A shady phytoplankton paradox: when phytoplankton increases under low light

Light is a fundamental driver of ecosystem dynamics, affecting the rate of photosynthesis and primary production. In spite of its importance, less is known about its community-scale effects on aquatic ecosystems compared with those of nutrient loading. Understanding light limitation is also important for ecosystem management, as human activities have been rapidly altering light availability to aquatic ecosystems. Here we show that decreasing light can paradoxically increase phytoplankton abundance in shallow lakes. Our results, based on field manipulation experiments, field observations, and models, suggest that, under competition for light and nutrients between phytoplankton and submersed macrophytes, alternative stable states are possible under high light supply. In a macrophyte-dominated state, as light decreases phytoplankton density increases, because macrophytes (which effectively compete for nutrients released from the sediment) are more severely affected by light reduction. Our ...光照是驱动生态系统动态变化的核心因子，可直接影响光合作用（photosynthesis）速率与初级生产力（primary production）。尽管其生态重要性不言而喻，但相较于营养负荷（nutrient loading），学界对其在水生生态系统中的群落尺度效应的认知仍较为匮乏。厘清光照限制效应对于生态系统管理亦具有重要意义——当前人类活动正快速改变水生生态系统的光照可获得性。本研究发现，光照减弱可出乎意料地提升浅水湖泊中的浮游植物（phytoplankton）丰度。本研究基于野外操控实验（field manipulation experiments）、野外原位观测与数值模型开展分析，结果显示：在浮游植物与沉水植物（submersed macrophytes）对光照和养分的竞争场景下，当光照供给充足时，生态系统存在两种交替稳定态（alternative stable states）。在沉水植物占优的稳定态中，随着光照减弱，浮游植物种群密度反而上升——这是由于沉水植物可有效竞争沉积物（sediment）释放的养分，其自身受光照减弱的负面影响更为显著。本研究的……