Predator-Driven Nutrient Recycling in California Stream Ecosystems

Nutrient recycling by consumers in streams can influence ecosystem nutrient availability and the assemblage and growth of photoautotrophs. Stream fishes can play a large role in nutrient recycling, but contributions by other vertebrates to overall recycling rates remain poorly studied. In tributaries of the Pacific Northwest, coastal giant salamanders (Dicamptodon tenebrosus) occur at high densities alongside steelhead trout (Oncorhynchus mykiss) and are top aquatic predators. We surveyed the density and body size distributions of D. tenebrosus and O. mykiss in a California tributary stream, combined with a field study to determine mass-specific excretion rates of ammonium (N) and total dissolved phosphorus (P) for D. tenebrosus. We estimated O. mykiss excretion rates (N, P) by bioenergetics using field-collected data on the nutrient composition of O. mykiss diets from the same system. Despite lower abundance, D. tenebrosus biomass was 2.5 times higher than O. mykiss. Mass-specific excretion summed over 170 m of stream revealed that O. mykiss recycle 1.7 times more N, and 1.2 times more P than D. tenebrosus, and had a higher N:P ratio (8.7) than that of D. tenebrosus (6.0), or the two species combined (7.5). Through simulated trade-offs in biomass, we estimate that shifts from salamander biomass toward fish biomass have the potential to ease nutrient limitation in forested tributary streams. These results suggest that natural and anthropogenic heterogeneity in the relative abundance of these vertebrates and variation in the uptake rates across river networks can affect broad-scale patterns of nutrient limitation.

溪流中消费者介导的养分循环，可影响生态系统的养分可利用性，以及光合自养生物（photoautotrophs）的群落组成与生长。溪流鱼类在养分循环中扮演着关键角色，但其他脊椎动物对整体养分循环速率的贡献仍未得到充分研究。在太平洋西北地区的支流中，沿海巨蝾螈（Dicamptodon tenebrosus）与硬头鳟（Oncorhynchus mykiss）均以较高密度共存，且二者均为顶级水生捕食者。我们在加利福尼亚州的一条溪流支流中，对沿海巨蝾螈与硬头鳟的种群密度及体尺分布展开了调查，并结合野外实验测定了沿海巨蝾螈的单位体重铵态氮（N）与总溶解磷（P）排泄速率。我们利用同一流域内采集的硬头鳟日粮养分组成实地数据，通过生物能学（bioenergetics）方法估算了硬头鳟的氮、磷排泄速率。尽管沿海巨蝾螈的种群丰度更低，但其生物量却是硬头鳟的2.5倍。对170米河段内的单位体重排泄总量进行核算后发现，硬头鳟的氮循环量是沿海巨蝾螈的1.7倍，磷循环量则为其1.2倍；其氮磷比（N:P，8.7）也高于沿海巨蝾螈的6.0，以及两物种综合的7.5。通过模拟生物量的权衡变化，我们估算得出：当群落生物量结构从蝾螈主导转向鱼类主导时，有望缓解森林溪流支流中的养分限制状况。上述结果表明，这两类脊椎动物相对丰度的自然与人为异质性，以及河网中养分吸收速率的空间差异，均可影响大尺度的养分限制格局。