Interactive effects of rising temperature and nutrient enrichment on aquatic plant growth, stoichiometry, and palatability

The abundance and stoichiometry of aquatic plants are crucial for nutrient cycling and energy transfer in aquatic ecosystems. However, the interactive effects of multiple global environmental changes, including temperature rise and eutrophication, on aquatic plant stoichiometry and palatability remain largely unknown. Here, we hypothesized that (1) plant growth rates increase faster with rising temperature in nutrient-rich than nutrient-poor sediments; (2) plant carbon (C): nutrient ratios (nitrogen (N) and phosphorus (P)) respond differently to rising temperatures at contrasting nutrient conditions of the sediment; (3) external nutrient loading to the water column limits the growth of plants and decreases plant C:nutrient ratios; and that (4) changes in plant stoichiometry affect plant palatability. We used the common rooted submerged plant Vallisneria spiralis as a model species to test the effects of temperature and nutrient availability in both the sediment and the water column on plant growth and stoichiometry in a full-factorial experiment. The results confirmed that plants grew faster in nutrient-rich than nutrient-poor sediments with rising temperature, whereas external nutrient loading decreased the growth of plants due to competition by algae. The plant C: N and C: P ratios responded differently at different nutrient conditions to rising temperature. Rising temperature increased the metabolic rates of organisms, increased the nutrient availability in the sediment and enhanced plant growth. Plant growth was limited by a shortage of N in the nutrient-poor sediment and in the treatment with external nutrient loading to the water column, as a consequence, the limited plant growth caused an accumulation of P in the plants. Therefore, the effects of temperature on aquatic plant C:nutrient ratios did not only depend on the availability of the specific nutrients in the environment, but also on plant growth, which could result in either increased, unaltered or decreased plant C:nutrient ratios in response to temperature rise. Plant feeding trial assays with the generalist consumer Lymnaea stagnalis (Gastropoda) did not show effects of temperature or nutrient treatments on plant consumption rates. Overall, our results implicate that warming and eutrophication might interactively affect plant abundance and plant stoichiometry, and therefore influence nutrient cycling in aquatic ecosystems.水生植物的丰度与化学计量比（stoichiometry）对于水生生态系统的养分循环与能量传递至关重要。然而，包括温度升高与富营养化（eutrophication）在内的多重全球环境变化，其对水生植物化学计量比与适口性（palatability）的交互效应，目前仍尚未得到充分阐释。本研究提出四项假说：其一，相较于贫营养沉积物，富营养沉积物中的植物生长速率随温度升高的增幅更为显著；其二，当沉积物养分条件存在差异时，植物碳（C）:养分比值（氮（N）、磷（P））对温度升高的响应模式各不相同；其三，水体中的外源养分输入会抑制植物生长，并降低植物的C:养分比值；其四，植物化学计量比的改变会影响其适口性。本研究以常见沉水植物苦草（Vallisneria spiralis）为模式物种，采用全因子实验（full-factorial experiment），探究沉积物与水体中的温度与养分有效性对植物生长及化学计量比的调控效应。实验结果证实：随温度升高，富营养沉积物中的植物生长速率显著高于贫营养沉积物；但水体外源养分输入则因藻类竞争抑制了植物生长。植物C:N与C:P比值在不同养分条件下对温度升高的响应存在显著差异。温度升高可提升生物体代谢速率，提高沉积物养分有效性并促进植物生长。在贫营养沉积物组与水体外源养分输入处理组中，植物生长受氮限制，进而导致植物体内磷的积累。由此可见，温度对水生植物C:养分比值的调控不仅取决于环境中特定养分的有效性，同时与植物生长状态密切相关，这使得温度升高下植物C:养分比值可能呈现升高、不变或降低三种变化趋势。针对广食性消费者静水椎实螺（Lymnaea stagnalis，腹足纲Gastropoda）的植物摄食试验表明，温度与养分处理均未对植物取食率产生显著影响。综上，本研究结果显示，气候变暖与富营养化可能通过交互作用影响植物丰度与化学计量比，进而对水生生态系统的养分循环过程产生调控作用。