Heatwave-mediated decreases in phytoplankton quality negatively affect zooplankton productivity

Abstract Climate change is expected to increase the frequency of extreme temperature events. The effect of heatwaves on phytoplankton is of particular concern because they are a key source of C, N, P, and essential fatty acids to aquatic ecosystems. Laboratory studies have demonstrated that phytoplankton grown at warmer temperatures are a lower-quality food source, but how heatwaves affect phytoplankton quality at the community scale is currently unclear. Here we address this knowledge gap by growing natural assemblages of freshwater phytoplankton at ‘ambient’, ‘constant warming’, or ‘heatwave’ conditions. We next fed these phytoplankton communities to natural assemblages of zooplankton to test the prediction that zooplankton that consume heatwave-exposed phytoplankton will exhibit reductions in biomass. Our experiment demonstrated that zooplankton that consumed ‘heatwave’ phytoplankton attained lower community biomass than those fed ‘constant warming’ or ‘ambient’ phytoplankton. Additionally, despite receiving similar total heat input, phytoplankton exposed to ‘heatwave’ conditions contained lower C, N, P, and fatty acid concentrations compare to phytoplankton grown in ‘constant warming’ conditions. Correlations between zooplankton biomass and all measured phytoplankton traits revealed that decreases in zooplankton biomass were best explained by low quantities of C, N, and monounsaturated fatty acids in ‘heatwave’ phytoplankton. Our study demonstrates that the effects of heatwaves on phytoplankton quality are clearly distinct from those caused by constant warming temperatures, and that heatwave-mediated decreases in resource quality have immediate effects on consumer productivity. 
MethodsThe study was a laboratory experiment using wild-caught phytoplankton and zooplankton communities. All data were collected and analyzed by the authors except that phytoplankton were identified by Biologica Environmental Services in Victoria British Columbia, Canada. Phytoplankton taxonomy data were analyzed by the authors.

摘要 气候变化预计将提升极端温度事件的发生频次。热浪对浮游植物（phytoplankton）的影响尤为值得关注，因其是水生生态系统中碳（C）、氮（N）、磷（P）及必需脂肪酸的关键来源。已有实验室研究证实，在较高温度下培养的浮游植物为质量更低的食物源，但目前学界尚不清楚热浪在群落尺度上会如何影响浮游植物的质量。本研究通过将淡水浮游植物自然群落分别置于‘环境对照’‘持续增温’或‘热浪’三种条件下培养，填补了这一研究空白。随后将这些浮游植物群落投喂给自然群落的浮游动物（zooplankton），以验证如下预测：取食经热浪暴露的浮游植物的浮游动物，其生物量会出现下降。本实验结果显示，取食‘热浪组’浮游植物的浮游动物，其群落生物量显著低于取食‘持续增温组’或‘环境对照组’浮游植物的浮游动物群落。此外，尽管总热输入相近，经‘热浪’条件处理的浮游植物，其碳、氮、磷及脂肪酸浓度均低于‘持续增温组’浮游植物。对浮游动物生物量与所有测得的浮游植物性状间的相关性分析显示，浮游动物生物量的下降，可由‘热浪组’浮游植物中较低的碳、氮及单不饱和脂肪酸含量得到最佳解释。本研究表明，热浪对浮游植物质量的影响与持续增温所造成的影响显著不同，且热浪介导的资源质量下降会对消费者生产力产生直接影响。
方法 本研究为实验室实验，采用野外采集的浮游植物与浮游动物群落开展。所有数据均由作者团队收集与分析，仅浮游植物的鉴定工作由加拿大不列颠哥伦比亚省维多利亚的Biologica Environmental Services完成。浮游植物分类学数据由作者团队分析完成。