Interacting temperature, nutrients and zooplankton grazing control phytoplankton size-abundance relationships in eight Swiss lakes

lake data set for the study "Interacting temperature, nutrients and zooplankton grazing control phytoplankton size-abundance relationships in eight Swiss lakes". Abstract: Biomass distribution among size classes follows a power law where the Log-abundance of taxa scales to Log-size with a slope that responds to environmental abiotic and biotic conditions. The interactions between ecological mechanisms controlling the slope of locally realized size-abundance relationships are however not well understood. Here we tested how warming, nutrient levels, and grazing affect the slope of phytoplankton community size-abundance relationships in decadal time-series from eight Swiss lakes of the peri-alpine region, which underwent environmental forcing due to climate change and oligotrophication. We expected rising temperature to have a negative effect on slope (favoring small phytoplankton), and increasing nutrient levels and grazing pressure to have a positive effect (benefitting large phytoplankton). Using a random forest approach to extract robust patterns from the noisy data, we found that the effects of temperature (direct and indirect through water column stability), nutrient availability (phosphorus and total biomass), and large herbivore (copepods and daphnids) grazing and selectivity on slope were non-linear and interactive. Increasing water temperature or total grazing pressure, and decreasing phosphorus levels, had a positive effect on slope (favoring large phytoplankton, which are predominantly mixotrophic in the lake dataset). Our results therefore showed patterns that were opposite to the expected long-term effects of temperature and nutrient levels, and support a paradigm in which i) small phototrophic phytoplankton appear to be favored under high nutrients levels, low temperature and low grazing, and ii) large mixotrophic algae are favored under oligotrophic conditions when temperature and grazing pressure are high. The effects of temperature were stronger under nutrient limitation, and the effects of nutrients and grazing were stronger at high temperature. Our study shows that the phytoplankton local size-abundance relationships in lakes respond to both the independent and the interactive effects of resources, grazing and water temperature in a complex, unexpected way, and observations from long-term studies can deviate significantly from general theoretical expectations.本数据集服务于题为《瑞士八个湖泊中温度、营养盐与浮游动物捕食交互作用调控浮游植物大小-丰度关系》（Interacting temperature, nutrients and zooplankton grazing control phytoplankton size-abundance relationships in eight Swiss lakes）的研究课题。摘要：不同粒径等级间的生物量分布服从幂律分布，即类群的对数丰度与对数粒径呈尺度缩放关系，其斜率响应环境中非生物与生物因子的变化。然而，调控局域尺度下实测大小-丰度关系斜率的各类生态机制间的交互作用，目前尚未得到充分阐释。本研究基于阿尔卑斯山前区8个瑞士湖泊的十年时间序列数据，探究增温、营养盐水平与捕食压力如何影响浮游植物群落的大小-丰度关系斜率——这些湖泊均经历了气候变化与贫营养化（oligotrophication）带来的环境胁迫。我们曾提出如下假说：温度升高会对斜率产生负向影响（偏向小型浮游植物），而营养盐水平与捕食压力的提升则会产生正向影响（利于大型浮游植物）。本研究通过随机森林（random forest）方法从含噪数据中提取稳健的模式，发现温度（通过水柱稳定性产生直接与间接作用）、营养盐可获得性（磷与总生物量）以及大型植食性浮游动物（桡足类与枝角类）的捕食与选择性对斜率的影响均呈非线性且存在交互作用。水温升高、总捕食压力提升，以及磷浓度降低，均会对斜率产生正向影响（利于大型浮游植物——本数据集内的大型浮游植物多为混合营养型（mixotrophic））。因此本研究结果与此前针对温度与营养盐长期效应的预期相悖，并支持如下理论范式：其一，在高营养盐、低温与低捕食压力条件下，小型光合自养（phototrophic）浮游植物更具优势；其二，在贫营养化且温度与捕食压力较高的环境中，大型混合营养藻类更占优势。营养盐限制条件下，温度的影响效应更强；而高温环境中，营养盐与捕食压力的影响效应更为显著。本研究表明，湖泊浮游植物的局域大小-丰度关系会以复杂且超出预期的方式响应资源、捕食与水温的独立及交互作用，且长期监测得到的观测结果可能与通用理论预期存在显著偏差。