Suffocating or Surviving: Why Life Along the Seafloor Can Persist Despite Low Oxygen Conditions
Primary Source: Reimers, C. E., Henkel, S. K., Fogaren, K. E., Chace, P. J., Hughes, A., & Wakefield, W. W. (2025). Dynamic benthic oxygen fluxes lessen hypoxia effects on open continental shelves. Limnology and Oceanography. https://doi.org/10.1002/lno.70197
Secondary Sources:
https://coastalscience.noaa.gov/crp/hypoxia/
https://oceanservice.noaa.gov/hazards/hypoxia
Yahalomi, D., Atkinson, S. D., Neuhof, M., Sally Chang, E., Philippe, H., Cartwright, P., Bartholomew, J. L., & Huchon, D. (2020). A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome. PNAS, 117(10), 5358–5363. https://doi.org/10.5061/dryad.v15dv41sm
Featured Image Source: Dan Howard/NOAA/CBNMS., Public domain, via Wikimedia Commons
Living on land, it is easy to take breathing for granted. We exist within an atmosphere (generally) full of the oxygen we need to respire and turn the food we eat into energy. Things aren’t always so easy for animals living in the ocean. The vast majority of oceanic animals require oxygen (only one animal – the parasitic Henneguya salminicola – has been discovered that lives without breathing) which primarily comes from two sources: dissolution of atmospheric oxygen into the surface ocean and the production of oxygen in the sea water by photosynthetic plankton, algae, or aquatic plants. As animals living in the ocean respire, they remove oxygen, which is why we generally see lower oxygen concentrations in the deep ocean relative to the surface. But as long as the oxygen is replenished by dissolution and photosynthesis, the animals in the ocean can survive.
Today, global warming is heating up the surface ocean, which makes it harder for oxygen to dissolve in, and nutrient runoff from fertilizer and wastewater are leading to excess respiration, meaning oxygen is being removed faster than it can be replenished. These processes reduce the oxygen content of seawater and can create “dead zones” in coastal areas around the world. These “dead zones” are described as hypoxic, meaning their dissolved oxygen concentration is ≤ 62.5 µmol/L. Around this concentration of oxygen, fish and other marine animals will begin to avoid these areas. Continued exposure to low oxygen can affect animals’ feeding, growth, and reproductive rates, and even lead to death.
The continental shelf (the shallow area of the ocean extending directly from the coastline) along Oregon and Washington is one location that experiences seasonal hypoxia, the prevalence of which has increased over the last 50 years. Hypoxia here is caused by the combination of two factors: summer upwelling, when winds blowing along the coast force low-oxygen waters from the deeper ocean up to the shallow shelf, and local oxygen consumption through respiration. The Oregon-Washington shelf seafloor is home to commercially important marine invertebrates, most notably the Dungeness crab. Catch rates are positively related to the oxygen concentrations along the seafloor, meaning that as hypoxia occurs more frequently, the crab population will decrease. However, a group of scientists from Oregon State University has recently discovered that natural physical processes along the shelf may be relieving the stress caused by hypoxia.
Measuring hypoxia and its effects
In their study, the scientists measured the oxygen of the bottom water (the water right above the sediment), counted the number and different types of macrofauna (large animals) living along the seafloor, and determined how fast oxygen was consumed at the seafloor-ocean interface. Macrofaunal counts were done using two methods: (1) collecting cores of sediment and sieving them to separate the animals from the sediment grains before assigning them into major taxonomic groups (e.g. crustaceans, mollusks, echinoderms, etc.) and (2) capturing images of the seafloor via cameras mounted to the frame they used to deploy their equipment. By doing this, they were able to assess both the density of the communities at each sampling site (number of individuals per unit area) and the community richness (number of different types of animals per unit area). Oxygen consumption at the seafloor was determined (1) by taking sediment cores and incubating them to measure how fast oxygen is removed from the water overlying the sediment and (2) using a technique called eddy-covariance, where velocity of the bottom-water movement, or current, is measured and paired with oxygen concentrations.
Bottom water oxygen concentrations along the Oregon-Washington shelf during this study ranged from 17-75 µmol/L, meaning these waters were severely to borderline hypoxic (threshold for hypoxia = 62.5 µmol/L). However, despite the inhospitable conditions, the sediment macrofaunal communities were surprisingly dense (31-113 organisms per core) and rich (11-28 types of animals). Additionally, the cameras captured many images of fishes, anemones, and sea stars. The cameras also recorded abundant evidence of recent activity on the seafloor, like mounds, burrow openings, and tracks, left behind in the sediment by animals. Importantly, they did not see any evidence of animals that usually live within the sediment exiting their burrows to live on the surface of the sediment, a typical response to hypoxic stress. Clearly, this hypoxic zone is not as dead as expected. But how can so many animals survive in such a harsh environment?
Turbulent oceans help bottom-dwellers breathe
The scientists noticed that the speed of water movement at the bottom of the ocean was significantly influenced by physical processes at the ocean surface, like waves and tides. They hypothesized that the waves and tides mixed the water all the way down to the seafloor and, in doing so, were providing oxygen from the surface ocean to the animals living at the bottom. As evidence of this, the scientists showed that oxygen consumption rates derived from sediment core incubations, not subject to physical mixing, were significantly lower than those measured at the actual seafloor, where mixing is occurring. Therefore, the authors of this study put forth that the dynamic physical conditions of the water along the Oregon-Washington shelf result in the delivery of dissolved oxygen to the sea floor, which in turn sustains the communities of animals living there, even under hypoxic conditions.
This news is certainly a glimmer of hope that animals living in some hypoxic zones aren’t necessarily doomed, but it does not mean hypoxia won’t wreak havoc on coastal ecosystems. While the Oregon-Washington shelf communities observed in this study were doing surprisingly well given the low oxygen concentrations in bottom water, community richness was far lower than measured in a 2003 study when bottom water oxygen concentrations were higher. The decline of coastal ecosystems, such as this one, may have devastating repercussions for the many communities of people living along coastlines who rely on the ocean for survival. So, what can we do to mitigate the effects of hypoxia?
For one, we must better understand the compounding effects of climate change, nutrient input, and ocean physics in driving the formation of hypoxic zones. Limiting nutrient discharge from both agricultural runoff and wastewater can reduce the extent and severity of hypoxic zones. The development and maintenance of forecasting models will help coastal communities prepare for hypoxic events. As mentioned at the top of this article, the warming of the oceans around the globe means that, in general, not as much oxygen can dissolve into sea water, making it easier for hypoxic zones to form. By working as a global community to reduce greenhouse gas emissions and slow global warming, the ocean ecosystems that keep so many of us alive will have a much better chance of thriving into the future.
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