobenthos abundance after exposure to 20,000 and 5,000 ppm CO2 for 77 to 375 hr during three experiments carried out at 2,000 m depth in Nankai Trough, north-western Pacific. Error bars represent one standard deviation (Ishida et al. 2005).

long history, with an emphasis on freshwater organisms (Wolff et al., 1988). Observed consequences of lowered water pH (at constant pCO2) include changes in production/productivity patterns in algal and heterotrophic bacterial species, changes in biological calcification/ decalcification processes, and acute and sub-acute metabolic impacts on zooplankton species, ocean bottom species, and fish. Furthermore, changes in the pH of marine environments affect: (1) the carbonate system, (2) nitrification (Huesemann et al., 2002) and speciation of nutrients such as phosphate, silicate and ammonia (Zeebe and Wolf-Gladrow, 2001), and (3) speciation and uptake of essential and toxic trace elements. Observations and chemical calculations show that low pH conditions generally decrease the association of metals with particles and increase the proportion of biologically available free metals (Sadiq, 1992; Salomons and Forstner, 1984). Aquatic invertebrates take up both essential and non-essential metals, but final body concentrations of metals vary widely across invertebrates. In the case of many trace metals, enhanced bioavailability is likely to have toxicological implications, since free forms of metals are of the greatest toxicological significance (Rainbow, 2002).

6.7.2.3 Acute CO2 sensitivity: oxygen transport in squid and fish

CO2 accumulation and uptake can cause anaesthesia in many animal groups. This has been observed in deep-sea animals close to hydrothermal vents or experimental CO2 pools. A narcotic effect of high, non-determined CO2 levels was observed in deep-sea hagfish after CO2 exposure in situ (Tamburri et al., 2000). Prior to anaesthesia high CO2 levels can exert rapid effects on oxygen transport processes and thereby contribute to acute CO2 effects including early mortality.

Among invertebrates, this type of CO2 sensitivity may be highest in highly complex, high performance organisms like squid (reviewed by Portner et al., 2004). Blue-blooded squid do not possess red blood cells (erythrocytes) to protect their extracellular blood pigment (haemocyanin) from excessive pH fluctuations. Acute CO2 exposure causes acidification of the blood, will hamper oxygen uptake and binding at the gills and reduce the amount of oxygen carried in the blood, limiting performance, and at high concentrations could cause death. Less oxygen is bound to haemocyanin in squid than is bound to haemoglobin in bony fish (teleosts). Jet-propulsion swimming of squid demands a lot of oxygen. Oxygen supply is supported by enhanced oxygen binding with rising blood pH (and reduced binding of oxygen with falling pH - a large Bohr effect3). Maximizing of oxygen transport in squid thus occurs by means of extracellular pH oscillations between arterial and venous blood. Therefore, finely controlled extracellular pH changes are important for oxygen transport. At high CO2 concentrations, animals can asphyxiate because the blood cannot transport enough oxygen to support metabolic functions. In the most active open ocean squid (Illex illecebrosus), model calculations predict acute lethal effects with a rise in pCO2 by 6500 ppm and a 0.25 unit drop in blood pH. However, acute CO2 sensitivity varies between squid species. The less active coastal squid (Loligo pealei) is less sensitive to added CO2.

In comparison to squid and other invertebrates, fish (teleosts) appear to be less sensitive to added CO2, probably due to their lower metabolic rate, presence of red blood cells (erythrocytes containing haemoglobin) to carry oxygen, existence of a venous oxygen reserve, tighter epithelia, and more efficient acid-base regulation. Thus, adult teleosts (bony fish) exhibit a larger degree of independence from ambient CO2. A number of tested shallow-water fish have shown relatively high tolerance to added CO2, with short-term lethal limits of adult fish at a pCO2 of about 50,000 to 70,000 ppm. European eels (Anguilla anguilla) displayed exceptional tolerance of acute hypercapnia up to 104,000 ppm (for review see Ishimatsu et al., 2004, Portner et al., 2004). The cause of death in fish involves a depression of cardiac functions followed by a collapse of oxygen delivery to tissues (Ishimatsu et al., 2004). With mean lethal CO2 levels of 13,000 to 28,000 ppm, juveniles are more sensitive to acute CO2 stress than adults. In all of these cases, the immediate cause of death appears to be entry of CO2 into the organism (and not primarily some other pH-mediated effect).

3 The Bohr Effect is an adaptation in animals to release oxygen in the oxygen starved tissues in capillaries where respiratory carbon dioxide lowers blood pH. When blood pH decreases, the ability of the blood pigment to bind to oxygen decreases. This process helps the release of oxygen in the oxygen-poor environment of the tissues. Modified after ISCID Encyclopedia of Science and Philosophy. 2004. International Society for Complexity, Information, and Design. 12 October 2004 <http://www.iscid.org/encyclopedia/Bohr_Effect>.

Fish may be able to avoid contact to high CO2 exposure because they possess highly sensitive CO2 receptors that could be involved in behavioural responses to elevated CO2 levels (Yamashita et al., 1989). However, not all animals avoid low pH and high concentrations of CO2; they may actively swim into CO2-rich regions that carry the odour of potential food (e.g., bait; Tamburri et al., 2000, Box 6.6).

Direct effects of dissolved CO2 on diving marine air breathers (mammals, turtles) can likely be excluded since they possess higher pCO2 values in their body fluids than water breathers and gas exchange is minimized during diving. They may nonetheless be indirectly affected through potential CO2 effects on the food chain (see 6.7.5).

6.7.2.4 Deep compared with shallow acute CO2 sensitivity Deep-sea organisms may be less sensitive to high CO2 levels than their cousins in surface waters, but this is controversial. Fish (and cephalopods) lead a sluggish mode of life with reduced oxygen demand at depths below 300 to 400 m. Metabolic activity of pelagic animals, including fish and cephalopods, generally decreases with depth (Childress, 1995; Seibel et al, 1997). However, Seibel and Walsh (2001) postulated that deep-sea animals would experience serious problems in oxygen supply under conditions of increased CO2 concentrations. They refer to midwater organisms that may not be representative of deep-sea fauna; as residents of so-called 'oxygen minimum layers' they have special adaptations for efficient extraction of oxygen from low-oxygen waters (Sanders and Childress, 1990; Childress and Seibel, 1998).

6.7.2.5 Long-term CO2 sensitivity

Long-term impacts of elevated CO2 concentrations are more pronounced on early developmental than on adult stages of marine invertebrates and fish. Long-term depression of physiological rates may, over time scales of several months, contribute to enhanced mortality rates in a population (Shirayama and Thornton, 2002, Langenbuch and Portner, 2004). Prediction of future changes in ecosystem dynamics, structure and functioning therefore requires data on sub-lethal effects over the entire life history of organisms.

The mechanisms limiting performance and long-term survival under moderately elevated CO2 levels are even less clear than those causing acute mortality. However, they appear more important since they may generate impacts in larger ocean volumes during widespread distribution of CO2 at moderate levels on long time scales. In animals relying on calcareous exoskeletons, physical damage may occur under permanent CO2 exposure through reduced calcification and even dissolution of the skeleton, however, effects of CO2 on calcification processes in the deep ocean have not been studied to date. Numerous studies have demonstrated the sensitivity of calcifying organisms living in surface waters to elevated CO2 levels on longer time scales (Gattuso et al. 1999, Reynaud et al., 2003, Feeley et al., 2004 and refs. therein). At least a dozen laboratory and field studies of corals and coralline algae have suggested reductions in calcification rates by 15-85% with a doubling of CO2 (to 560 ppmv) from pre-industrial levels. Shirayama and Thornton (2002) demonstrated that increases in dissolved CO2 levels to 560 ppm cause a reduction in growth rate and survival of shelled animals like echinoderms and gastropods. These findings indicate that previous atmospheric CO2 accumulation may already be affecting the growth of calcifying organisms, with the potential for large-scale changes in surface-ocean ecosystem structure. Due to atmospheric CO2 accumulation, global calcification rates could decrease by 50% over the next century (Zondervan et al., 2001), and there could be significant shifts in global biogeochemical cycles. Despite the potential importance of biogeochemical feedback induced by global change, our understanding of these processes is still in its infancy even in surface waters (Riebesell, 2004). Much less can be said about potential ecosystem shifts in the deep sea (Omori et al., 1998).

Long-term effects of CO2 elevations identified in individual animal species affects processes in addition to calcification (reviewed by Ishimatsu et al., 2004, Portner and Reipschlager, 1996, Portner et al, 2004, 2005). In these cases, CO2 entry into the organism as well as decreased water pH values are likely to have been the cause. Major effects occur through a disturbance in acid-base regulation of several body compartments. Falling pH values result and these affect many metabolic functions, since enzymes and ion transporters are only active over a narrow pH range. pH decreases from CO2 accumulation are counteracted over time by an accumulation of bicarbonate anions in the affected body compartments (Heisler, 1986; Wheatly and Henry, 1992, Portner et al, 1998; Ishimatsu et al. 2004), but compensation is not always complete. Acid-base relevant ion transfer may disturb osmoregulation due to the required uptake of appropriate counter ions, which leads to an additional NaCl load of up to 10% in marine fish in high CO2 environments (Evans, 1984; Ishimatsu et al., 2004). Long-term disturbances in ion equilibria could be involved in mortality of fish over long time scales despite more or less complete compensation of acidification.

Elevated CO2 levels may cause a depression of aerobic energy metabolism, due to incomplete compensation of the acidosis, as observed in several invertebrate examples (reviewed by Portner et al. 2004, 2005). In one model organism, the peanut worm Sipunculus nudus, high CO2 levels caused metabolic depression of up to 35% at 20,000 ppm pCO2. A central nervous mechanism also contributed, indicated by the accumulation of adenosine in the nervous tissue under 10,000 ppm pCO2. Adenosine caused metabolic depression linked to reduced ventilatory activity even more so when high CO2 was combined with oxygen deficiency (anoxia; Lutz and Nilsson, 1997). Studies addressing the specific role of adenosine or other neurotransmitters at lower CO2 levels or in marine fish during hypercapnia are not yet available.

The depression of metabolism observed under high CO2 concentrations in marine invertebrates also includes inhibition of protein synthesis - a process that is fundamental to growth and reproduction. A CO2 induced reduction of water pH to 7.3 caused a 55% reduction in growth of Mediterranean mussels (Michaelidis et al. 2005; for review see Portner et al. 2004,

2005). Fish may also grow slowly in high CO2 waters. Reduced growth was observed in juvenile white sturgeon (Crocker and Cech, 1996). In this case, the stimulation of ventilation and the associated increase in oxygen consumption indicated a shift in energy budget towards maintenance metabolism, which occurred at the expense of growth. This effect was associated with reductions in foraging activity. A harmful influence of CO2 on reproductive performance was found in two species of marine copepods (Acartia steuri, Acartia erythrea) and sea urchins (Hemicentrotus purcherrimus, Echinometra mathaei). While survival rates of adult copepods were not affected during 8 days at pCO2 up to 10,000 ppm, egg production and hatching rates of eggs were significantly reduced concomitant to an increased mortality of young-stage larvae seen at water pH 7.0 (Kurihara et al., 2004). In both sea urchin species tested, fertilization rates decreased with pCO2 rising above1000 ppm (below water pH 7.6; Kurihara et al., 2004). Hatching and survival of fish larvae also declined with water pCO2 and exposure time in all examined species (Ishimatsu et al., 2004).

6.7.3 From physiological mechanisms to ecosystems

CO2 effects propagate from molecules via cells and tissues to whole animals and ecosystems (Figure 6.26; Table 6.4). Organisms are affected by chemistry changes that modulate crucial physiological functions. The success of a species can depend on effects on the most sensitive stages of its life cycle (e.g., egg, larvae, adult). Effects on molecules, cells, and tissues thus integrate into whole animal effects (Portner et al., 2004), affecting growth, behaviour, reproduction, and development of eggs and larvae. These processes then determine the ecological success (fitness) of a species, which can also depend on complex interaction among species. Differential effects of chemistry changes on the various species thus affect the entire ecosystem. Studies of CO2 susceptibility and affected mechanisms in individual species (Figure 6.26) support development of a cause and effect understanding for an entire ecosystem's response to changes in ocean chemistry, but need to be complemented by field studies of ecosystem consequences.

Boat Anode Bonding

Figure 6.26 Effects of added CO2 at the scale of molecule to organism and associated changes in proton (H+), bicarbonate (HCO3-) and carbonate (CO32-) levels in a generalized and simplified marine invertebrate or fish. The blue region on top refers to open water; the tan region represents the organism. Generalized cellular processes are depicted on the left and occur in various tissues like brain, heart or muscle; depression of these processes has consequences (depicted on the right and top). Under CO2 stress, whole animal functions, like growth, behaviours or reproduction are depressed (adopted from Portner et al., 2005, - or + denotes a depression or stimulation of the respective function). Black arrows reflect diffusive movement of CO2 between compartments. Red arrows reflect effective factors, CO2, H+, HCO3- that modulate functions. Shaded areas indicate processes relevant for growth and energy budget.

Figure 6.26 Effects of added CO2 at the scale of molecule to organism and associated changes in proton (H+), bicarbonate (HCO3-) and carbonate (CO32-) levels in a generalized and simplified marine invertebrate or fish. The blue region on top refers to open water; the tan region represents the organism. Generalized cellular processes are depicted on the left and occur in various tissues like brain, heart or muscle; depression of these processes has consequences (depicted on the right and top). Under CO2 stress, whole animal functions, like growth, behaviours or reproduction are depressed (adopted from Portner et al., 2005, - or + denotes a depression or stimulation of the respective function). Black arrows reflect diffusive movement of CO2 between compartments. Red arrows reflect effective factors, CO2, H+, HCO3- that modulate functions. Shaded areas indicate processes relevant for growth and energy budget.

Table 6.4 Physiological and ecological processes affected by CO2 (note that listed effects on phytoplankton are not relevant in the deep sea, but may become operative during large-scale mixing of CO2). Based on reviews by Heisler, 1986, Wheatly and Henry, 1992, Claiborne et al., 2002, Langdon et al., 2003 Shirayama, 2002, Kurihara et al., 2004, Ishimatsu et al., 2004, 2005, Portner et al. 2004, 2005, Riebesell, 2004, Feeley et al., 2004 and references therein.

Table 6.4 Physiological and ecological processes affected by CO2 (note that listed effects on phytoplankton are not relevant in the deep sea, but may become operative during large-scale mixing of CO2). Based on reviews by Heisler, 1986, Wheatly and Henry, 1992, Claiborne et al., 2002, Langdon et al., 2003 Shirayama, 2002, Kurihara et al., 2004, Ishimatsu et al., 2004, 2005, Portner et al. 2004, 2005, Riebesell, 2004, Feeley et al., 2004 and references therein.

Affected processes

Organisms tested

• Calcareous benthos and plankton

• Sipunculids

• Crustaceans

• Echinoderms/gastropods

• Sipunculids

N-metabolism

• Sipunculids

• Sipunculids

• Crustaceans

Ion homeostasis

• Fish, crustaceans

• Sipunculids

• Echinoderms/gastropods

Reproductive performance

• Echinoderms

• Copepods

Cardio-respiratory functions

• Fish

Photosynthesis

• Phytoplankton

Growth and calcification

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