Combined stable‐isotope and fatty‐acid analyses demonstrate that large wood increases the autochthonous trophic base of a macroinvertebrate assemblage

Abstract

Summary Large wood (LW), defined as pieces of wood greater than 10 cm in diameter and 1 m long, is well known to alter river hydromorphology and the availability of potential food resources for consumers. However, there has been a lack of studies investigating whether these can cause shifts in the trophic base, which may explain alterations to the total abundance and taxonomic structure of the macroinvertebrate assemblage. We aimed to determine how the presence of LW altered the trophic base of the macroinvertebrate consumer assemblage in a lowland river, and to provide a methodological comparison of two assimilation‐based food web methods: stable‐isotope analysis (SIA) and fatty‐acid biomarker profiles (FA). To do so, we quantified the contribution of trophic resources to the diets of macroinvertebrates colonising the surface of LW, present in this study as single logs, and surrounding bed sediments with those from bed sediments of a nearby control site with minimal amounts of LW. SIA showed that the macroinvertebrate food web, even for non‐filter feeding taxa, was mostly sustained by seston exported from a lake 1 km upstream, highlighting a high degree of lake‐river coupling. The presence of wood altered the trophic base from being predominantly seston‐supported to one with increased support from epixylic autochthonous production (i.e. periphyton and bryophytes on wood). Terrestrial matter (i.e. leaves and grass) and organic sediments were a relatively unimportant fraction of the trophic base (<10%) in all locations. FA did not directly track the influence of seston, but instead differentiated between overall allochthonous (terrestrial) and autochthonous (aquatic) components of the trophic base. In particular, FA analysis demonstrated the higher nutritional value of autochthonous primary producers, and provided supporting evidence that most consumers, even seston‐feeders, were primarily supported by autochthonous resources and not by allochthonous matter. FA indicated shifts in some taxa‐specific diets not detected by stable isotopes alone. Our study demonstrated that the combined use of stable isotopes and fatty acids provides new insights into determining the trophic base of a complex food web with trophic resources of both terrestrial/aquatic and lacustrine/riverine origins. In addition, directly comparing results from both stable‐isotope and fatty‐acid analyses provided additional information on selective feeding by seston‐feeding taxa on autochthonous and allochthonous fractions of the seston. The presence of LW in the river channel decreased lake‐river coupling by providing alternative basal resources, primarily through increasing high‐quality autochthonous production on wood and by providing a superior substratum for net‐spinning caddisflies to feed on a fraction of the seston richer in essential fatty acids. River management strategies that incorporate instream LW therefore have the potential to alter energy flows and enhance ecosystem productivity by increasing the quantity and quality of available basal food resources.