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.

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Seasonal hypoxia (≤2 mg dissolved oxygen L−1) can have detrimental effects on marine food webs. Recent studies indicate that some jellyfish can tolerate low oxygen and may have a competitive advantage over other zooplankton and fishes in those environments. We assessed community structure and distributions of cnidarian and ctenophore jellyfish in seasonally hypoxic Hood Canal, WA, USA, at four stations that differed in oxygen conditions. Jellyfish were collected in June through October 2012 and 2013 using full-water-column and discrete-depth net tows, concurrent with CTD casts to measure dissolved oxygen (DO). Overall, southern, more hypoxic, regions of Hood Canal had higher abundances and higher diversity than the northern regions, particularly during the warmer and more hypoxic year of 2013. Of fifteen species identified, the most abundant—the siphonophore Muggiaea atlantica and hydrozoan Aglantha digitale—reached peak densities > 1800 Ind m−3 and 38 Ind m−3, respectively. M. atlantica were much more abundant at the hypoxic stations, whereas A. digitale were also common in the north. Vertical distributions explored during hypoxia showed that jellyfish were mostly in the upper 10 m regardless of the oxycline depth. Moderate hypoxia seemed to have no detrimental effect on jellyfish in Hood Canal, and may have resulted in high population densities, which could influence essential fisheries and trophic energy flow.

A zooplankton time series from the eastern Strait of Juan de Fuca in the Salish Sea was explored for its relationships to climate variability and survival of Puget Sound salmon populations. Vertical net tows were conducted $ monthly over the upper 40 m from 2003 to 2018. Species with warm-water associations that occur year-round in Puget Sound, and species with cold-water associations that dominate on the continental shelf during summer upwelling, were important members of the community at different times of year. Shifts in advection from Puget Sound and the coastal ocean were indicated by the abundance of Puget Sound-dominant copepods Ditrichocorycaeus anglicus and Paracalanus spp. in summer through mid-fall. In contrast, the coastal upwelling species Pseudocalanus mimus, Calanus marshallae, and Acartia longiremis were most abundant in fall through early spring, despite being the seasons they are least abundant on the continental shelf. Axis 1 of a Nonmetric Multidimensional Scaling ordination of the copepod community revealed clear seasonal cycles in community structure and longer-term variance that correlated with the Pacific Decadal Oscillation and local temperatures. Axis 2 correlated to smolt-to-adult survival of Puget Sound coho salmon, but not with measured environmental factors. Copepod size composition and biogeographic affinities shifted along that axis. The mechanisms driving the shifts are not clear, but may indicate changes in trophic dynamics, or cryptic responses to environmental changes. These patterns suggest the importance of large-scale climate influences and local environmental processes that affect estuarine circulation, population growth, and links to economically important fisheries.

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