Life history traits and spawning behavior modulate ecosystem‐level effects of nutrient subsidies from fish migrations

Abstract

AbstractMigratory animals can have profound impacts on ecosystem structure and function. In streams, salmon are well known for their contrasting influences on primary productivity through nutrient delivery, which enhances potential productivity, and substrate disturbance during nest building, which reduces algal biomass and primary production. However, most migratory fish species neither disturb the substrate significantly nor die en masse after spawning, hence their influence on ecosystems may differ from that observed in salmon streams. To determine the influence of nutrient subsidies from migrations of iteroparous fish whose broadcast spawning does not disturb the substrate substantially, we compared nutrient limitation, nutrient concentrations, and stream metabolism during spawning migrations of suckers (Catostomus spp.) in Lake Michigan tributaries with and without migration barriers. Although suckers deliver both nitrogen and phosphorus as eggs and waste excretion, only nitrogen concentrations were elevated during the migration (NH4‐N rose 44% relative to sites without a sucker run). Nutrient diffusing substrates demonstrated P‐limitation during the migration at sites spanning a wide range of sucker abundance, suggesting that high demand for likely masked P inputs from fish. Time series analyses indicated that gross primary production (GPP) increased with sucker excretion, but not with egg deposition after accounting for abiotic conditions. In contrast, egg deposition, but not excretion, was associated with a slight increase in ecosystem respiration (ER), suggesting that sucker gametes provide labile carbon that contributes to ER. The effects of suckers contrast with ecosystem responses to salmon migrations, which elevate ER but have mixed effects on GPP. This disparity reflects the fact that suckers fertilize streams without attendant disturbance effects. Our results suggest that basic differences in life history and behavior of migratory fish determine the direction and magnitude of their ecosystem effects. As a result, broad trait‐based predictions of the ecosystem role of migratory fishes may become possible as more species are studied.