It is well-known that nutrients shape phytoplankton communities in marine systems, but in coastal waters allochthonous dissolved organic matter (ADOM) may also be of central importance. We studied how humic substances (proxy of ADOM) and other variables influenced the nutritional strategies, size structure and pigment content of the phytoplankton community along a south–north gradient in the Baltic Sea. During the summer, the proportion of mixotrophs increased gradually from the phosphorus-rich south to the ADOM-rich north, probably due to ADOM-fueled microbes. The opposite trend was observed for autotrophs. The chlorophyll a (Chl a): carbon (C) ratio increased while the levels of photoprotective pigments decreased from south to north, indicating adaptation to the darker humic-rich water in the north. Picocyanobacteria dominated in phosphorus-rich areas while nanoplankton increased in ADOM-rich areas. During the winter–spring the phytoplankton biomass and concentrations of photoprotective pigments were low, and no trends with respect to autotrophs and mixotrophs were observed. Microplankton was the dominant size group in the entire study area. We conclude that changes in the size structure of the phytoplankton community, the Chl a:C ratio and the concentrations of photoprotective pigments are indicative of changes in ADOM, a factor of particular importance in a changing climate.

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外来物质：塑造波罗的海浮游植物群落的重要因素

众所周知，营养物质塑造了海洋系统中的浮游植物群落，但在沿海水域，外来溶解有机物 (ADOM) 也可能具有核心重要性。我们研究了腐殖质（ADOM 的替代物）和其他变量如何影响波罗的海南北梯度浮游植物群落的营养策略、大小结构和色素含量。在夏季，混合营养菌的比例从富含磷的南部到富含 ADOM 的北部逐渐增加，这可能是由于以 ADOM 为燃料的微生物。对于自养生物观察到相反的趋势。叶绿素 a (Chl a): 碳 (C) 比增加，而光保护色素的水平从南到北减少，表明对北方较暗的富含腐殖质的水的适应。Picocyanobacteria 在富含磷的区域占主导地位，而在富含 ADOM 的区域中纳米浮游生物增加。在冬春季，浮游植物生物量和光保护色素的浓度很低，没有观察到自养生物和混合养生物的趋势。微浮游生物是整个研究区的主要大小组。我们得出结论，浮游植物群落大小结构、Chl a:C 比率和光保护色素浓度的变化表明 ADOM 的变化，ADOM 是气候变化中特别重要的一个因素。微浮游生物是整个研究区的主要大小组。我们得出结论，浮游植物群落大小结构、Chl a:C 比率和光保护色素浓度的变化表明 ADOM 的变化，ADOM 是气候变化中特别重要的一个因素。微浮游生物是整个研究区的主要大小组。我们得出结论，浮游植物群落大小结构、Chl a:C 比率和光保护色素浓度的变化表明 ADOM 的变化，ADOM 是气候变化中特别重要的一个因素。