Seasonal and spatial variation of phytoplankton assemblages, biomass and cell size from spring to summer across the north-eastern New Zealand continental shelf

The composition, biomass and cell size of phytoplankton taxonomic groups were determined in the Hauraki Gulf and adjacent shelf of north-eastern New Zealand. In early spring, on the inner shelf, over-winter mixing and upwelling supported a bloom dominated by large, chain-forming diatoms in a moderately turbulent water column. The bloom declined in late spring because of nutrient limitation, and the assemblage evolved initially toward small diatoms, and eventually to co-occurrence of dinoflagellates, small nanoflagellates and picophytoplankton in early and late summer. Mid- to outer-shelf biomass was much lower than inshore, and was dominated by small or motile taxa which were probably limited by grazing and light. In early summer, strong upwelling displaced inner shelf phytoplankton to beyond the shelf edge, whilst enriching the shelf with nutrients. However, shelf phytoplankton biomass increased only after the relaxation of upwelling. The Hauraki Gulf was strongly stratified from early spring through late summer. The flora were seasonally less variable than on the shelf, with a thecate dinoflagellate-dominated flora in early spring, replaced post-bloom by the co-occurrence of presumably low-nutrient-adapted autotrophic and/or heterotrophic dinoflagellates (most of which were non-thecate, and some toxic), nanoflagellates and picophytoplankton. The succession in floristics was consistent with a change from an autotrophic toward a heterotrophic ecosystem from spring to summer. Implications for secondary production, and vertical and lateral organic carbon export on the shelf, are discussed.