Data from: Experimental test of phytoplankton competition for nutrients and light in poorly mixed water columns

A recent theory of the vertical distribution of phytoplankton considers how interacting niche construction processes such as resource depletion, behavior, and population dynamics contribute to spatial heterogeneity in the aquatic environment. In poorly mixed water columns with opposing resource gradients of nutrients and light, theory predicts that a species should aggregate at a single depth. This depth of aggregation, or biomass maximum, should change through time due to depletion of available resources. In addition, the depth of the aggregation should be deeper under low amounts of nutrient loading and shallower under higher amounts of nutrient loading. Theory predicts total biomass to exhibit a saturating relationship with nutrient supply. A surface biomass maximum limited by light and a deep biomass maximum limited by nutrients or co-limited by nutrients and light is also predicted by theory. To test this theory, we used a motile phytoplankton species (Chlamydomonas reinhardtii) growing in cylindrical plankton towers. In our experiment, the resource environment was strongly modified by the movement, self-shading, nutrient uptake, and growth of the phytoplankton. Supporting predictions, we routinely observed a single biomass maximum at the surface throughout the course of the experiment and at equilibrium under higher nutrient loading. However, at equilibrium, low nutrient loading led to a non-distinct biomass maximum with the population distributed over most of the water column instead of the distinct subsurface peak predicted by theory. Also supporting predictions, total biomass across water columns was positively related to nutrient supply but saturating at high nutrient supply conditions. Further supporting predictions, we also found evidence of light limitation for a surface biomass maximum and nutrient limitation for the deep biomass when no surface maximum was present. In addition, the light level leaving the bottom of the water column declined through time as the phytoplankton grew and was negatively related to nutrient loading. Nutrients were strongly depleted where biomass was present by the end of the experiment. This experimental study shows that the vertical distribution of phytoplankton may be driven by intraspecific resource competition in space.

近期有关浮游植物垂直分布的理论，聚焦于资源消耗、行为特征与种群动态等相互作用的生态位构建（niche construction）过程，如何推动水生环境形成空间异质性。在营养与光照呈反向资源梯度且混合作用较弱的水柱中，该理论预测单一物种会聚集于单一深度。该聚集深度（即生物量峰值（biomass maximum））会因可用资源的消耗随时间发生变化。此外，在低营养负荷条件下，聚集深度会更深；而高营养负荷下则更浅。该理论还预测，总生物量与营养供给之间会呈现饱和关系（saturating relationship）。理论同时预测，存在两类生物量峰值模式：一类是受光照限制的表层生物量峰值，另一类是受营养限制或受营养与光照共同限制的深层生物量峰值。为验证该理论，我们选取运动型浮游植物物种莱茵衣藻（Chlamydomonas reinhardtii），在圆柱形浮游培养塔中开展培养实验。本实验中，浮游植物的运动、自身遮光、营养摄取与生长过程，会显著改变实验体系内的资源环境。实验结果部分支持理论预测：整个实验周期内，我们始终观察到表层存在单一生物量峰值；在高营养负荷条件下达到平衡状态时，也呈现该特征。但在低营养负荷条件下达到平衡时，并未出现理论预测的清晰次表层生物量峰值，种群反而均匀分布于大部分水柱中。另有实验结果支持理论预测：水柱总生物量与营养供给呈正相关关系，但在高营养供给条件下会趋于饱和。进一步的实验证据还表明，当不存在表层生物量峰值时，表层生物量峰值会受光照限制，深层生物量峰值则受营养限制。此外，随着浮游植物的生长，水柱底部透出的光照强度随时间逐渐降低，且与营养负荷呈负相关关系。实验结束时，存在浮游植物生物量的区域，其营养物质已被显著消耗。本实验研究表明，浮游植物的垂直分布可能由空间尺度下的种内资源竞争驱动。