Chlorophyll a is commonly used as a surrogate for algal biomass in lakes and oceans even though phytoplankton can readily adjust intracellular chlorophyll concentration to changes in the environment through a cellular process termed photoacclimation. Marine studies have documented fluctuations in cellular chlorophyll to year‐round changes in light intensity and nutrients using the chlorophyll : carbon ratio to estimate cellular pigment density. However, similar freshwater studies are almost exclusively limited to chlorophyll : carbon ratios at the summertime deep chlorophyll maximum (DCM) depth in less productive water. This study expands direct estimates of freshwater photoacclimation to all seasons of the year utilizing optically derived estimates of algal carbon and chlorophyll fluorescence from high‐frequency (daily) data. We describe patterns of photoacclimation at daily, seasonal, and interannual time scales over a 5‐yr period. We show that increases in chlorophyll density at the cellular level not only occurred at the DCM depth during summer stratification but were a dominant and recurring feature when the lake was vertically mixing in fall, winter, and spring. Pigment density within the mixed layer was negatively correlated with light level, indicating a photoacclimation driven response to reduced light intensity. Predicting how cellular pigment density is affected by the surrounding environment in a replicated range of lake systems is needed if chlorophyll is to be accurately used as an estimate of algal biomass in freshwaters.

中文翻译：

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5年多聚寡营养湖中浮游植物光驯化的季节性模式：叶绿素作为生物量估计的意义

叶绿素A尽管浮游植物可以通过称为光驯化的细胞过程轻松调整细胞内叶绿素浓度以适应环境的变化，但通常用作湖泊和海洋中藻类生物量的替代品。海洋研究已经记录了细胞叶绿素的波动与光强度和营养物质的全年变化的关系，使用叶绿素：碳比率来估计细胞色素密度。然而，类似的淡水研究几乎完全局限于夏季深叶绿素最大（DCM）深度的叶绿素：碳比率，在生产力较低的水中。这项研究利用高频（每日）数据对藻类碳和叶绿素荧光的光学估计，将淡水光驯化的直接估计扩展到一年中的所有季节。我们描述了 5 年期间每日、季节和年际时间尺度的光适应模式。我们发现，细胞水平上叶绿素密度的增加不仅发生在夏季分层期间的 DCM 深度，而且是秋季、冬季和春季湖泊垂直混合时的主要和反复出现的特征。混合层内的颜料密度与光水平呈负相关，表明光适应驱动对光强度降低的响应。如果要准确地使用叶绿素来估计淡水中的藻类生物量，则需要预测复制的一系列湖泊系统中细胞色素密度如何受到周围环境的影响。