Productivity dominated by picoplankton in a macro-tidal tropical estuary, Darwin Harbour

Productivity around the tropical coast of North Australia has been considered to be dominated by phytoplankton. The phytoplankton encompasses a large taxon which, dependent on the source definition, can vary significantly. Traditionally, they would only include pelagic photosynthetic organisms (autotrophic), yet more recently the definition is being revised to include heterotrophic organisms. Recent advancements in oceanography over the past three decades have found the prevalence in the global ocean of cyanobacteria of sizes ranging from 0.5–1.5 µm. Two cyanobacterial genera <i>Synechoccocus</i> and <i>Prochlorococcus</i> are now recognised to dominate phytoplankton cell counts and biomass specifically in the oligotrophic tropical and subtropical oceans. This study is the first to identify the importance of picoplankton as a foremost contributor to productivity in a macro-tidal ecosystem in Northern Tropical Australia. Picoplankton was found to be the dominant group of primary producers (&gt; 99% or 6 × 10⁹ cells/L) within the phytoplankton in Darwin Harbour during the dry season of 2013. The larger phytoplankton &gt; 2 µm contributed (&lt; 0.05% or 2 × 10² cells/L) of the population. This study further defines the influences on this dynamic system that would allow the picoplankton community to thrive, highlighting the complexity of this nitrogen-limited system, where nutrient cycling, including nitrogen fixation mediated possibly through the microbial community, plays a pertinent role. Other environmental influences include spring tides, spatial variation and increased turbidity which benefits heterotrophy over autotrophy in this ecosystem.

澳大利亚北部热带沿岸海域的生产力历来被认为以浮游植物（phytoplankton）为主导。浮游植物是一个庞大的类群，其界定范围因定义来源不同而存在显著差异。传统上其范畴仅包含远洋光合自养生物，而近年来该定义正被修订，将异养生物纳入其中。近三十年来海洋学领域的一系列研究进展显示，全球海洋中广泛存在尺寸介于0.5–1.5微米的蓝细菌（cyanobacteria）。目前学界已确认，聚球藻属（*Synechococcus*）和原绿球藻属（*Prochlorococcus*）是贫营养热带及亚热带海域浮游植物细胞数量与生物量的主要贡献类群。本研究首次证实了超微型浮游生物（picoplankton）在澳大利亚北部热带大型潮汐生态系统生产力中的核心地位：2013年旱季对达尔文港（Darwin Harbour）的调查显示，浮游植物群落中的优势初级生产者为超微型浮游生物，其占比超过99%，细胞密度达6×10⁹ 个/升；而尺寸大于2微米的大型浮游植物仅占总种群的不足0.05%，细胞密度仅为2×10² 个/升。本研究进一步阐明了驱动该动态生态系统中超微型浮游生物群落繁盛的各类影响因素，凸显了这一氮限制生态系统的复杂性——其中包括可能由微生物群落介导的固氮作用在内的养分循环，在此系统中发挥着关键作用。其他环境影响因素包括大潮、空间异质性以及浊度升高，后者会使该生态系统中的异养作用相较于自养作用更具优势。