All animals need oxygen. Land animals have it easy, with all this air just floating about free for the breathing, but marine animals rely on oxygen that is dissolved in the water. Oxygen can only dissolve into the ocean from the surface, so it’s a limited resource. That’s why there’s natural low-oxygen habitats, like deep in the mud where oxygen can’t penetrate, and unnatural low-oxygen habitats, like in the Gulf of Mexico Dead Zone or in the Chesapeake. The unnatural low-oxygen habitats are caused by sewage and fertilizer pollution – the nutrients cause an algae bloom, the algae dies, falls to the bottom, decays, and the decaying process sucks out all the oxygen. (In the US Northwest, there’s a different process involving naturally low-oxygen waters getting blown onshore by the wind.) This process is called hypoxia.
During a hypoxic event, everything that can’t get out of the way can die. Fish and crabs flee into shallow waters (which are closer to the surface, so they have more oxygen) and there can be massive die-offs of everything from mussels to sea stars. So hypoxia is considered to be a kind of toxin by regulatory agencies such as the EPA. Like all toxins, it’s important to know what dose is harmful. In the case of hypoxia, it’s considered to be less than 2 milligrams of oxygen per liter of water.
However, a recent analysis in PNAS from Raquel Vaquer-Sunyer and Carlos Duarte has found that significant harm occurs way before oxygen sinks to 2 mg/L. They analyzed 872 experiments that examined oxygen tolerance in 206 species of marine bottom-dwelling organisms. Though the median lethal concentration (LC50, when half of the organisms die) was around 2 mg/L, crustaceans were far more sensitive – some species died when oxygen was double that of hypoxia. Fish and crustacean showed physical distress and avoidance behavior at three times the hypoxic limit, with poor overfished cod becoming stressed at an airy 10 mg/L.
The researchers also looked at the amount of time different groups could tolerate 2 mg/L hypoxia before half of them died. (LT50, median lethal time). Once again, fish and crustaceans were most sensitive, with flounder only able to tolerate 23 minutes of hypoxia before expiring. Animals that don’t move very much were more tolerant, with molluscs being able to take over 100 hours of hypoxia. In all cases, larvae were far more sensitive than adults.
So why is this important? The key is in the variance – the little bars sticking out of the box plot. That means that while some species in each group are relatively tolerant of hypoxia, others are so sensitive that they die far before oxygen levels fall to 2 mg/L. Essentially, hypoxia is more “toxic” to ecosystems than we have thought – damage to the ecosystem can occur well before oxygen levels fall to hypoxic levels.
There are more than 400 hypoxic “dead zones” worldwide, and they’re growing. A recent paper in Science found that they cover an area the size of Oregon and are doubling in size every decade. And this only counts the areas that meet ther 2 mg/L definition of hypoxic – Vaquer-Sunyer and Duarte’s work shows that damage could be occuring on an even more massive scale.
To end on a cheery note (because I’m just filled with California sunshine!), the good news about hypoxia is that it has a clear cause and a clear solution. Stop putting fertilizer into the ocean and the problem goes away. The Black Sea began to recover when the collapse of eastern European economies in the 1990s meant that they could no longer afford fertilizer. So there you go – the economic crisis in the US could reduce the size of the Gulf of Mexico Dead Zone! Now, don’t you feel better?
R. Vaquer-Sunyer, C. M. Duarte (2008). Thresholds of hypoxia for marine biodiversity Proceedings of the National Academy of Sciences, 105 (40), 15452-15457 DOI: 10.1073/pnas.0803833105