Algal oxygen boost could help hot rock pool dwellers on sunny days

Abstract

If you're down on the beach poking around in a rock pool, it's often the start of a summer holiday, but for Tristan McArley, from the University of Auckland, New Zealand, it's just another day at the office. He collects fish from rock pools to learn how the animals cope as the temperatures soar and the water oxygen levels dwindle, usually impairing the ability of fish to go about their daily chores. However, on one particularly brilliant summer day, McArley noticed that the rock pools were literally fizzing. Algae trapped in the pools were photosynthesising and pumping out oxygen, boosting the gas in the stagnant water to the point where it could hold no more and the oxygen escaped as bubbles. It occurred to McArley that a surfeit of oxygen is just another situation that the fish encounter naturally when it is sunny, ‘[yet] very few studies had looked into the influence of higher than normal levels of oxygen in rock pool fish’, he says.Knowing that European perch are able to extend their metabolic capacity (the difference between their maximum and minimum metabolic rates, which permits them to perform routine activities) when oxygen levels are boosted, McArley and his colleagues Anthony Hickey and Neill Herbert, also from Auckland, wondered how local triplefin fish deal with an overabundance of oxygen on blistering summer days.‘These fish often occupy quite small rock pools so they are easily caught with hand nets’, says McArley, who transported 2 g mottled twisters (Bellapiscis medius) and common triplefins (Forsterygion lapillum) back to the Leigh Marine Laboratory. McArley then began measuring the fish's oxygen consumption – initially when swimming at their top speed in 21°C water at either normal (100% air saturation) or 200% air saturation (super oxygenated) to record their maximum oxygen consumption. Next, he allowed the fish to settle down gradually until they reached the minimum oxygen consumption necessary for survival. After that, McArley increased the temperature gradually from 21°C to 29°C to find out how the fish's oxygen consumption increased as the temperature rose. Finally, he logged when the fish toppled over as they were warmed in normal and super-oxygenated water, to find the highest temperature that they can tolerate.The metabolic capacity of the fish in 200% air saturated water increased, allowing them to continue going about their lives despite the higher temperatures, which usually impair fish activities. Considering the sunny conditions that cause oxygen levels to increase in rock pools, McArley says, ‘It may be that whenever these species face acutely high temperatures, they do so under super-oxygenated conditions’. At the highest temperatures, the super-oxygenated fish were barely more resilient than the fish in normally oxygenated water, toppling over at about 31°C, ‘pointing to factors other than the ability to supply oxygen to tissues as being responsible for setting the absolute thermal limits in these species’, says McArley. However, he was surprised when he measured the metabolic rates of the super-oxygenated twisters and triplefins at rest; the metabolic rates were higher than those of the fish in normal water. ‘This suggests there are also some additional metabolic costs of super-oxygenation to these fish’, he says.It seems that fish in hot rock pools on sunny days may not be as compromised as everyone had thought and McArley adds, ‘It is important to study the impacts of stressors such as acute heat stress under ecologically relevant conditions’. However, he warns that the algal oxygen boost is unlikely to protect sizzling rock pool species in the long run as climate change takes an unrelenting grip.