Basal resource quality and energy sources in three habitats of a lowland river ecosystem

Abstract

Understanding energy flow through ecosystems and among sub-habitats is critical for understanding patterns of biodiversity and ecosystem function. It can also be of considerable applied interest in situations where managing for connectivity among habitats is important for restoring degraded ecosystems. Here, we describe patterns of basal resource quality and identify primary basal energy sources in three habitats-river channels, anabranches and wetlands-of a lowland river floodplain in the Murray River catchment, Australia during a period of disconnected surface flow. We used a combination of stable isotope and fatty acid analyses to determine which basal resources were assimilated by the backswimmer Anisops thienemanni and the Eastern mosquitofish Gambusia holbrooki and assessed food quality across the three habitats. Seston was a primary basal resource for both animals in all three habitats, but was of higher quality within floodplain habitats than in the river channel. Although floodplain seston contained higher concentrations of essential fatty acids, fatty acid profiles of animals from different habitats remained similar. Our research suggests that inundation of floodplains and subsequent reconnection to the river could be valuable to afford riverine animals the opportunity to access high quality resources, but highlights a need to quantitatively assess the transfer of essential fatty acids between trophic levels to determine how much riverine animals are in fact limited by poorer quality food resources. We demonstrate the importance of estimating the quality of organic matter fluxes into food webs, and the potential role of targeted environmental flows to re-establish high quality energy pathways in riverine ecosystems. Food webs represent an important facet of ecosystem function and describe the energy pathways between resources and animals (Hladyz et al. 2012). Within freshwater ecosystems, research has focused on understanding the contribution and importance of terrestrially-and aquatically-derived carbon to food webs (e.g., Rees et al. 2020). Less attention has been oriented towards determining the quality of food resources (e.g., Guo 2018) and the dominant pathways for energy to reach higher trophic levels. Understanding which resources underpin food webs and factors influencing resource availability to animals is key to improving our capacity to gauge ecosystem health (Holland et al. 2020). Food quality, in its coarsest form, may be assessed by ecological stoichiometry (e.g., C : N) however, animals can be limited by availability of complex organic compounds such as amino acids (Dwyer et al. 2018), sterols and fatty acids (Twining et al. 2016). Within freshwater ecosystems, algae and, in particular, diatoms are considered high-quality food resources for herbivorous taxa due to their high concentration of inorganic nutrients (nitrogen and phosphorous) and longchain polyunsaturated fatty acids (e.g. Guo 2018). Some omega-3 (ω3) and omega-6 (ω6) polyunsaturated fatty acids, such as eicosapentaenoic (EPA;...

Dietary fatty acid profiles shape crayfish biosynthesis and performance: Implications for riverine food webs

“…These results have nutritional implications for riverine predators that as a direct result of river regulation and the reduction of large floodplain inundation events, have reduced access to floodplain prey. Previous work has shown that floodplain habitats can provide higher quality basal food resources for primary consumers than riverine habitats (McInerney et al, 2020), and here we identify one mechanism by which that may be extended to subsequent trophic levels (see Figure S5). Riverine crayfish contained the lowest proportion of LIN among all crayfish in the study.…”

“…Eight river and eight wetland sites were selected from the lower Ovens River floodplain, northeastern Victoria, Australia. Each river site was defined as a 200 m reach and wetlands known to permanently retain water were selected (for detailed descriptions of river and wetland habitats, see McInerney et al., 2020). Crayfish were sampled using collapsible shrimp traps baited with cow liver contained within plastic capsules to prevent consumption by crayfish.…”

“…ALA also was retained in higher proportions by detritus-fed crayfish, possibly reflecting selective retention of algal ω3 polyunsaturated FA from biofilms on aged detritus (Kühmayer et al, 2020) by crayfish fed the poor quality diet. ALA also is abundant in terrestrial plants (Guo et al, 2017(Guo et al, , 2020 and the high proportion within the diets of wild crayfish could suggest some level of dietary reliance on macrophytes which are a particularly rich source of ALA in these habitats (McInerney et al, 2020).…”

“…The resulting detritus appears to provide an abundant and productive food resource for consumers (McInerney et al., 2017). Such observations are at odds with our understanding of the nutritional value of detritus, which is characterised by what is thought to be a low‐quality FA profile; lack of LC‐PUFAS and dominance of saturated FAs indicative of heterotrophic decomposers (e.g., 10:0, 15:0, 17:0 and their branched iso and anteiso‐homologues, McInerney et al., 2020).…”

The dark side of rocks: An underestimated high‐quality food resource in river ecosystems

“…Biofilms are mostly composed of algae and readily develop on the surface of leaves, woody debris and rocks. In particular, biofilms growing on rocks, also called epilithon, have been found to better support stream food webs than do terrestrial leaves due to their high PUFA content (Guo et al., 2016a; Lau et al., 2009; McInerney et al., 2020). However, in most previous studies, the method to sample these biofilms has not been uniform; a few studies collected biofilms on the upper side of the rock (light‐exposed biofilms; in short ‘light biofilms’), whereas most studies do not clarify whether the biofilms were only from the upper side or a combination of the upper side and underside of rocks.…”