The effects of ocean acidification on noncalcifying marine organisms are still poorly understood. Although a number of studies have linked increased CO2 levels to significant developmental defects in marine animals, few have been undertaken with ocean acidification in mind. As a result, the CO2 levels examined are often unrealistically high, reducing the relevance and predictive power of these findings (Rosa and Seibel, 2008).
Although there are many scenarios that could link ocean acidification to increased mortality among marine animals, few direct connections have been established. Several recent studies suggest that ocean acidification can impact the innate ability of some fishes to detect olfactory cues, which are vital to their ability to avoid predation and identify suitable habitat. Clownfish that had been exposed to acidic conditions (pH 7.8 or 1000 ppm CO2, equivalent to pessimistic predictions for 2100) during their egg and larval stages were shown to be strongly attracted to the smell of predators and unable to distinguish between predator and non-predator species (Dixson et al., 2009). Clownfish raised in acidic conditions also exhibited significantly different responses – when compared to a control group – to a variety of vegetative olfactory cues that might be used to identify suitable habitat. In addition, these fish seemed less responsive the olfactory cues released by their parents – a condition that might lead to increased inbreeding (Munday et al., 2008). Olfactory cues are thought to be important to many reef fish species. The inability to identify predators or locate suitable habitat could lead to significant increases in predation and mortality rates, with cascading effects throughout reef ecosystems.
It is important to bear in mind that ocean acidification is not the only threat to marine ecosystems. Its effects will be compounded by the impact of global warming and marine pollution, likely resulting in especially vulnerable populations. One recent study examined the synergistic effects of increasing ocean temperature, the associated expansion of the oxygen-minimum zone, and ocean acidification on the jumbo squid, a large pelagic top predator found in the Eastern Tropical Pacific and a commercially important species – targeted for human consumption and use as bait. As a result of the low oxygen-carrying capacity of its blood and the highly pH sensitive blood-oxygen binding observed in most active squids, the jumbo squid was expected to be especially sensitive to ocean acidification and global warming. Indeed, the study concluded that ocean acidification (in this case, defined as CO2 levels of approximately 1000 ppm and a pH level of 7.8) will substantially reduce the jumbo squid’s metabolic rates and activity levels, by 31% and 45%, respectively (Rosa and Seibel, 2008). The authors hypothesize that the squid’s available habitat will decrease as a consequence of these effects, with possible associated alterations in its behavior and diet, as well as its growth and reproductive patterns. In addition to their commercial importance, jumbo squids are an essential component in the diets of a variety of birds, fishes, and mammals. Any decrease in the jumbo squid population, therefore, would likely have a cascading effect throughout the Eastern Tropical Pacific ecosystem.