Out of breath in the ocean: Impacts of low-oxygen on zooplankton migratory behavior

I am a researcher at the University of Oslo where I study how krill respond to hypoxia in the Oslofjord. I’m using a combination of active acoustics data and in-situ sampling to observe how oxygen loss may alter the behavior of zooplankton, the small animals that play a central role in marine ecosystems: as prey, predators, and carbon flux mediators. 

I received two years of funding for this research from a Rubicon grant by the Netherlands Research Council.

Out of breath in the ocean: Impacts of low oxygen on zooplankton migratory behavior

As the oceans become warmer, they lose oxygen. Declining marine oxygen will result in degraded ecosystems, impacting marine food webs, biological productivity, and the global carbon pump. However, it remains difficult to make wide-ranging predictions due to knowledge gaps of how low-oxygen impacts animal behavior. 

Close-up of a krill during a cruise in the Oslofjord.

Zooplankton are central to marine ecosystems: they serve as the trophic link between primary producers (phytoplankton) and higher trophic levels (e.g. fish) and they transport carbon from the surface to deeper waters. This huge carbon flux is mediated by a daily migration cycle and fecal pellet production. Mesozooplankton, such as abundant krill and copepods, spend the night grazing at the surface on smaller plankton and then retreat to deeper waters during the day to hide from predators, where they expel their grazed surface carbon through respiration and fecal pellets. The expansion of deep low-oxygen zones may decrease the depth to which zooplankton migrate thus reducing their ability to transport carbon to deep waters.

Echogram during a cruise showing plankton, fish, and the bottom topography of the Oslofjord.

Effects of seasonal hypoxia are well-studied in shallow coastal systems, like the Chesapeake Bay and the Baltic Sea. However, hypoxia also occurs in deep water offshore or in deep coastal areas like fjords, where the impact of low oxygen on pelagic communities is less understood. A fjord’s complex bottom topography often causes seasonal stratification and restricted water circulation, resulting in frequent hypoxia in deep waters. Not only will researching hypoxia in fjords will lead to greater understanding of low-oxygen impacts in these economically and ecologically important systems, deep fjord basins often have oceanic deep-water fauna and salinities, so studying hypoxia in coastal fjords can shed light on the impacts of deep low-oxygen zones in less accessible offshore areas.

In this project, I want to use use improved acoustic technologies to assess 1) how low oxygen is changing zooplankton migratory behavior in a Norwegian fjord. I will also investigate how these changes will impact 2) zooplankton carbon export and 3) the diet of their fish predators. 

Here I am testing an echosounder at the Drøbak field station. 

This project will produce a novel and nuanced picture of the behavioral responses of zooplankton to low oxygen, which would help disentangle the still unquantified consequences of low oxygen for ecologically important fjord ecosystems, and, more broadly, the global carbon pump.