Organic matter resources fuelling food webs in a human-modified lowland river: importance of habitat and season

Abstract

The role of hydromorphological degradation and temporal variation for food webs in humanmodified rivers is still not fully evaluated. We tested the hypothesis that man-made engineering structures alter macroinvertebrate resource use in the Elbe River (Germany) in relation to seasonal variation. Stable isotopes (d 13 C, d 15 N) and mixing models revealed that dietary contributions of benthic organic matter (BOM) and phytoplankton were driven by engineering structure. Contributions of biofilm were driven by season, while contributions of terrestrial particulate organic matter (t-POM) were driven by both engineering structure and season. Contributions of t-POM were larger than those of phytoplankton in spring and summer, but not in autumn, which adds to the debate about the sources of organic matter fuelling riverine benthic food webs. Resource availability was not systematically related to resource use, indicating that factors other than resource limitation were responsible for the observed results. By demonstrating that human alterations determine consumer resource use independently from resource availability, our study links hydromorphological modifications to fluxes of matter in riverine food webs. Future studies should quantify organic matter fluxes from various autochthonous and allochthonous pathways in human-modified and natural rivers to allow for a robust synthesis of how hydromorphological modifications alter benthic food webs. Keywords Elbe River Á Food web Á Stable isotopes Á Mixing model Á Non-native species Á River engineering Handling editor: María del Mar Sánchez-Montoya

Habitat availability determines food chain length and interaction strength in food webs of a large lowland river

“…Standing stocks of terrestrial POM (t-POM) were not quantified during the study, and we instead took values from the literature for similarsized rivers (Benfield, 1997) (Table 1). Detailed information on sampling processing and isotope analysis, as well as mixing model analysis for macroinvertebrate primary consumers, are provided elsewhere (Brauns et al, 2019). For this study, we quantified the proportion of assimilated prey for omnivorous or macroinvertebrate predators initially at the mesohabitat-scale at each engineering structure.…”

“…Our previous work showed that macroinvertebrate secondary production is lower at hydromorphologically altered shores (Brabender, Weitere, Anlanger, & Brauns, 2016) and that the trophic basis of macroinvertebrate primary consumers varies with the degree of hydromorphological alteration (Brauns, Brabender, Gehre, Rinke, & Weitere, 2019).…”

“…The present research aimed to test the extent to which qualitative and quantitative properties of benthic food webs differ among riverine shorelines subjected to different hydromorphological alterations. Our research builds on a previously described gradient of habitat availability associated with three different engineering structures commonly found in large navigable rivers (Brabender et al, 2016;Brauns et al, 2019). Here, we constructed binary and ingestion webs for these structures based on benthic macroinvertebrate consumers and their resources and compared how food chain length, food web complexity, food web robustness, ingestion rates, and the strength of consumer-resource interactions differ with shoreline engineering practices.…”

Trophic niche differentiation mirrors intra-island population structure of Galápagos marine iguanas (Amblyrhynchus cristatus)

“…sites or subspecies. We tested which of these factors was more important using the 'overlapping' package [40] in R as outlined by Brauns et al [41]. We used the 95% credibility intervals of dietary proportions across the studied locations and two subspecies, and calculated the percentage overlap of these intervals between all combinations of sites as well as subspecies.…”