The Chatham Rise supports some of New Zealand's most economically valuable commercial fish stocks, fuelled by the highly productive waters of the Subtropical Frontal Zone (STFZ). Climate change-related shifts in phytoplankton community structure and function are predicted and may affect Chatham Rise productivity and deep-sea carbon sequestration by the biological carbon pump. However, little is known about how two major phytoplankton groups, diatoms and coccolithophores, will respond, since the seasonality of their productivity and export fluxes to the seafloor are not well known in the SW Pacific sector. This study is the first to report on the seasonal cycle of phytoplankton species fluxes in the STFZ, using a 12-month sediment trap record (1996–1997) on the northern (subtropical-influenced) and southern (subantarctic-influenced) flanks of the Chatham Rise. Diatom and coccolith flux assemblages were characterised and compared with biogeochemical fluxes from the same sites over the same time-frame. Northern phytoplankton flux composition was typical of the subtropics, with most particulate organic carbon (POC) and biogenic silica export associated with diatom fluxes. Fluxes to the northern trap were largely composed of coastal and benthic diatom taxa, reinforcing previous interpretations suggesting lateral advection and sediment resuspension along and across the Chatham Rise slope, influenced also by the Wairarapa Eddy. At the southern site, phytoplankton fluxes were on average an order of magnitude higher than at the northern site and comprised a combination of subantarctic and frontal zone diatom taxa. The majority of the annual diatom flux in the southern trap occurred during a spring Pseudo-nitzschia “pulse” associated with high biogenic silica and POC fluxes, which are inferred to be from subsurface chlorophyll maxima. High coccolith flux at the southern STFZ site was consistent with ecosystem processes observed from other subantarctic export regimes. Phytoplankton successions inferred from the sediment trap records suggest that coccolithophores precede diatoms in spring, which is contrary to expectations from classical studies where the opposite temporal trend is suggested. While maximum diatom species flux co-occurred with peak POC flux at both sites in spring, the estimated contribution of organic carbon associated with diatoms to POC flux was negligible (<0.05%). Estimated coccolithophore POC flux was also low, suggesting that either other phytoplankton groups and/or zooplankton-mediated processes (e.g. fecal pellet fluxes) must contribute more substantially to POC export in the STFZ. Our findings provide new insights into phytoplankton export flux pathways to the seafloor in the Chatham Rise region, with implications for regional biogeochemistry, paleoceanographic and export modelling studies.

查塔姆河谷（Chatham Rise）以亚热带额叶带（STFZ）的高产水为生，为一些新西兰最具经济价值的商业鱼类种群提供了支持。预测了与气候变化有关的浮游植物群落结构和功能的变化，这些变化可能会影响Chatham Rise的生产力和生物碳泵对深海固碳的影响。但是，人们对两个主要的浮游植物群（硅藻和鳞茎藻类）将如何作出反应还知之甚少，因为在太平洋西南地区，生产力的季节性和向海底的出口通量并不为人所知。这项研究是第一个报告STFZ浮游植物物种通量的季节性周期的报告，使用了查塔姆崛起的北部（亚热带影响）和南部（亚南极影响）侧面的12个月沉积物陷阱记录（1996-1997）。表征了硅藻和椰油的通量组合，并与相同时间范围内相同地点的生物地球化学通量进行了比较。北部的浮游植物通量组成是亚热带地区的典型特征，大多数颗粒有机碳（POC）和生物硅的出口都与硅藻通量有关。北部陷阱的通量主要由沿海和底栖的硅藻类群组成，强化了先前的解释，表明沿Chatham Rise斜坡及横跨整个Chatham Rise斜坡的横向对流和沉积物再悬浮，也受到了Wairarapa涡流的影响。在南部站点，浮游植物的通量平均比北部地区高出一个数量级，包括亚极角质和额带硅藻类群的组合。南部陷阱的年硅藻通量大部分发生在春季假性尼兹卡与高生物硅和POC通量有关的“脉冲”，推测是来自地下叶绿素最大值。STFZ南部站点的高球藻通量与从其他亚极南极出口体制观察到的生态系统过程一致。从沉积物陷阱记录推论的浮游植物演替表明，在春季硅藻藻先于硅藻藻，这与经典研究的预期相反，经典研究提出了相反的时间趋势。虽然春季两个地点的最大硅藻通量都与峰值POC通量同时出现，但与硅藻相关的有机碳对POC通量的估计贡献可忽略不计（<0.05％）。估计的浮石藻POC通量也很低，表明其他浮游植物群和/或浮游动物介导的过程（例如 粪便颗粒通量）必须对STFZ中的POC出口做出更大的贡献。我们的发现为查塔姆崛起地区浮游植物出口通量通向海底的途径提供了新见解，对区域生物地球化学，古海洋学和出口建模研究具有重要意义。