Spatial and seasonal dynamics of microbial loop fluxes were investigated in contrasting productivity regimes in the Indian sector of the Southern Ocean. Observations carried out in late summer (February–March 2018; project MOBYDICK) revealed higher microbial biomasses and fluxes in the naturally iron‐fertilized surface waters of Kerguelen island in comparison to surrounding off‐plateau waters. Differences were most pronounced for bacterial heterotrophic production (2.3‐fold), the abundance of heterotrophic nanoflagellates (HNF; 2.7‐fold). Independent of site, grazing by HNF was the main loss process of bacterial production (80–100%), while virus‐induced mortality was low (< 9%). Combining these results with observations from previous investigations during early spring and summer allowed us to describe seasonal patterns in microbial food web fluxes and to compare these to carbon export in the iron‐fertilized and high‐nutrient, low‐chlorophyll (HNLC) Southern Ocean. Our data suggest an overall less efficient microbial food web during spring and summer, when respiration and viral lysis, respectively, represent important loss terms of bacterially‐mediated carbon. In late summer, primary production is more efficiently transferred to bacterial biomass and HNF and thus available for higher trophic levels. These results provide a new insight into the seasonal variability and the quantitative importance of microbial food web processes for the fate of primary production in the Southern Ocean.

中文翻译：

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不同于南部海洋生产力状况的季节性微生物食物网动态

微生物环通量的空间和季节动态进行了调查，以对比南印度洋地区的生产力状况。夏末（2018年2月至3月； MOBYDICK项目）进行的观测显示，与周围的高原以外水域相比，克尔格伦岛的天然铁肥地表水中微生物的生物量和通量更高。细菌异养产生的差异最为明显（2.3倍），异养纳米鞭毛的丰度最高（HNF； 2.7倍）。HNF放牧是细菌生产的主要损失过程（80-100％），而病毒引起的死亡率却很低（<9％），而与地点无关。将这些结果与早春和夏季以前的调查结果相结合，使我们能够描述微生物食物网通量的季节性模式，并将其与铁肥和高营养低叶绿素（HNLC）南部海洋中的碳出口进行比较。我们的数据表明，在春季和夏季，微生物的食物网总体效率较低，这时呼吸和病毒裂解分别代表了细菌介导的碳的重要损失。在夏末，初级生产更有效地转移到细菌生物量和HNF，因此可用于更高的营养水平。这些结果为季节性变化和微生物食物网过程对南大洋初级生产命运的定量重要性提供了新的认识。