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

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 μm mesh) had no effect on FA concentration or composition of decomposing litter. Omega-3 polyunsaturated FAs were either nearly absent (20:5ω3) or depleted preferentially over other FAs (18:3ω3). During decomposition the polyunsaturated FA linoleic acid (18:2ω6, 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., 16:1ω7) and fungal (e.g., 18:1ω9) FA markers increased over time in autumn (and 16:1ω7 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.