Use of different sources of organic matter by macroinvertebrates from headwater streams: Natural and semi-experimental approach combining stable isotopes and trophic transfers of fatty acids and sterols

In headwater streams, macroinvertebrates play an essential role in the flow of matter and energy from basal organic sources to the upper trophic level (fish fauna). Macroinvertebrates development depends on a limited number of basal sources from allochthonous leaf litter to autochthonous primary production (benthic microalgae and bryophytes). The riparian forest and tree cover of these small streams produce a substantial input of allochthonous detrital carbon, which is regarded as the basis for ecosystem functioning according to the RCC (river continuum concept). However, the concept outlined by the RPM (riverine productivity model) and recent studies tend to emphasise the importance of autochthonous primary production and particularly that of diatoms, which appear to be able to sustain secondary production thanks to its nutritional quality that is clearly higher than that of detrital material. In this context, the aim of this phd-thesis was to study the trophic relationships between the main basal sources and a selection of macroinvertebrates typical of headwater streams. The focus was on the importance of primary producers and benthic microalgae, in the trophic functioning of these environments, in particular by providing essential molecules (polyunsaturated fatty acids and sterols). First, in a mesocosm experiment, we studied the use of available organic resources by the scraper, Rhithrogena semicolorata and the shredder, Gammarus pulex, in the presence or absence of autotrophic biofilms. By using an approach based on stable isotopes analysis (¹³C and ¹⁵N) of bulks, fatty acids and sterols, we demonstrated that microalgae (diatoms) were assimilated by both species and their ingestion was essential to Rhithrogena semicolorata in relation to the supply of essential fatty acids (20:5ω3 and 20:4ω6). For Gammarus pulex, the feeding on microalgae was not essential when leaf litter was not recalcitrant but promoted a higher somatic growth. In a second time, a seasonal monitoring was undertaken on a mid-mountain forested stream (Massif Central, France). In early spring, using a Bayesian mixture model (SIAR), we observed that most of the energy assimilated by nine macroinvertebrate taxa (of different trophic guilds) originated from epilithic biofilms, bryophytes and fine benthic detritus particles. Using stable carbon isotope signatures of fatty acids (CSIA, C¹³FA), we showed that current fatty acids (16:0, 18:1ω9, 18:2ω6 and 18:3ω3) were supplied in proportion to the different organic sources assimilated by invertebrates. In contrast, the essential fatty acids (20:4ω6 and 20:5ω3) were derived from autochthonous sources and obtained through different feeding strategies, such as omnivory. The follow-up during summer and autumn, but on a smaller number of macroinvertebrate taxa (five), confirmed the observations from the spring. Epilithic and epixylous biofilms were still the source of essential fatty acids and notably 20:4ω6 and 20:5ω3 assimilated by the different macroinvertebrate taxa, irrespective of their usual trophic guild. Furthermore, we demonstrated that although bryophytes and fine detrital particles were significant food sources, the recalcitrant leaf litter from the adjacent beech forest was little assimilated by macroinvertebrate shredders. Lastly, the analysis of sterol transfer, both under natural conditions and during the mesocosm experiment, suggested that no nutritional constraints linked to these compounds occurred among all the macroinvertebrate species analysed. (author)