Oceans on acid: climate change's "evil twin"
As we continue to burn fossil fuels, produce cement, and alter land – like clearing forests that would normally convert atmospheric carbon dioxide into oxygen - the levels of carbon dioxide in the atmosphere continue to increase. Between 1995 and 2004, high-precision instruments have recorded an average increase of 1.9 ppm (parts per million) in atmospheric carbon dioxide. Between 2005 and 2014, there has been a global average increase of 2.1 ppm. As of 17 February 2015, Carbon dioxide levels were 399.97 ppm, a vast increase over 1959 levels – the first year with full high-precision instrument data measurements – of 315.97 ppm, and a vast increase over pre-industrial levels of around 280 ppm.
With failures of national, regional, and local governments, as well as industry and the public, to take meaningful steps to reduce emissions, levels are only set to increase. Climate change is perhaps the most well-known consequence of our continued emissions, but of increasing concern is climate change’s “evil twin” – ocean acidification.
Not all carbon dioxide remains in the atmosphere, with around 30% being soaked up by the ocean. When carbon dioxide enters the ocean, it forms carbonic acid, which in turn breaks down into bicarbonate (HCO¬3) and hydrogen (H+) ions. Bicarbonate ions can break down even further into carbonate ions (carbon dioxide3) and a hydrogen ion. It is the increase in the concentration of hydrogen ions that increases acidity levels in the ocean. This chemical reaction is entirely natural, and it has always taken place - just not always at such magnitude, and with such rapidity.
Since the Industrial Revolution, surface waters of the ocean have declined by 0.1 pH units. This deceptively small change represents a 26 - 30% increase in ocean acidity. Projected atmospheric carbon dioxide levels suggest that by the end of the century, ocean pH will drop by 0.3 – 0.4 units - an increase in acidity by 100 – 150%. Numerous studies have indicated acidification will have impacts on marine life, particularly calcareous organisms like molluscs and corals.
There is a growing body of evidence that fish are also at risk. Laboratory experiments that attempt to determine how fish may or may not be impacted by ocean acidification typically simulate past, present, or predicted future acidity levels, and even beyond into what is generally considered impossible. Whilst studies that suggest high carbon dioxide levels can be lethal to adult fish typically simulate levels far above those predicted in the future, more realistic scenarios suggest that long-term sub-lethal effects are possible.
Fish may not have calcareous shells that can be dissolved in acidic waters, but a number of studies have indicated that they can still suffer physiological impacts. One area of research interest concerns otoliths – bony structures located behind the brain of fish that help with sound detection, balance, and orientation. Otoliths are primarily composed of aragonite, a form of calcium carbonate which is highly soluble in acidic waters.
Work by Sean Binami, a PhD student at Rosenstiel School of Marine and Atmospheric Science, University of Miami, focusing on larval cobia (Rachycentron canandum) has suggested that predicted end-of-century carbon dioxide levels may increase otolith mass. Moving into the extreme (and possibly more unlikely) carbon dioxide scenarios, the researchers noted a reduction in the size of larvae and delays in their development, as well as increases in the mass, size, and density of their otoliths. Exactly what this means for these large tropical pelagic fish remains unclear. The changes in otoliths may increase sound detection, which may or may not be of benefit to their health and survival.
Otoliths aren’t the only sensory organ to show alterations under ocean acidification, nor do the effects remain purely physiological. Danielle Dixson, a coral reef fish specialist who is now based at Georgia Institute of Technology, focused her efforts on odour tracking behaviour of the smooth dogfish (Mustelus canis). Under current-day simulations, captive dogfish preferentially tracked the odour of squid (one of their prey species) that was introduced by Danielle and her fellow researchers into their tanks. However, as simulated carbon dioxide levels reached end-of-century projections, the dogfish’s preference switched, and they actively avoided the odour. Furthermore, when the dogfish did encounter food, they responded less aggressively than when simulated acidity levels represented those currently found.
Exactly why this occurs is a subject of research. Similarly, work by Philip Munday from the Australian Research Council Centre of Excellence for Coral Reef Studies suggested that by the end of the century, disruption to the olfactory sensory organs of larval orange clownfish (Amphiprion percula) may prevent them from locating reef habitat and suitable sites for settlement. There are other behavioural changes that have been noted. Papua New Guinea is home to a coral reef system located next to natural volcanic seeps, which increases the carbon dioxide concentration in the surrounding water to levels projected at the end of the century. The wild reef fish studied (two damselfish and two cardinalfish species) in the reefs close to the seeps exhibited quite different behaviour from those in reefs further away. They also exhibited signs of olfactory impairment, spending considerable time in waters containing the smell of a predator. They also appeared bolder, emerging six times faster from their shelter after disturbance than their counterparts living in less acidic waters.
The fact that community structure and fish diversity didn’t really differ between the reefs next to and away from the reefs isn’t down to acclimatisation to the more acidic conditions, the researchers note, but a result of a combination of recruitment from outside the seep areas, and fewer predators in the seep-associated reefs. Without recruitment from other sources and fewer predators, the days would have been numbered for these four wild species of fish living in a rather unique environment.
Perhaps the most imminent and widespread of the threats from ocean acidification to fish comes indirectly, such as from the loss of habitat.
Shallow water coral reefs may occupy less than 0.2% of the ocean floor, but they support an estimated 25% of marine fish through either part or all of their life cycle. With chronic ocean acidification impacting calcifying organisms like corals, loss of essential habitat for fish - and indeed the 9 – 12% of the world's fisheries that depend on shallow-water coral reefs - is a huge concern. Ocean acidification in the deep sea may be expected to lag surface water acidification by a few centuries, but it is still a threat to deep sea corals – and perhaps more so than for shallow water counterparts.
Both deep and shallow water corals can suffer impairment to growth, reproduction, metabolism, and survival from acidification. Unfortunately for deep sea corals, the saturation level of calcium carbonate decreases with depth, meaning that their skeletons are more likely to dissolve than those at the surface water. Like the more familiar shallow water coral reefs, deep sea corals also provide vital habitat for some fish species through part or all of their life cycle. This includes commercially important species, such as walleye Pollock and sea bass.
Of equal concern are potential effects coming through the food web. Acidification is likely to alter marine food webs, but unravelling exactly how, and what alterations will mean for different species is a complex task. Ocean acidification is likely to have the greatest impact on calcareous organisms – including those that are near the base of food webs. Focusing on the Puget Sound, Shallin Busch, a research ecologist at the Northwest Fisheries Science Centre, built a conceptual model that indicated changes in a few key groups within the food web can have ecological ramifications. For example, benthic crustaceans, with their calcified shells threatened by acidification, are a key prey item for a number of benthic fish species, some of which feed almost exclusively on crustaceans. The model predicted that loss of benthic crustaceans would likely lead to declines in such fish species, simply because there would not be enough prey to support their population.
For those fish that also feed on other species that are less susceptible to acidification, such as soft infauna, they may become more dominant predators in the community, which in turn can have impacts on the populations and distributions of other species. Busch’s model also suggested decreases in fishery yields of Pacific herring, primarily as a result of a decline in the prey copepods, small planktonic crustaceans that lie at the lower levels of the food web. The situation could have been much worse for the fisheries. Ocean acidification may directly impact copepods, but it also depresses populations of some of their other predators and resource competitors, like macrozooplankton and euphausiids. The herring didn’t lose quite as much prey as they could have if only copepods were impacted.
There are still several knowledge gaps on how marine fishes will respond to ocean acidification - both directly and indirectly, particularly in relation to sustained, realistic levels of future acidification. Ocean acidification isn’t just of concern to conservationists and scientists, but to fisheries and those who both enjoy and depend on fish for their livelihoods.
This article was written for The Marine Professional.
