Positive effect of shredders on microbial biomass and decomposition in stream microcosms

Abstract

Summary 1. Animals play a major role in nutrient cycling via excretory processes. Although the positive indirect effects of grazers on periphytic algae are well understood, little is known about top‐down effects on decomposers of shredders living on leaf litter. 2. Nutrient cycling by shredders in oligotrophic forest streams may be important for the microbial‐detritus compartment at very small spatial scales (i.e. within the leaf packs in which shredders feed). We hypothesised that insect excretion may cause local nutrient enrichment, so that microorganism growth on leaves is stimulated. 3. We first tested the effect of increasing concentration of ammonium (+10, +20 and +40 μg NH4+ L−1) on fungal and bacterial biomass on leaf litter in a laboratory experiment. Then we performed two experiments to test the effect of the presence and feeding activity of shredder larvae. We used two species belonging to the trichopteran family Sericostomatidae: the Palaearctic Sericostoma vittatum and the Neotropical Myothrichia murina, to test the effect of these shredders on fungal and bacterial biomass and decomposition on leaves of Quercus robur and Nothofagus pumilio, respectively. All experiments were run in water with low ammonium concentrations (2.4 ± 0.34 to 14.47 ± 0.95 μg NH4+ L−1). 4. After 5 days of incubation, NH4 concentrations were reduced to near‐ambient streamwater concentrations in all treatments with leaves. Fungal biomass was positively affected by increased ammonium concentration. On the other hand, bacteria abundance was similar in all treatments, both in terms of abundance (bacteria cells mg−1 leaf DW) and biomass. However, there was a tendency towards larger mean cell size in treatments with 20 μg NH4 L−1. 5. In the experiment with S. vittatum, fungal biomass in the treatment with insects was more than twice that in the control after 15 days. Bacteria were not detected in treatments with insects, where hyphae were abundant, but they were abundant in treatments without larvae. In the decomposition experiment run with M. murina, leaf‐mass loss was significantly higher in treatments with larvae than in controls. 6. Our hypothesis of a positive effect of shredders on fungal biomass and decomposition was demonstrated. Insect excretion caused ammonium concentration to increase in the microcosms, contributing to microbial N uptake in leaf substrata, which resulted in structural and functional changes in community attributes. The positive effect of detritivores on microbes has been mostly neglected in stream nutrient‐cycling models; our findings suggest that this phenomenon may be of greater importance than expected in stream nutrient budgets.

Effects of invasive clam (Corbicula fluminea) die‐offs on the structure and functioning of freshwater ecosystems

“…The presence of invertebrates enhanced the response of fungal biomass and also leaf mass loss. Invertebrate excretion can stimulate fungal biomass by increasing ammonia concentration in leaf substrata (Villanueva, Albariño, & Canhoto, 2012). These results are consistent with those obtained in the live treatment.…”

Functional feeding groups of aquatic insect families in Latin America: a critical analysis and review of existing literature

“…The main function of shredders is the breakdown of large particles of plant material into smaller pieces that are then transported downstream or available to other stream consumers (Wallace & Webster, 1996). Shredders also make nutrients available to microbial consumers (Díaz Villanueva et al, 2012). In general, we refer to them as consumers of coarse particulate organic matter (CPOM) and producers of fine particulate organic matter (FPOM).…”

“…Grazing by macroinvertebrates is also beneficial to microbes, as they recycle nutrients back into the environment (Vanni, 2002). As primary consumers in aquatic food webs, macroinvertebrates represent an important link between basal resources (e.g., algae and detritus) and upper trophic levels (e.g., fish) or microbial communities (Díaz Villanueva, Albariño & Canhoto, 2012). However, their role is not only to make energy available to upper trophic levels, they also regulate energy flow along the aquatic food web (Chew, 1974;Wallace & Webster, 1996).…”

Top‐down and bottom‐up control of litter decomposers in streams

“…The opposite, that is stimulation of fungal growth and sporulation in the presence of shredders, was observed on recalcitrant substrata (Gonçalves et al ., ). This has been attributed to ammonium excreted by invertebrates (Villanueva, Albariño & Canhoto, ). Other authors reported no changes in fungal biomass, sporulation rates and community composition in the presence of shredders (Ferreira & Graça, ; Chung & Suberkropp, ).…”