Changes in the nutritional quality of decaying leaf litter in a stream based on fatty acid content

Abstract

We examined the nutritional quality of decaying leaf litter in a third-order forested stream, using measurements of fatty acid (FA) composition over time. We measured changes in concentrations of total, polyunsaturated, microalgal, and microbial marker FAs in mixed-species leaf packs in spring and autumn and effects of including/excluding macroinvertebrates. Initial concentrations of total FAs in litter were significantly less in spring (5.2 mg/g) than in autumn (6.9 mg/g; F = 6.3; P = 0.03), but total FA concentrations in litter placed in the stream declined significantly over 120 days in both spring (62%; F = 10.9; P \ 0.001) and autumn (56%; F = 19.4; P = 0.0001). Quantities of most FAs declined at a greater rate than that of bulk leaf matter. The presence or absence of macroinvertebrates (5 mm vs. 250 lm mesh) had no effect on FA concentration or composition of decomposing litter. Omega-3 polyunsaturated FAs were either nearly absent (20:5x3) or depleted preferentially over other FAs (18:3x3). During decomposition the polyunsaturated FA linoleic acid (18:2x6, common in fungi), declined in concentration more rapidly than other FAs in the spring, but in autumn declined at slower rates, perhaps suggesting greater fungal activity in autumn. Quantities of bacterial (e.g., 16:1x7) and fungal (e.g., 18:1x9) FA markers increased over time in autumn (and 16:1x7 also in spring). Our data provide no evidence for increasing nutritional FA quality of litter during decay and microbial colonization, based on total and polyunsaturated FAs, despite measured increases in bacterial and fungal FA over time. Routine measurements of FA composition of litter could provide insights into the nutrition of allochthonous matter and the importance of fungi and bacteria during decomposition.

Feeding strategies for the acquisition of high‐quality food sources in stream macroinvertebrates: Collecting, integrating, and mixed feeding

“…Differences in the FA composition among aquatic macroinvertebrates, periphyton, and terrestrial leaves suggest that periphyton is the main LC‐PUFA (in particular EPA) source for macroinvertebrates. Although submerged leaves were not available in our study streams in summer and FA of biofilms attached to submerged leaves were not analyzed, previous studies report that submerged leaves as a whole contained no or very little EPA, < 1% (Brett et al ; Torres‐Ruiz and Wehr ; Guo et al c ), which is also confirmed by our leaf FA (Table ). Moreover, seasonal change is unlikely to affect the low‐EPA content of submerged leaves (Torres‐Ruiz and Wehr ).…”

“…Although submerged leaves were not available in our study streams in summer and FA of biofilms attached to submerged leaves were not analyzed, previous studies report that submerged leaves as a whole contained no or very little EPA, < 1% (Brett et al ; Torres‐Ruiz and Wehr ; Guo et al c ), which is also confirmed by our leaf FA (Table ). Moreover, seasonal change is unlikely to affect the low‐EPA content of submerged leaves (Torres‐Ruiz and Wehr ). Macroinvertebrate primary consumers in our study streams showed limited ability to synthesize EPA from dietary ALA, suggesting they obtained EPA from food sources that are rich in EPA (Brett and Müller‐Navarra ; Wacker et al ; Torres‐Ruiz et al ).…”

Species traits reveal effects of land use, season and habitat on the potential subsidy of stream invertebrates to terrestrial food webs

“…Not only do aquatic insects often emerge with very high abundance and biomass, but they also carry nutrients and essential lipids produced within aquatic environments (Baxter et al 2005;Gratton et al 2008;Gladyshev et al 2009;Torres-Ruiz and Wehr 2010). These aquatic insect subsidies support a multitude of organisms in riparian communities, including arthropods (Collier et al 2002;Burdon et al 2013), birds (Gray 1993;Poulin et al 2010) and bats (Fukui et al 2006).…”

Benthic algae stimulate leaf litter decomposition in detritus‐based headwater streams: a case of aquatic priming effect?

“…There is evidence that small amounts of labile C exudates released by primary producers can substantially accelerate the decomposition of slow-turnover pools of organic matter (often referred to as recalcitrant organic matter), a process called the ''priming effect'' in both terrestrial (Kuzyakov et al 2000, Fontaine andBarot 2005) and aquatic (Guenet et al 2010, Bianchi 2011 ecosystems. Living primary producers are rich in these essential compounds, and consumption of litter-associated primary producers by detritivores may help meet their nutritional requirements and maintain high detritus-processing activity (Franken et al 2005, Torres-Ruiz andWehr 2010). Under high-nutrient conditions (Fig.…”