Minor food sources can play a major role in secondary production in detritus‐based ecosystems

Abstract

In many ecosystems, detritus is the dominant source of energy and the driver of ecosystem functioning. In particular, in forested headwater streams, allochthonous detritus (e.g. leaf litter, dead wood) constitute the main energy source for detritivores and living primary producers contribute marginally to ecosystem metabolism and energy flows. We hypothesised that a low consumption of benthic diatoms, a high‐quality resource, could be of major importance for the growth of detritivores. In particular, these resources might represent an essential source of polyunsaturated fatty acids (PUFAs). In a microcosm experiment, three food resources were manipulated: alder (Alnus glutinosa: Betulaceae) leaf litter, fungal mycelium and a common benthic diatom. They were offered to juveniles of Gammarus fossarum (Crustacea: Amphipoda) as food resources, either alone or in combination, with each resource type being enclosed in agarose pellets. Juveniles were fed for 5 weeks in controlled conditions. Survival, feeding and growth rates were monitored. The fatty acids content of food resources and gammarids were also quantified. Our results showed that detritus alone permits survival, but not the significant growth of detritivores. The presence of diatoms in food resources was necessary to ensure a significantly positive mass growth of detritivores over the 5‐week experiment. More importantly, detritivores that did not receive algae in their food were generally unable to maintain their PUFA levels when compared to juveniles collected in the field. Gut‐content analysis of field‐collected G. fossarum showed that low amounts of benthic algae were always visible, indicating that most individuals fed at least for a small part on benthic algae. These minor, but high quality, food sources might therefore be essential for ensuring the growth and survival of detritivores. Our results clearly highlight the need to consider the functional importance of such minor food sources.

Litter P content drives consumer production in detritus‐based streams spanning an experimental N:P gradient

“…Therefore, the patterns of secondary production reported in this study are likely driven by release from P limitation of consumer growth, via microbially modulated shifts in detrital stoichiometry, rather than any release from biochemical constraints like those observed by Crenier et al. ().…”

“…, Demi ), minor sources of high‐quality food (i.e., diatoms) may play a particularly important role in consumer growth by providing essential biochemicals (i.e., polyunsaturated fatty acids; Crenier et al. ). As such, small changes in patterns of diatom consumption following nutrient enrichment may be an additional driver in the patterns of consumer production reported here.…”

Periphytic algae decouple fungal activity from leaf litter decomposition via negative priming

“…Algae can also increase overall microbial biomass in the litter-periphyton complex, and because algae are N-and phosphorus (P)-rich relative to litter, this increases nutrient uptake and reduces C:N and C:P ratios (Danger et al, 2013;Halvorson et al, 2016). Algae also add essential polyunsaturated fatty acids that may translate to enhanced detritivore feeding and growth (Crenier et al, 2017). These algal-mediated interactions may be a missing link to understanding decomposition and other aquatic ecosystem processes, especially as riparian canopy openness varies seasonally and spatially, increases under anthropogenic influence, and alters energy and nutrient transfer through aquatic food webs (Allan, 2004;Bechtold, Rosi, Warren, & Keeton, 2016;Norman et al, 2017;Warren et al, 2016).…”

Urbanisation alters fatty acids in stream food webs

“…In our study of a large catchment, macroinvertebrates had greater proportions of Σω3 FAs and lower proportions of Σother FAs than those in periphyton, whereas their proportions of Σω6 were similar. These patterns probably resulted from macroinvertebrates having the ability to selectively consume and retain periphyton‐derived FAs even if food sources have low amounts of them, especially ALA, EPA, and ARA (Brett et al., ; Crenier et al., ; Guo, Bunn, Brett, & Kainz, ). These compounds also indicate higher food quality than shorter‐chained or saturated FAs (Guo et al., ; Torres‐Ruiz et al., ).…”