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.

Introduced lake trout alter nitrogen cycling beyond Yellowstone Lake

“…We logged terms (base 10) and used ordinary least squares (OLS) regression (SPSS, version 12.0.1) to create a predictive allometric cutthroat trout and lake trout excretion model based on individual dry mass. The slope of our cutthroat trout and lake trout model (0.70 using reduced major axis linear regression for comparison only) was similar to other slopes measured for Bonneville cutthroat trout (0.71; Oncorhynchus clarkii utah), kokanee salmon (0.96; Oncorhynchus nerka; Wheeler et al 2014), rainbow trout (0.62; Oncorhynchus mykiss), brook trout (0.47; Salvelinus fontinalis; Paulson 1980, Sereda et al 2008, and brown trout (0.75; Salmo trutta; Villéger et al 2012). Reduced major axis linear regression allows biologically interpreting the slope parameter in a symmetric relationship (Warton et al 2006) whereas our predictive allometric OLS model allowed us to estimate the excretion rate based on the trout mass.…”

“…Cutthroat trout transported more NH 4 þ (281-540 lg N m À2 h À1 ) and P (35-95 lg N m À2 h À1 ) to Clear Creek when these fish were abundant compared to spawning Bonneville cutthroat trout and Kokanee salmon in Utah streams (Wheeler et al 2014). Finally, cutthroat trout transported .13 times more nutrients to Clear Creek compared to waterfowl who transported terrestrial N (205 lg N m À2 h À1 ) and P (25 lg P m À2 h À1 ) to wintering wetlands in New Mexico (Post et al 1998).…”

“…Similarly, longfin dace (Agosia chryogaster) excreted 5-10% of the N demanded by stream microbes in Sycamore Creek, Arizona, USA (Grimm 1988a, Vanni 2002. Cutthroat trout likely supplied more N to stream microbes when they were abundant, similar to spawning Bonneville cutthroat trout and Kokanee salmon (46-188% of NH 4 þ demand; Wheeler et al 2014). Fish excretion can dominate fluxes, such as in Rio Las Marias, a Venezuelan stream that had extremely high fish biomass (McIntyre et al 2008) and in Costa Rican streams where banded tetra (Astyanax aeneus) composed 18% of fish biomass but excreted 90% of P demanded by microbes (Small et al 2011).…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Wheeler et al (2015) studied how stream hydrology affected consumer excretion subsidies, and found that the ratio of fish migrant biomass to system size which was measured by discharge, was related to spatiotemporal hydrologic variation [21]. The excretion subsidies that were produced by potamodromous fishes were changed with the maximum influence of consumer feces occurred during low flow periods [21].…”

Nonnative fish stocking alters stream ecosystem nutrient dynamics

“…Trout are often stocked at much higher biomass levels than resident fish and therefore can alter the location, concentration, and storage duration of in-stream nutrients. The NH 4 + -N process subsidy provided by stocking of hatchery fish (39-85% of demand) falls mostly within the range provided by migratory native fish populations (46-188%; Wheeler et al 2015). At release densities, stocked trout provided a substantial NH 4 + -N process subsidy (up to ~85%) that was orders of magnitude greater than the native fish community subsidy.…”