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Off the eastern coast of New Zealand, warm, saline, nutrient-poor Subtropical Waters (STW) are separated from cool, fresher, relatively nutrient-rich Sub-Antarctic Waters (SAW) by the Subtropical Convergence (STC). The Chatham Rise, a submarine rise, restricts the latitudinal movement of the STC as well as mixing of STW and SAW Due to this restriction, this sector of the STC is characterized by sharp gradients in temperature, macro- (nitrate, silicate and phosphate) and micro- (iron) nutrient concentrations. Shipboard incubations were conducted during austral spring 2000 and 2001 to test the hypothesis that these gradients affect the taxonomic composition and/or growth rates of phytoplankton on either side of and at the STC. Maximum chlorophyll a concentrations during 2000 were 0.39 μg L -1 , but were an order of magnitude higher in 2001. During both years, STC phytoplankton were dominated by diatoms (77% of the total chlorophyll a during austral spring 2000 and 70% during spring 2001), whereas cryptophytes and prasinophytes dominated STW assemblages (27 and 36% during 2000, and 63 and 17% during 2001). Chlorophyll in the SAW was dominated by procaryotes and photosynthetic nanoflagellates during 2000 (17% procaryotes, 68% nanoflagellates), and by diatoms during the austral spring 2001 cruise (53%). Growth rates of the phytoplankton assemblage were determined by 14 C-labeling of chlorophyll a and photosynthetic pigments. During 2000, temperature-normalized growth rates were near maximal at the STC, and decreased on average to less than half of the maximum north and south of that front, whereas in 2001 both absolute and relative growth rates were low at all stations. Growth rates did not closely parallel biomass of the various taxa, suggesting that nutrient limitation and/or grazing were significantly impacting standing stocks. It appeared that growth was strongly influenced by nutrients and light, but that biomass was more strongly influenced by grazing The STC is a globally important region of enhanced biomass and productivity; however, the phytoplankton assemblage reflects control by both top-down and bottom-up processes that makes a predictive understanding of the area's biogeochemical cycles extremely difficult.

Taxonomic composition and growth rates of phytoplankton assemblages at the Subtropical Convergence east of New Zealand