Low oxygen (hypoxia) and low pH are known to have profound physiological effects on zooplankton, the microscopic animals of the sea. It is likely that many individual zooplankton change their behaviors and locations in the water column to reduce or avoid these stresses. However, changes in swimming or avoidance, and their consequences for zooplankton distributions and interactions with their predators and prey, are poorly understood. 

Individual swimming behavior is a primary mechanism underlying patterns in zooplankton population distributions as most species select water column positions with favorable biological, physical, and chemical conditions. As anthropogenic climate change and eutrophication are leading to declines in oxygen and pH throughout the world’s oceans, species-specific movement responses to de-oxygenation and acidification are likely ways through which short-term, localized impacts of these stressful conditions on individual zooplankton will be magnified or suppressed, ultimately affecting population, community, and ecosystem-level dynamics.

This study was designed to provide information on small-scale behavioral responses of zooplankton to oxygen and pH using video systems deployed in the field in a seasonally hypoxic estuary combined with  focused laboratory experiments to explore changes in zooplankton behavior. An underwater video camera was custom designed and built for this project. We deployed it on an oceanographic mooring in a low-oxygen region of Puget Sound, Washington for several months in 2017 and 2018. Using videos recorded by this camera, we found that copepods (a type of small crustacean zooplankton) swam significantly slower in stressful (hypoxic and acidified) waters relative to non-stressful waters. Copepods were also less likely to show escape responses in stressful waters, with the smallest copepods (1-2 mm in length) being 67% less likely to exhibit an escape behavior (jumping) than in non-stressful conditions. In the field, copepod abundances increased in stressful waters, possibly because their slower swimming speeds made it less likely that they would find their way into less stressful depths. On the other hand, amphipods (a larger type of crustacean zooplankton) did not swim differently in stressed versus unstressed conditions. But the abundance of amphipods was lower in stressful waters, suggesting they were able to avoid those conditions. These changes indicate potentially important effects of hypoxia and ocean acidification on zooplankton and support the conclusion that ocean conditions affect the small-scale behaviors of zooplankton, but that different types of zooplankton respond differently. 

In the laboratory, movements of an important species of copepod (Calanus pacificus) were observed in oxygen and pH gradients by filming individual organisms in acrylic water columns equipped with computerized camera systems. The columns were filled with seawater with either low oxygen or low pH conditions in the bottom half of the column and unstressful seawater in the top half. Experiments exploring changes due to low oxygen stress and low pH stress were examined separately to determine whether those stressors, which commonly co-occur in the ocean, have different effects on the organisms. We found that levels of low oxygen commonly measured in Puget Sound caused mortality or avoidance by the copepods, whereas low levels of pH that are also common in the field did not. These observations determined that oxygen is the primary factor driving copepod response to stressful conditions, and helps explain why copepods had previously been observed in low numbers in hypoxic and acidified environments in the field.

This study was the first to report on in situ swimming behaviors of zooplankton. The technology and analytical protocols we developed represent a significant advance in our ability to study small scale behaviors of plankton. These results deepen our understanding of how zooplankton respond to low oxygen and pH conditions in ways that could profoundly affect marine ecosystems and fisheries through changes in their populations and distributions. 

In addition to the scientific achievements, this project trained graduate and undergraduate students and engaged grades K-12 students and teachers in under-served coastal Washington communities by developing ocean technology-based citizen-scientist activities and curricular materials in plankton ecology, ocean change, construction and use of biological sensors, and quantitative analysis of environmental data. K-12 students and teachers learned to construct and deploy low-cost instruments for monitoring change in nearshore environments and other habitats of local ecological and economic importance. Students are able to access and analyze data from their sensor deployments through online servers, connecting them to their science in real time.

Last Modified: 02/16/2024
Modified by: Julie E Keister