School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, USA.
Ecology. 2011 Feb;92(2):373-85. doi: 10.1890/09-1694.1.
In coastal areas of the North Pacific Ocean, annual returns of spawning salmon provide a substantial influx of nutrients and organic matter to streams and are generally believed to enhance the productivity of recipient ecosystems. Loss of this subsidy from areas with diminished salmon runs has been hypothesized to limit ecosystem productivity in juvenile salmon rearing habitats (lakes and streams), thereby reinforcing population declines. Using five to seven years of data from an Alaskan stream supporting moderate salmon densities, we show that salmon predictably increased stream water nutrient concentrations, which were on average 190% (nitrogen) and 390% (phosphorus) pre-salmon values, and that primary producers incorporated some of these nutrients into tissues. However, benthic algal biomass declined by an order of magnitude despite increased nutrients. We also measured changes in stream ecosystem metabolic properties, including gross primary productivity (GPP) and ecosystem respiration (ER), from three salmon streams by analyzing diel measurements of oxygen concentrations and stable isotopic ratios (delta O-O2) within a Bayesian statistical model of oxygen dynamics. Our results do not support a shift toward higher primary productivity with the return of salmon, as is expected from a nutrient fertilization mechanism. Rather, net ecosystem metabolism switched from approximately net autotrophic (GPP > or = ER) to a strongly net heterotrophic state (GPP << ER) in response to bioturbation of benthic habitats by salmon. Following the seasonal arrival of salmon, GPP declined to <12% of pre-salmon rates, while ER increased by over threefold. Metabolism by live salmon could not account for the observed increase in ER early in the salmon run, suggesting salmon nutrients and disturbance enhanced in situ heterotrophic respiration. Salmon also changed the physical properties of the stream, increasing air-water gas exchange by nearly 10-fold during peak spawning. We suggest that management efforts to restore salmon ecosystems should consider effects on ecosystem metabolic properties and how salmon disturbance affects the incorporation of marine-derived nutrients into food webs.
在北太平洋的沿海地区，产卵鲑鱼的年回游为溪流带来了大量的营养物质和有机物质，通常被认为可以提高受纳生态系统的生产力。从鲑鱼数量减少的地区流失这种补贴，据推测会限制幼鲑鱼养殖栖息地（湖泊和溪流）的生态系统生产力，从而加剧种群数量的下降。利用阿拉斯加一条支持中等鲑鱼密度的溪流五年至七年的数据，我们表明鲑鱼可以预测性地增加溪流水中的营养物质浓度，这些浓度平均比鲑鱼出现前高出 190%（氮）和 390%（磷），并且初级生产者将其中一些营养物质纳入组织。然而，尽管营养物质增加，底栖藻类生物量却减少了一个数量级。我们还通过分析氧浓度和稳定同位素比值（氧-氧同位素比值）的昼夜测量值，在一个氧动态贝叶斯统计模型中，测量了来自三条鲑鱼溪流的溪流生态系统代谢特性的变化，包括总初级生产力（GPP）和生态系统呼吸（ER）。我们的结果不支持随着鲑鱼的回归，向更高的初级生产力转变的观点，这与营养物质施肥机制的预期相反。相反，净生态系统代谢从接近净自养（GPP>或= ER）状态转变为强烈的净异养状态（GPP< ER），这是由于鲑鱼对底栖生境的生物扰动。随着鲑鱼季节性的到来，GPP 下降到<鲑鱼出现前的 12%，而 ER 增加了三倍多。在鲑鱼洄游初期，活鲑鱼的新陈代谢无法解释 ER 的观察到的增加，这表明鲑鱼的营养物质和干扰增强了原位异养呼吸。鲑鱼还改变了溪流的物理性质，在产卵高峰期，空气-水气体交换增加了近 10 倍。我们认为，恢复鲑鱼生态系统的管理工作应考虑对生态系统代谢特性的影响，以及鲑鱼干扰如何影响海洋衍生营养物质纳入食物网的情况。
