We compare estimates of biomass and communlty structure based upon depth-stratified sampling from 0 to 900 m at a site on the continental slope south of Tasmania. Australia, using an EK500 acoustic system with vessel-mounted and deeply towed 38 kHz split-beam transducers and a midwater trawl with opening-closing codend system. Multivariate analyses of the target strength (TS) distributions and of dominant species in the tows provided consistent views of community structure: an epipelagic communlty that was differentiated and mid-depth and deep communities not differentiated on a die1 basis. Peaks in the TS distributions and the estimated TS of key biological groups were congruent, although the in situ TS data indicated that fishes with swimbladders in the size range of myctophids have lower TS (up to 5 dB) than previously predicted for physoclists, possibly due to the smaller volume of gas-filled swimbladders among mesopelagic fishes. There was a progressive increase in modal TS distributions with depth, consistent with the larger mean s~z e of both squids and fishes in the deeper strata. Bionlass was lowest in the dayt~me near-surface stratum, but there were no other trends of biomass with depth. The acoustic estimate of biomass based upon the trawl-derived assessment of community composition was 7-fold higher than the trawl-based volume-swept biomass estimate. This acoustic biomass estimate was 3-fold less than one based upon an assessment of community composition derived from TS distributions. The higher acoustic biomass estimates are consistent with regional estimates of primary production and simple trophodynamic calculations.

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...