Hypoxia, a low level of oxygen that limits the physiological functions of animals, is a topic that fascinates many biologists. As climate change progresses, the frequency of hypoxic episodes in aquatic environments is increasing, putting fish species under stress and even reducing populations in some cases. But it is not only fish that suffer the ill effects of hypoxia. Karen Wishner and colleagues from the University of Rhode Island, USA, and the University of Washington, USA, have discovered that some zooplankton species (such as copepods) are now living close to their physiological bottleneck. Zooplankton that live in regions of the ocean with very low dissolved oxygen, known as oxygen minimum zones, are remarkably susceptible to even the tiniest dip in oxygen. Their numbers decline with even the smallest (≤1%) decrease in dissolved oxygen levels. Even though these creatures are minute, they are a crucial component of oceanic food webs – providing nutrition for larger creatures further up the network – so understanding their distribution has far-reaching implications for the entire oceanic ecosystem.
Wishner and her colleagues undertook an expedition to one of the major oxygen minimum zones in the eastern tropical North Pacific. They surveyed the region between 325 and 650 m, to measure the amount of oxygen in the water and the numbers of zooplankton in the same area to determine the population size. The team discovered that regions with relatively high oxygen levels (8–10 μmol l−1) had remarkably high zooplankton levels relative to regions where the oxygen concentration was 5 μmol l−1 or less; and they found no zooplankton at all in the zones with the least oxygen. They suspected that these zooplankton species are living close to their physiological limit, where they can just maintain their minimum metabolic needs in regions where oxygen is very low, making them very vulnerable to even the smallest decline in oxygen.
Aiming to understand the physiological mechanisms that potentially dictate the distribution of zooplankton, the team conducted a shipboard metabolic experiment to measure the oxygen levels at which these zooplankton species cannot sustain the minimum metabolic needs. They then calculated the ratio of the amount of oxygen recorded in regions where zooplankton live in to the amount of oxygen required to sustain the zooplankton, known as the metabolic index, which estimates the amount of excess oxygen available to sustain life at a specific location in the ocean. When the metabolic index is less than 1, animals have insufficient oxygen to metabolize aerobically and have to initiate anaerobic metabolism, and life becomes unsustainable. Despite the challenges of measuring the metabolic rates of these vanishingly small animals on their ship, the authors discovered that there is only sufficient oxygen in the oxygen minimum zone for the zooplankton to sustain a metabolic rate that is barely twice their resting metabolic rate. Putting this into context, fish typically double their resting metabolic rate when digesting a meal.
This discovery is alarming, although not entirely surprising. The vertebrate population has crashed by 60% since 1970, likely as a result of human activity. Many species, terrestrial and aquatic, are currently experiencing serious ecological stress. It appears that the zooplankton living in the Pacific oxygen minimum zone are also suffering, which could potentially trigger the collapse of the ocean ecosystem that they underpin.