Biogeographic and trophic drivers of mesozooplankton distribution on the northeast continental shelf and in Hauraki Gulf, New Zealand

The northeast continental shelf of New Zealand and adjacent Hauraki Gulf were surveyed from early spring to late summer, to identify biogeographic and trophic factors influencing mesozooplankton community distribution and abundance. The outer shelf supported low-abundance oceanic species associated with the East Auckland Current which were also transported to the inner shelf during upwelling. Outer gulf sites, where gulf and shelf waters converged, supported a characteristic frontal assemblage. The inner gulf supported an abundant and diverse neritic community, with greater seasonal variability than offshore. Over spring to summer, succession of mesozooplankton taxa reflected an ecosystem shift from net autotrophic to heterotrophic states. These biogeographic and trophic effects defined the grazing impact mesozooplankton had on lower trophic levels, with grazing often matching primary production in spring and early summer. Greater mesozooplankton abundances in the gulf were supported by higher volume-specific primary productivity than on the shelf. The results have important implications for higher trophic levels, including larval fish survival, adult fish distributions and whale feeding.