Abstract
Floodplain aquatic ecosystems experience temporal changes in basal production sources and inputs from allochthonous sources that influence energy flow. We analyzed stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) from 449 fish muscles representing 18 species to characterize energy pathways in two rivers of the lower Okavango Delta, Botswana. We sampled fish during wet and dry seasons and used mixing models to estimate proportions of major basal production sources assimilated by fishes. We estimated trophic position (TP) for each fish population and assessed the association between TP and the degree that fish biomass was supported by a single production source. During the wet season, fishes in the Boteti River assimilated material through food chains originating mainly from aquatic macrophytes. During the dry season, terrestrial C3 plants and terrestrial C4 grasses assumed greater importance. In the Boro River, terrestrial C3 plants were inferred to be an important production source supporting fishes during the wet season, with aquatic macrophytes becoming more important during the dry season. In both rivers and seasons, the degree of dependence on a single source was negatively related to TP, supporting the hypothesis that organisms positioned higher in the food web tend to be supported by multiple food chains and basal sources, whereas organisms lower in the food web may be supported by many or few food chains. We showed that consumers in river food webs shift foraging habits in response to seasonal patterns of connectivity and habitat availability, thus promoting long-term population and community stability.
Highlights
Food webs in river-floodplain systems are complex with multiple spatial compartments linked via movements of water and organisms (Winemiller 2004; Layman et al 2012)
Not a dominant input, C4 grasses assumed greater importance in the Boteti food web during the dry season, suggesting that at least some fish and/or macroinvertebrates that are consumed by carnivorous fishes changed their foraging strategies seasonally in response to shifts in resource availability
The dynamics of seston production are undocumented in this system, our results are consistent with the idea that both autochthonous and allochthonous resources should be major inputs to aquatic food webs in riverfloodplain ecosystems (Humphries et al 2014)
Summary
Food webs in river-floodplain systems are complex with multiple spatial compartments linked via movements of water and organisms (Winemiller 2004; Layman et al 2012). Aquatic food webs are supported by both algal-grazer and detrital-microbial energy pathways (Winemiller 1990,1996; Vadeboncoeur et al 2003; Moline et al 2004; McIntosh et al 2017). Whereas macrophytes may be abundant within streams and riparian areas, they usually do not directly contribute much material to the biomass of aquatic organisms at positions higher in food webs (Bunn et al 2003) This appears to be due to the inability of most aquatic macroinvertebrates and vertebrates to digest and absorb nutrients from macrophyte tissues (Renaud et al 1999; Cotner and Biddanda 2002). A few studies suggest that heterotrophy and the brown food web may predominate in ecosystems with a high abundance and diversity of consumers but low aquatic primary productivity (Legendre and Rassoulzadegan 1995; Cotner and Biddanda 2002)
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