In a new research study, oceanographers at the University of Rhode Island (URI) have discovered that a slight decrease in oxygen levels in the ocean may have huge consequences for small marine organisms called ‘zooplankton’.
These organisms are one of the keep components of food web in the midwater, the expanse of the ocean below the surface and above seafloor. Within the midwater, there are large regions with low level of oxygen known as ‘oxygen minimum zones (OMZs). In coastal ‘dead zones’ where the levels of oxygen can suddenly go down, marine life not adapted to these conditions die. On the other hand, zooplankton in OMZs are specially acclimated to live where other marine organisms, particularly predators cannot.
However, due to increasing climate change, the OMZs are expanding, and even small changes in the regions of low oxygen levels can push zooplankton beyond their unique physiological limits.
According to oceanography professor Karen Wishner, animals in the ocean’s OMZ have adapted to very low oxygen level for over millions of years, but they are living at the extreme limits of their physiological potential. The new study demonstrates that these organisms are sensitive to even minute changes in oxygen, and their number decreases in abundance when oxygen level drop slightly, she added.
The researchers also found more natural variability in the levels of oxygen in OMZs compared to results of previous research. It had a direct effect on the distribution of various kinds of zooplankton, as they respond to even lower than 1% reduction in oxygen levels. Findings of the new study are detailed in a research paper and published in the journal Science Advances.
Although zooplankton have adapted to conditions in the OMZs for over millions of years, climate change may cause these zones to expand, leading to major unexpected changes in the midwater ecosystem. For example, if the OMZs expands into shallower water, zooplankton may become more susceptible to its predators, leading to population crash, which will have negative impacts on the overall food chain.
Wishner said that due to global warming, loss of oxygen in the ocean water has been envisaged in the future, and these organisms may not be able to adapt and persist. Being the key components of food web in the oceanic ecosystems, decrease in zooplankton population will impact top predators such as whales, the researchers said.