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Mesoscale eddies drive variability in phytoplankton functional trait composition and primary productivity (PP) relative to adjacent waters. Offshore waters in southeast Australia are subject to substantial mesoscale eddies that form when the East Australian Current (EAC) travels poleward along the coast, forming distinctive habitats in the upper ocean. Eddies provide an important enrichment mechanism in the nitrogen-limited waters of the Tasman Sea, yet there is limited knowledge of PP within cold- and warm-core eddies in the region and how physico-chemical and biological factors affect phytoplankton communities in this variable environment. We addressed the scarcity of observations by quantifying net PP using 13C isotopic enrichment incubations of surface waters over 10 degrees of latitude, comparing phytoplankton species composition in five different environments: a coastal shelf station, an oceanic cold and warm-core eddy, and a coastal dipole. Cold-core (cyclonic) eddies were significantly more productive than their warm-core (anticyclonic) counterparts (∼35 versus 0 mg C m−3 d−1), with centric diatoms the most prominent phytoplankton group, with a relatively high centric:pennate ratio. The diffuse attenuation coefficient, K d (PAR), and silicate were the best overall predictors of phytoplankton composition, explaining 88% of variation based on pigment analysis and size fractionation. Variance in net PP did not correlate significantly with physico-chemical parameters frequently used in PP models (temperature, K d [PAR], Chl a), yet inclusion of size-fractionated Chl a generated substantial improvement in our statistical model (from 36 to 77%). We show that cold-core eddies play a key role in regulating PP in eastern Australian waters and highlight a need for eddy-resolving models to incorporate descriptors of phytoplankton size structure to improve the accuracy of PP forecasts in eddy intensive regions. • Mesoscale eddies modulate primary productivity dynamics along the East Australian coast. • Knowledge of phytoplankton size structure improves prediction of primary productivity. • Phytoplankton community structure can be predicted from light and silicate levels. • Centric diatoms are the most prominent phytoplankton group in East Australian cyclonic eddies. [ABSTRACT FROM AUTHOR]

Larval fish have been shown to vertically migrate with ontogeny, a behaviour that influences their dispersal by exposing the larvae to currents with different velocities at different stages of development. Most studies have focused on studying the vertical migration of tropical reef fishes, that have been shown in general to migrate downwards with ontogeny. Here we investigated if similar ontogenetic vertical migration patterns also occur for seven families of temperate reef fish along the eastern coast of Australia, sampling at three depth ranges (0–1 m, 5–50 m, 51–100 m) across sixteen stations along the continental shelf, and eight within cold-core eddies. Larval fish were identified to the lowest taxonomic level possible, their standard length measured, and ontogenetic stage classified to preflexion, flexion or postflexion. Generalised linear models were used to determine if the larval fish exhibited patterns of vertical migration with ontogeny and length. Four families (Labridae, Serranidae, Scorpaenidae, and Synodontidae) exhibited a downwards migration with ontogeny similar to vertical distribution patterns seen in tropical reef fish studies, Mullidae displayed different vertical migration patterns within eddies compared to coastal waters, and two families showed no ontogenetic vertical migration (Pomacentridae and Scaridae). For all families other than Scaridae, the larvae inside an eddy were longer suggesting larval retention. The vertical migration patterns observed, while varying among taxa, show that reef-associated larval fish off the eastern Australian coast do change their depth with ontogeny. The results of this study will be useful in parameterising connectivity modelling studies for these temperate reef-associated families. • There was a general pattern of larval fish increasing depth with age • Stage-dependent vertical distributions were not consistent across families • There was no effect of eddies on vertical distribution • The effect of environmental variables was not uniform across families [ABSTRACT FROM AUTHOR]

The biogeography of the deep-sea benthic fauna is uncertain due to the vast size and incomplete exploration of these environments. While shallow water assemblages are differentiated into tropical, temperate and polar faunas, it is unknown whether these units extend to lower depths. Here we use model-based and multivariate statistics to analyse megafaunal benthic samples along a 2300 km transect off the eastern Australian continental margin. We show that a temperate-tropical transition between 33-30°S occurs at both lower bathyal (~2500 m) and abyssal (~4000 m) depths. This transition occurs despite almost uniform temperature, salinity and dissolved oxygen concentrations occurring across latitudes at these depths. Conversely, the patterns are consistent with the flux of organic matter to the seafloor, which varies from being relatively high in the productive temperate off SE Australia to low levels in more-oligotrophic tropical waters. Biodiversity is not uniform across the deep-sea and regional-scale heterogeneity needs to be incorporated into marine park designs. • There is a significant temperate-tropical transition in benthic assemblages at bathyal-abyssal depths off eastern Australia • The transition is congruent with variation in net primary productivity but not gradients of temperature, salinity or oxygen • The deep-sea habitats are not uniform and assemblage composition differs at regional scales [ABSTRACT FROM AUTHOR]