Migratory fish excretion as a nutrient subsidy to recipient stream ecosystems

Abstract

Summary Consumer excretion can be an important component of nutrient cycling in aquatic ecosystems. Uncertainty concerning the functional role of many freshwater organisms remains, including those with migratory life‐history strategies that may introduce nutrients to recipient systems. We quantified the magnitude, variability and importance of excretion by migratory fish in the context of stream nutrient cycling. In 2011–12, we collected data from tributaries of a central Utah reservoir used by two potamodromous fishes (Bonneville cut‐throat trout – BCT, Oncorhynchus clarkii utah; kokanee salmon – KOK, Oncorhynchus nerka) with temporally separated spawning migrations. To quantify the contribution of the two migratory freshwater fishes to nutrient cycling, we extrapolated measurements of per capita nitrogen (N) and phosphorus (P) excretion rates to the population level within the local environmental context of two tributaries. We observed differences in excretion subsidies between species and tributaries. BCT excretion rates and ratios were significantly greater than those for KOK. Estimates of the ratio of population‐level migrant excretion to tributary nutrient export were highly variable through time and between tributaries. These estimates were influenced by spatiotemporal hydrologic variation and positively related to ratios of migrant biomass to discharge. During migrations, daily migrant excretion loading comprised a maximum of 6–859% and 1–388% of tributary NH4‐N and soluble reactive phosphorus (SRP) export, respectively. Measurements of nutrient uptake suggested that migrant excretion could meet a substantial portion of ecosystem nutrient demand. Migrant excretion fluxes comprised 46–188% of ecosystem NH4‐N demand and varied between streams and species. In contrast, the proportion of SRP demand supplied by migrant excretion (34–37%) was relatively invariant. These results demonstrate an important role for potamodromous fishes as nutrient sources in recipient ecosystems. Furthermore, our data support predictions about when and where effects of fish‐derived nutrients will be strongest, thereby advancing the understanding of context‐dependent migratory fish effects in riverine ecosystems. Although widespread and common, potamodromous fishes are overlooked but important organisms capable of substantially affecting stream nutrient cycling.