Characterization of a Southern Ocean deep chlorophyll maximum: response of phytoplankton to light, iron, and manganese enrichment

Southern Ocean phytoplankton growth is limited by low iron (Fe) supply and irradiance, impacting the strength of the biological carbon pump. Unfavourable upper ocean conditions such as low nutrient concentrations can lead to the formation of deep chlorophyll or biomass maxima (DCM/DBM). While common in the Southern Ocean, these features remain under-studied due to their subsurface location. To increase our understanding of their occurrence, we studied the responses of phytoplankton communities from a Southern Ocean DCM to increasing light, Fe, and manganese (Mn) levels. The DCM communities were light- and Fe-limited, but light limitation did not increase phytoplankton Fe requirements. The greatest physiological responses were observed under combined Fe/light additions, which stimulated macronutrient drawdown, biomass production and the growth of large diatoms. Combined Mn/light additions induced subtle changes in Fe uptake rates and community composition, suggesting species-specific Mn requirements. These results provide valuable information on Southern Ocean DCM phytoplankton physiology.

南大洋浮游植物的生长受低铁（Fe）供应与光照辐照度共同限制，这会影响生物碳泵（Biological Carbon Pump）的作用强度。上层海洋的不利环境条件（如低营养盐浓度）可促成深层叶绿素最大值层或生物量最大值层（DCM/DBM）的形成。这类特征虽在南大洋广泛分布，但因其处于次表层，尚未得到充分研究。为深化对这类特征发生机制的认知，我们针对取自南大洋深层叶绿素最大值层的浮游植物群落，探究了其对光照、铁（Fe）与锰（Mn）浓度升高的响应。该群落同时受光照与铁限制，但光照限制并未提升浮游植物对铁的需求。联合添加铁与光照的实验组观测到最显著的生理响应：该处理可促进大量营养盐消耗、生物量生成以及大型硅藻的生长。联合添加锰与光照的实验组仅引发铁摄取速率与群落组成的细微变化，这表明浮游植物存在物种特异性的锰需求。本研究结果为南大洋深层叶绿素最大值层浮游植物的生理特性提供了极具价值的参考数据。