Experimental Observations

Experimental approaches have, so far, been carried out in controlled laboratory experiments on single organisms or in larger volume sediment or seawa-ter mesocosms enriched with CO2 containing mixed populations. There are many important biological processes within the lifecycle of an organism or even more so in an ecosystem. Therefore an impact on a process, be it at the cellular level or ecosystem level, may have a negative impact on the ultimate successful functioning of the ecosystem. An experimental approach is a key tool in determining the weak links in these processes.

4.3.1. Primary Production

The Royal Society [29] concluded that unlike land plants, most marine phyto-plankton are thought to have mechanisms to actively concentrate CO2 so that changes in seawater pH and CO2 have little (<10%) if any direct effect on their growth rate or their elemental composition. However, whilst taxon specific differences in CO2 sensitivity have been observed in laboratory culture [30] it is currently unknown whether a reduction of the advantage of possessing a CO2 concentrating mechanism will impact phytoplankton species diversity in the natural environment. This is a possibility and, should it occur, may impact the contribution of different functional groups, primary production, food web structure and marine biogeochemical cycles. The cocco-lithophore Emiliania huxleyi seem to be an exception to this generalisation, having low affinity for inorganic carbon such that it could be carbon limited in today's ocean, with increasing CO2 resulting in increased productivity [31].

4.3.2. Calcification

Although there is variability amongst experiments, with some studies showing no change or even increased calcification [49,50], most calcifying species studied to date, representing the major marine calcifying groups (coccolitho-phores, pteropods, foraminifera, corals, calcareous macroalgae, mussels, oysters, echinoderms and crustacean), show reduced net calcification rates in response to elevated CO2 [reviewed in Refs. 32-25]. For example, a mean decrease of 16% (double pre-industrial CO2 concentration (2x CO2)) and 20% (triple pre-industrial CO2 concentration (3x CO2)) for coccolithophores; 6% (2x CO2) and 9% (3xCO2) for foraminifera; 24% (2x CO2) and 41% (3x CO2) for Scleractinian corals; 25% (2x CO2) for coralline red algae; 25% (2x CO2) and 37% (3x CO2) for mussels; and 10% (2x CO2) and 15% (3x CO2) for oysters.

This variability between major groups of organisms may result primarily from the different mechanisms used to carry out calcification. Coccolitho-phores, for example, carry out calcification in an intracellular compartment which may be buffered against external changes by their own homeostatic mechanisms. Foraminifera and corals carry out calcification in an enclosed yet extracellular space, relying on membrane transporters to regulate conditions. In more complex multicellular organisms, such as crustaceans and molluscs, metabolic energy balance as well as whole animal acid-base regulation (see below) may be more important in determining the responses of calcification to decreased seawater pH. To maintain a calcified structure, when exposed to a more acidic environment for a short time, an organism may have to divert energy from other metabolic processes in order to compensate for dissolution. Other metabolic processes may also be impacted by CO2 so this compensation may not always be possible over longer time periods. Evidence strongly indicates that dissolution rates will, over the timescale associated with ocean acidification, become greater than the rate at which organisms can grow and calcify, resulting in an inevitable reduction in biogenic CaCO3.

4.3.3. Acid-Base Regulation and Internal Physiology Much is known about the short-term effects of very high concentrations of CO2 (higher than we will see due to ocean acidification) on respiration and acid base balance in marine invertebrates and fish [9]. These early experiments were important in the discovery that CO2 in seawater readily diffuses across animal surfaces, lowering the pH of internal fluids and that many animals have developed compensation mechanisms to regulate their internal pH. We now know that for normal function of an organism, internal pH must be kept within relatively narrow ranges because processes such as enzyme function, protein phosphorylation, chemical reactions and the carrying capacity of haemoglobin for O2 are all influenced by pH and that these can be regulated for short periods of exposure to high CO2. Evidence so far indicates that fish are tolerant to these short-term high CO2 exposures but organisms such as squid, may be more vulnerable (reviewed in [9,33]). However, we do not yet know the impact of long term exposure to the relatively lower levels of CO2 they will experience in the future from ocean acidification.

4.3.4. Fertilisation, Embryo Development, Larval Development and Settlement

Physiological impacts induced by lowered pH have the potential to affect an animal at any stage in its life cycle; however, adults tend to have more protection as well as better mechanisms to deal with a fluctuating environment with early life stages tending to be more vulnerable. Many benthic marine invertebrates produce free-swimming larvae, which spend time developing through several larval stages in the plankton before settling into the adult form (Fig. 5). Large numbers of larvae are often produced because of high rates of mortality, for example, coastal estuarine bivalves experience more than 98% mortality during settlement [36]. Oyster [37], echinoderm [38,39] and fish larvae [40] as well as barnacle, tube worm and copepod eggs [41, personal observation] have all been found to either be increasingly malformed or have slower rates of development at high CO2. Barnacle settlement has also been affected [42].

Assuming that some larvae are still viable and go on to settle on the shore, delayed development could leave juveniles susceptible to additional stresses such as wave impact and temperature and salinity variations. In addition, if they settle later, they may miss their survival window.

FIGURE 5 Barnacle life cycle, showing the pelagic larval stages I to VI, the cyprid larvae settling to become a benthic juvenile and finally an adult. From Desai and Anil [43].

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FIGURE 5 Barnacle life cycle, showing the pelagic larval stages I to VI, the cyprid larvae settling to become a benthic juvenile and finally an adult. From Desai and Anil [43].

4.3.5. Communication

Chemical cues are used for marine communication and can have strong influences on habitat selection and predator prey interactions as well as courtship and mating, species recognition, and symbiotic relationships [44]. Some of these cues are known to be susceptible to changes in pH during formation and detection or within the seawater itself. Settlement of oyster larvae can be induced or inhibited by the presence of weak bases or acids, respectively, possibly as a mechanism for suitable habitat selection (e.g. [45 47]). Weakly acidic environments also impaired the ability of juvenile salmon to detect and respond to alarm cues [48]. The normal response to predators by littorinid snails on rocky shores is to thicken their shells. However, under CO2-induced acidification the snails switched from thickening shells in the presence of predators to increased avoidance behaviour [49].

4.3.6. Interactions

The responses that occur within one individual can lead to changes in how it interacts with others and its environment. For example, burrowing brittlestars were found to have increased muscle wastage in their arms as compensation for increasing their calcified material under low pH conditions [50]. These brittlestars are important prey for commercial fish and aid nutrient and oxygen cycling between the sediment and the overlying water [51]. Reduced muscle may lead to reduced ability to feed themselves, lower quality of food for predators and reduce nutrient flow.

The importance of microbial and viral activity in the oceans is becoming ever more apparent; recent experiments show that bleaching of corals can be induced by increased viral activity [52], however, some evidence suggests that viral activity may decrease with increasing CO2 [53]. The ability of a host to have an immune response to viral attack is critical for its health. Preliminary evidence suggests that at lowered pH mussels are unable to induce normal immune responses [54].

Organisms that occupy the same ecosystem space or function, may be out-competed if they are less suited to surviving in a high CO2 ocean. This may lead to an overall loss of biodiversity or even regime shift but may not lead to a complete breakdown in all functions of the ecosystem [19]. Changes to populations and their interactions within communities could well influence the relative composition, productivity, timing, location and predominance of the major functional groups and thereby impact the rest of the food web.

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