Keister, J. E., Bonnet, D., Chiba, S., Johnson, C. L., Mackas, D. L., and Escribano, R. 2012. Zooplankton population connections, community dynamics, and climate variability. – ICES Journal of Marine Science, 69: 347–350.

Several hypotheses of how zooplankton communities respond to coastal hypoxia have been put forward in the literature over the past few decades. We explored three of those that are focused on how zooplankton composition or biomass is affected by seasonal hypoxia using data collected over two summers in Hood Canal, a seasonally-hypoxic sub-basin of Puget Sound, Washington. We conducted hydrographic profiles and zooplankton net tows at four stations, from a region in the south that annually experiences moderate hypoxia to a region in the north where oxygen remains above hypoxic levels. The specific hypotheses tested were that low oxygen leads to: (1) increased dominance of gelatinous relative to crustacean zooplankton, (2) increased dominance of cyclopoid copepods relative to calanoid copepods, and (3) overall decreased zooplankton abundance and biomass at hypoxic sites compared to where oxygen levels are high. Additionally, we examined whether the temporal stability of community structure was decreased by hypoxia. We found evidence of a shift toward more gelatinous zooplankton and lower total zooplankton abundance and biomass at hypoxic sites, but no clear increase in the dominance of cyclopoid relative to calanoid copepods. We also found the lowest variance in community structure at the most hypoxic site, in contrast to our prediction. Hypoxia can fundamentally alter marine ecosystems, but the impacts differ among systems.

Copepods dominate the zooplankton, but surprisingly little is known of their tolerance to the increasing threat of hypoxia. We measured abundances of the calanoid copepod Calanus pacificus in relation to oxygen concentrations in the field and established its tolerance to low dissolved oxygen (DO) in the laboratory. In situ distributions of female C. pacificus were assessed with depth-stratified net sampling in Hood Canal, a seasonally hypoxic sub-estuary of Puget Sound, Washington. No clear avoidance of DO levels from supersaturated down to 2.0 mg DO L⁻¹ was observed; DO levels <2 mg L⁻¹ were rare in our sampling. In the lab, at 13°C female C. pacificus exhibited 100% 24-h survival at DO levels down to 1.7 mg DO L⁻¹; below 1.5 mg DO L⁻¹, survival sharply declined, with 25% survival at 1.2 mg L⁻¹ and complete mortality within 1 h at 0.9 mg L⁻¹. Calanus pacificus is one of the dominant copepods throughout much of the North Pacific and an important trophic link in the ecosystem. Our measurements indicate that they have a steep threshold in oxygen tolerance that is similar to those measured for other calanoid copepods and which could limit their habitat in years of severe oxygen depletion.

Zooplankton exhibit diverse swimming behaviors to reposition themselves in the water column, feed, find mates, and avoid predation. Environmental stressors that modify behavior can have cascading effects on population distributions and predator–prey interactions. Understanding zooplankton population dynamics is challenging, largely because traditional methods for quantifying zooplankton distributions are costly, limited in scope, and require extended analysis by trained analysts. We developed a novel methodology that combined remotely deployed camera systems, machine learning‐based identification of zooplankton, and video‐based tracking technology to quantify copepod and amphipod in situ swimming behavior in Hood Canal, WA, USA, a seasonally hypoxic and acidified fjord. Behavioral analysis showed copepods of all sizes swam on average 24% slower in stressful (hypoxic and acidified) waters relative to non‐stressful waters. Copepods exhibited less frequent escape responses in stressful waters, with a 68% decrease in the amount of time spent “jumping” for copepods 1–2 mm in length. Interestingly, abundances of small copepods increased in stressful waters, with 56% more 1–2 mm long copepods in stressful vs. non‐stressful conditions. In contrast, amphipods' average “darting” speeds did not differ between environmental conditions, but the abundance of amphipods significantly decreased in stressful waters relative to non‐stressful waters, suggesting avoidance of stressful conditions. Changes in swimming behavior are informative metrics in understanding ecosystem impacts of environmental stress because swimming speed has individual, population, and community‐level implications. Our results suggest that, among copepods, in situ behaviors may be useful proxies in monitoring the impacts of climate change on coastal ecosystems.