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

Abstract

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.

Physical oceanography of New Zealand/Aotearoa shelf seas – a review

“…Extending this, Zeldis et al (2005) suggest that stronger winds drive more vertical mixing which is correlated with greater mixed-layer primary biomass and productivity. This ecosystem response to wind mixing may partially explain the correlation of sea temperatures with recruitment, previously observed for the Hauraki Gulf snapper stock (Zeldis et al 2005;Zeldis and Willis 2015).…”

Balance of Catchment and Offshore Nutrient Loading and Biogeochemical Response in Four New Zealand Coastal Systems: Implications for Resource Management

“…Net primary production (NPP) measured at the NIWA outer Firth monitoring site (Fig. 1) increases in spring and is maximal in summer (Chang et al 2003;Zeldis and Willis 2015), but with onset of summer water column stratification (see Fig. 9c) and decreased river loads (Fig.…”

Multi-locus DNA metabarcoding of zooplankton communities and scat reveal trophic interactions of a generalist predator

“…However, recent evidence shows that salps in the Southern Ocean form an important part of whale and seabird diet with higher levels of protein, carbon content and other nutrients than previously thought 71,72 . Thalia democratica is seasonally common in the Hauraki Gulf where it can be found in densities in excess of 300 per m 3 22,73 , and could form an important and regular part of the diet for Bryde’s whales, as it does for other predators 74 . It is unknown whether or how Bryde’s whales are choosing their prey in other populations 16 , but there is evidence that other baleen whales make decisions about prey aggregations before expending energy on a lunge feeding bout 64 .…”

“…1). Zooplankton were sampled ( n = 9) using a single vertical haul using 200 μm mesh plankton net for details see 22 . We assigned the samples to align with seasonal temperature regimes: cool-season (Austral winter and spring; 13–17 °C) and warm-season (Austral summer and autumn: 17–21 °C) 36 .…”

“…Given that Bryde’s whales are a generalist predator, foraging in surface waters 20 , we hypothesise that in the Hauraki Gulf the diet of Bryde’s whales follows seasonal shifts in zooplankton community composition. Zooplankton studies using manual sorting techniques and morphological identification have found that seasonal changes in salinity, water temperature and productivity shape the composition of the entire zooplankton community 21,22 . As the Gulf is shallow (<50 m depth), these broad seasonal trends are likely more important drivers of ecosystem dynamics than the limited impact of shorter-term factors such as diurnal variation in environmental conditions 22 .…”