Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa.
Department of Ecology and Resource Management, University of Venda, Thohoyandou 0950, South Africa.
Sci Total Environ. 2019 Feb 15;651(Pt 1):410-418. doi: 10.1016/j.scitotenv.2018.09.132. Epub 2018 Sep 11.
Within a given ecosystem, species persistence is driven by responses to the effects of biotic and abiotic stressors. Ongoing climatic shifts and increased pollution pressure have created the need to assess potential effects and interactions of physical and biotic factors on coastal ecosystem processes to project ecosystem resilience and persistence. In coastal marine environments, primary production dynamics are driven by the interaction between bottom-up abiotic effects and biotic effects induced by top-down trophic control. Given the many environmental and climatic changes observed throughout coastal regions, we assessed the effects of interactions among temperature, nutrients and grazing in a laboratory-based microcosm experiment. We did this by comparing chlorophyll-a (chl-a) concentrations at two temperatures in combination with four nutrient regimes. To test for subsequent cascading effects on higher trophic levels, we also measured grazing and growth rates of the calanoid copepod Pseudodiaptomus hessei. We observed different phytoplankton and zooplankton responses to temperature (17 °C, 24 °C) and nutrients (nitrogen only (N), phosphates only (P), nitrogen and phosphates combined (NP), no nutrient additions (C)). Contributions of predictors to model fit in the boosted regression trees model were phosphates (42.7%), copepods (23.8%), nitrates (17.5%) and temperature (15.9%), suggesting phosphates were an important driver for the high chl-a concentrations observed. There was an increase in total phytoplankton biomass across both temperatures, while nutrient addition affected the phytoplankton size structure prior to grazing irrespective of temperature. Phytoplankton biomass was highest in the NP treatment followed by the N treatment. However, the phytoplankton size structure differed between temperatures, with microphytoplankton being dominant at 24 °C, while nanophytoplankton dominated at 17 °C. The P and C treatments exhibited the lowest phytoplankton biomass. Copepod abundances and growth rates were higher at 17 °C than at 24 °C. This study highlights that bottom-up positive effects in one trophic level do not always positively cascade into another trophic level. It was, however, evident that temperature was a limiting factor for plankton abundance, productivity and size structure only when nutrients were limiting, with top-down pressure exhibiting minimal effects on the phytoplankton.
在给定的生态系统中，物种的持久性是由对生物和非生物胁迫因素的影响的反应驱动的。持续的气候变化和污染压力的增加，使得需要评估物理和生物因素对沿海生态系统过程的潜在影响和相互作用，以预测生态系统的恢复力和持久性。在沿海海洋环境中，初级生产力的动态是由底栖非生物效应与由顶级捕食控制引起的生物效应之间的相互作用驱动的。鉴于在整个沿海地区观察到的许多环境和气候变化，我们在基于实验室的微宇宙实验中评估了温度、养分和放牧相互作用对沿海生态系统过程的影响。我们通过比较两个温度下的叶绿素 a（chl-a）浓度，并结合四种养分状况来做到这一点。为了测试对更高营养级的后续级联效应，我们还测量了桡足类桡足类拟大眼(Pseudodiaptomus hessei)的摄食率和生长率。我们观察到浮游植物和浮游动物对温度（17°C、24°C）和养分（仅氮（N）、仅磷酸盐（P）、氮和磷酸盐组合（NP）、无养分添加（C））的不同反应。在提升回归树模型中，对模型拟合有贡献的预测因子包括磷酸盐（42.7%）、桡足类（23.8%）、硝酸盐（17.5%）和温度（15.9%），这表明磷酸盐是观察到高 chl-a 浓度的重要驱动因素。在两个温度下，总浮游植物生物量都有所增加，而养分添加在摄食之前影响浮游植物的大小结构，而与温度无关。在 NP 处理下，浮游植物生物量最高，其次是 N 处理。然而，浮游植物的大小结构在温度之间有所不同，在 24°C 时以微浮游植物为主，而在 17°C 时以纳米浮游植物为主。P 和 C 处理的浮游植物生物量最低。桡足类的丰度和生长率在 17°C 时高于 24°C。本研究表明，一个营养级别的正向底栖效应并不总是正向级联到另一个营养级。然而，当营养物质受到限制时，温度是浮游生物丰度、生产力和大小结构的限制因素，而顶级捕食压力对浮游植物的影响最小，这一点很明显。
