Although declining oxygen concentration has been reported for the oxygen minimum zones (OMZs) of the tropical oceans and the North Pacific Ocean, consistent with model predictions of the effects of global warming, its ecological impacts are poorly understood. We report the apparent impact of declining oxygen on midwater fishes within the OMZ of the southern California Current (CC). Principal component analysis of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) ichthyoplankton time series from 1951 to 2008 indicates that the dominant temporal pattern (principal component 1 [PC1]) represents the marked decline of the region's mesopelagic fishes during periods of reduced oxygen. Of the 27 taxa with loadings > 0.5 on PC1, 24 were mesopelagic. PC1 was strongly correlated with intermediate-water oxygen concentrations (r = 0.75, p < 0.05), which were about 20% lower in the past decade and the 1950s than in the period from 1970 to 1995. The abundance of mesopelagic fishes represented by PC1 was reduced, on average, by 63% between periods of high and low oxygen concentrations. We hypothesize that the underlying mechanism is the shoaling of the hypoxic boundary layer during periods of reduced oxygen, which renders the mesopelagic fauna more vulnerable to visually orienting predators. The mesopelagic fish fauna provides a vital trophodynamic link between the marine plankton and many higher predators. The decline of deepwater fish populations has profound implications for commercial fisheries, marine food webs and marine conservation: climate models predict a 20 to 40% decline in global deepwater oxygen concentrations over the coming century.KEY WORDS: Oxygen · Mesopelagic fishes · Oxygen-minimum zone · Climate change · California CurrentResale or republication not permitted without written consent of the publisher

With a mounting imperative to advance stewardship strategies that consider the special features of the deep ocean and ensure that this biome serves future generations, we must promote long-term, deep-ocean sustainability through precaution, knowledge creation, and governance development.

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Zooplankton are the intermediate trophic level between phytoplankton and fish, and are an important component of carbon and nutrient cycles, accounting for a large proportion of the energy transfer to pelagic fishes and the deep ocean. Given zooplankton's importance, models need to adequately represent zooplankton dynamics. A major obstacle, though, is the lack of model assessment. Here we try and stimulate the assessment of zooplankton in models by filling three gaps. The first is that many zooplankton observationalists are unfamiliar with the biogeochemical, ecosystem, size-based and individual-based models that have zooplankton functional groups, so we describe their primary uses and how each typically represents zooplankton. The second gap is that many modelers are unaware of the zooplankton data that are available, and are unaccustomed to the different zooplankton sampling systems, so we describe the main sampling platforms and discuss their strengths and weaknesses for model assessment. Filling these gaps in our understanding of models and observations provides the necessary context to address the last gap-a blueprint for model assessment of zooplankton. We detail two ways that zooplankton biomass/abundance observations can be used to assess models: data wrangling that transforms observations to be more similar to model output; and observation models that transform model outputs to be more like observations. We hope that this review will encourage greater assessment of zooplankton in models and ultimately improve the representation of their dynamics.Keywords: plankton net, bioacoustics, optical plankton counter, Continuous Plankton Recorder, size-spectra, ecosystem model, observation model, model assessment Everett et al. Challenges for Zooplankton Model Assessment THE IMPORTANCE OF ZOOPLANKTONAll marine phyla are part of the zooplankton-either permanently as holoplankton (e.g., copepods or arrow worms) or temporarily as meroplankton (e.g., crab or fish larvae). In this review we define zooplankton as all organisms drifting in the water whose locomotive abilities are insufficient to progress against ocean currents (Lenz, 2000). Their sizes range from flagellates (about 20 µm) to siphonophores up to 30 m long. Zooplankton are the intermediate trophic level between phytoplankton and fish and are an important component of carbon and nutrient cycles in the ocean. They account for a large proportion of the energy transfer to fish on continental shelves (Marquis et al., 2011), temperate reefs (Kingsford and MacDiarmid, 1988;Champion et al., 2015), seagrass meadows (Edgar and Shaw, 1995), and coral reefs (Hamner et al., 1988;Frisch et al., 2014). Zooplankton are also key in the transfer of energy between benthic and pelagic domains (Lassalle et al., 2013). Zooplankton are responsible for transferring energy to deep water through the sinking of fecal pellets and moribund carcases (Stemmann et al., 2000;Henschke et al., 2013Henschke et al., , 2016 or through diel vertical migration (Ariza et al., 2015) and can play...