Reconciling the role of organic matter pathways in aquatic food webs by measuring multiple tracers in individuals

Abstract

Few studies measure multiple ecological tracers in individual organisms, thus limiting our ability to differentiate among organic matter source pathways and understand consequences of dietary variation and the use of external subsidies in complex food webs. We combined two tracers, stable isotope (SI) ratios and fatty acids (FA), to investigate linkages among ecological compartments (water column, benthos, riparian zone) in food webs in waterholes of a dryland river network, the Border Rivers in southwestern Queensland, Australia. Comprehensive analyses of sources (plankton, periphyton, leaf litter, riparian grasses) and animals (benthic insects, mollusks, large crustaceans, fishes) for SI and FA showed that all three zones contribute to animal biomass, depending on species and life stage. Large fishes derived a subsidy from the riparian/floodplain zone, likely through the consumption of terrestrial and semi-aquatic insects and prawns that fed on detritivorous insects. Importantly, post-larval bony bream (Nematalosa erebi) and golden perch (Macquaria ambigua) were tightly connected to the water column, as evidenced by 13C-depleted, 15N-enriched isotope ratios and a high content of plankton-derived polyunsaturated fatty acids (eicosapentaenoic acid [EPA; 20:53] and docosahexaenoic acid [DHA; 22:6003]). These observations were consistent with expectations from nutritional requirements of fish early life stages and habitat changes associated with maturity. These results highlight the importance of high-quality foods during early development of fishes, and show that attempting to attribute food-web production to a single source pathway overlooks important but often subtle subsidies that maintain viable populations. A complete understanding of food-web dynamics must consider both quantity and quality of different available organic matter sources. This understanding can be achieved with a combined SI and FA approach, but more controlled dietary studies are needed to estimate how FA profiles are modified by animals when consuming a diverse range of diets of variable quality.

Polyunsaturated fatty acids in stream food webs – high dissimilarity among producers and consumers

“…[PUFA] consumer /[PUFA] algae ). Data used for subtropical rivers were fromJardine et al (2015) DHA is selectively and highly retained over other PUFA in fish, consistent with all reviewed datasets. It is still not clear if the high retention of DHA in fish is mainly driven by dietary DHA…”

How important are terrestrial organic carbon inputs for secondary production in freshwater ecosystems?

“…The stream ecology community has classically considered the allochthonous inputs to be the key basal resource because of their quantitative dominance over in‐stream primary production in low‐order systems and larger, turbid, and heterotrophic rivers (Vannote, Minshall, Cummins, Sedell & Cushing, ). However, more recently there has been a shift in emphasis to the disproportionate importance of autotrophic producers to animal consumer production in streams and larger rivers (Brito, Moulton, De Souza & Bunn, ; Bunn, Davies & Winning, ; Hayden, McWilliam‐Hughes & Cunjak, ; Jardine et al., ; Lau, Leung & Dudgeon, ,b; Lewis, Hamilton, Rodríguez, Saunders & Lasi, ; Neres‐Lima et al., ; Thorp & Bowes, ; Thorp & Delong, ). Conversely, lakes have classically been thought to be driven by autochthonous production (Carpenter, Kitchell & Hodgson, ), but recent studies have suggested that terrestrial carbon inputs support approximately 30%–70% of the zooplankton as well as zoobenthos and fish production (Berggren, Ziegler, St‐Gelais, Beisner & del Giorgio, ; Berggren et al., ; Carpenter et al., ; Cole et al., , ; Grey, Jones & Sleep, ; Jansson, Persson, De Roos, Jones & Tranvik, ; Karlsson, Jonsson, Meili & Jansson, ; Pace et al., , ; Tanentzap et al., ).…”

Quantifying terrestrial carbon in freshwater food webs using amino acid isotope analysis: Case study with an endemic cavefish

“…Allochthonous flow of terrestrial carbon into aquatic food webs (allochthony) is thought to be an underestimated component of the global carbon cycle (Boyero et al, ; Hanson, Pace, Carpenter, Cole, & Stanley, ). Allochthony is commonly studied using naturally occurring carbon and nitrogen isotopes (Brett et al, ; Jardine et al, ; Tanentzap et al, ), yet methodological constraints have thus far hindered the formulation of a clear consensus about the role of terrestrial organic carbon in aquatic ecosystems (Brett et al, ).…”