Calanus pacificus from Puget Sound, WA show avoidance behaviors when exposed to hypoxic, but not acidic, environmental stressors in the laboratory

Background/Question/Methods

: The intensity and extent of hypoxia and acidification is worsening in coastal marine systems with increasing temperatures, coastal development, and nutrient inputs. Bottom waters can be more stressful in terms of hypoxia and acidification than surface waters, especially in areas where limited mixing and warming of surface waters form a stratified water column. Changes in bottom water chemistry have been shown to impact a range of marine species including copepods, crustaceous zooplankton that are ubiquitous in the world's marine and freshwater systems and play major roles in aquatic food webs and biogeochemical cycling. Previous studies have shown that many copepod species have steep mortality thresholds when exposed to hypoxic waters. Hypoxic bottom waters have lower abundances of copepods than normoxic bottom waters, but little is known about the ability of copepods to alter their normal distributions and migration patterns to avoid stressful bottom waters. Even less is known about how pH affects the mortality rates and behaviors of marine copepods. We examined whether the calanoid copepod, Calanus pacificus, modifies behavior to avoid stressful bottom waters. In two sets of laboratory experiments, bottom waters were manipulated in 1-m tall acrylic tanks to mimic either low dissolved oxygen (<1.0 mg/L O2) or low pH (~7.5 pH) conditions experienced in situ in Puget Sound, WA. Swimming behaviors were recorded using two forward facing and four upward facing 5MP IR cameras. Copepod pixel coordinates were extracted from the videos and converted into 2-D physical space, then assembled into individual swimming trajectories to obtain vertical distributions, mortality counts, and swimming speeds.

Results/Conclusions

: When exposed to hypoxic bottom waters, the vertical distribution of C. pacificus was significantly higher in the water column and mortality counts increased compared to control treatments. Copepods exposed to acidic bottom waters showed no differences in vertical distribution or mortality between acidic (pH=7.5) and control (pH=8.2) treatments. The results of this study suggest that some C. pacificius are able to behaviorally avoid, and therefore reduce exposure to, hypoxic bottom waters. These behaviors may have important implications for calanoid copepods’ ability to persist in a changing ocean, as well as trophic level interactions and the overlap of predators and their prey.