Seasonal hydrology influences energy channels in food webs of rivers in the lower Okavango Delta

Describing species interactions and resource use can elucidate patterns of energy flow in ecosystems. Here, we analyzed stable isotope ratios (δ 13 C and δ 15 N) to infer seasonal variation in energy pathways and species foraging strategies in two rivers of the lower Okavango Delta, Botswana. We sampled fish during wet and dry seasons and estimated proportions of basal production sources assimilated by fish using Bayesian isotopic mixing models. We estimated modal trophic position for each fish population and assessed its correlation with the degree that biomass was supported by one production source. In the Boteti River during the wet season, biomass of most fishes was supported by food chains originating mainly from seston. During the dry season, C4 grasses assumed greater importance, and the degree of dependence on a single source was inversely related to trophic position. In the Boro River, seston was inferred to be an important production source supporting most species during both periods, and the degree of dependence on a single source was positively related to trophic position during the dry season, suggesting that species higher in the food web assimilated material disproportionately from one source during this period. Our findings support the idea that fish foraging habits shift in response to seasonal patterns of connectivity and habitat availability. We showed that consumers in river food webs are supported by multiple basal production sources and that seasonal hydrological pulsing, despite causing short-term changes in food web dynamics, may promote long-term population and community stability.