Shrimps and leaf decomposition: Effect on macroinvertebrates and meiofaunal assemblages

Macroconsumers are known to be critical structuring lower trophic levels and therefore regulating leaf decomposition. In freshwater, shrimps can affect the decomposition of organic matter. For instance, atyid shrimps brush the substrate with their modified chelae to feed, removing and resuspending sediments into the water column. This action removes meiofauna but enhances the presence of some macroinvertebrates, such as mayflies (Ephemeroptera) that feed by scraping on rocks. We assessed how the meiofaunal and macroinvertebrate assemblage respond to the presence or absence of shrimps in decomposing leaves. Prieta stream in Puerto Rico is a second order headwater stream dominated by shrimps. We used an electric exclusion technique to create two treatments: Shrimp presence and shrimp exclusion in pools. Each treatment included packs of 5 gram of leaves (Dacryodes excelsa) that were collected over time and replicated in 6 pools over 49 days. At each sampling date, we randomly selected one pack and collect the sediment for meiofauna and macroinvertebrates. Packs were dried and weighted to determine the percent mass loss. No significant differences were detected in the decomposition rate with or without shrimps. In contrast, significant differences were detected in macroinvertebrates richness and abundances, and meiofaunal assemblages when the shrimps were excluded. Shrimps are crucial organisms in streams, especially in small short streams, typical of oceanic islands. Even when decomposition rate was not affected, macro and meiofaunal assemblages were. These differences could overshadow the decomposition effect since macroinvertebrates and meiofaunal groups are known to feed on bacteria, fungi and organic matter. Indeed, macroconsumers can regulate lower trophic levels than could affect decomposition. Nevertheless, this effect can be overshadowed by higher abundances on lower trophic levels when macroconsumers are excluded.