Seasonality of a Floodplain Subsidy to the Fish Community of a Large Temperate River

This study examines the temporal and spatial variability of the floodplain trophic contribution to the fish community of Lake Saint-Pierre, a large (≈ 300 km²), shallow (≈ 3 m) widening of the St. Lawrence River (Québec, Canada). We hypothesized that floodplain contribution to fish diet would be maximal during the early growing period following the seasonal flood and at sites located closest to the floodplain. Carbon stable isotope ratios (δ¹³C) of invertebrates and 35 fish species were analyzed at the beginning and at the end of two growing seasons, at increasing distances from the floodplain. Floodplain contribution, in the form of exported aquatic invertebrates, was estimated at the fish community (littoral versus main river), species and individual fish level, using a stable isotopic mixing model and a linear mixed model selection. As hypothesized, fish captured in the early part of the growing season near the floodplain benefitted from the largest floodplain contribution, which decreased during the late growing season. In addition, floodplain contribution differed among fish species, being higher in small or immature littoral species (northern pike, yellow perch, blackchin and golden shiners) than for larger taxa inhabiting the main river channel (shorthead redhorse, white sucker and channel catfish). Our study revealed that floodplain production during the early growing season represents a key food resource to a large portion of the riverine food web, thus highlighting the essential role of the floodplain in human-altered temperate rivers.