Coping with low oxygen levels
February 24, 2015
Nutrients are brought to the Gulf of California by runoff, causing phytoplankton to thrive near the sea surface. The phytoplankton add oxygen to the surface layers as does the exchange between the ocean and the atmosphere. This makes the surface waters relatively oxygen-rich. When phytoplankton die, they sink down from the surface layers. Zooplankton and larger animals produce fecal material, and also sink from the surface when they die. This dead material sinking through the water column is called marine snow (and it really looks like snow falling through the sky). As the marine snow sinks, it is consumed by bacteria. The bacteria use oxygen to break down the marine snow, causing this deeper water to be depleted of oxygen. Deeper in the water column, much of this microbial breakdown has already happened, so the oxygen levels rise again where there is less consumption of particles and therefore, less consumption of oxygen. The oxygen minimum zone (OMZ) is this layer where the oxygen levels are lower than in waters above or below.
As the ocean warms, OMZs are expanding. (Read more about why OMZs are expanding here https://www.mbari.org/expeditions/Midwater2013-Fall/logbook/day2.htm). It is important to understand how animals that live in the OMZ will be affected by the expansion. More importantly, what will be the effect on animals that would not be able to survive in an oxygen-poor environment? By comparing the midwater and the OMZ over the last 25 years in Monterey Bay with that in the Gulf, we can make predictions about how communities will respond to lower oxygen levels.
The OMZ in the Gulf extends from 100 meters to 1,000 meters below the sea surface. Because the Gulf is a narrow marginal sea and the waters are warm, there is less mixing of oxygen-rich surface waters to deep waters below, contributing to the wide OMZ we see here. It is a good system to study to make predictions about the future of ecosystems in other parts of the world, where OMZs are just starting to expand.
Using both the mini ROV and ROV Doc Ricketts, we can document the presence/ absence of animals and their depth ranges at different oxygen levels. We can also collect them and measure their respiration rates in the lab. Kim Reisenbichler has built a system to do just that. Data collected from this system over the course of the expedition will help ecologists understand how animals cope with changes in oxygen concentrations in their ecosystem.
—Susan von Thun