Hydrology influences litter breakdown in low-order streams through physical abrasion and by affecting the abundances of detritivorous invertebrates. The degree to which the hydrological regime – mediated by precipitation changes – contributes to the temporal variability of litter breakdown is not well studied. In regions with high/increasing inter-annual fluctuations in precipitation, the temporal variability of reach-scale breakdown needs to be determined to better characterize reference conditions for stream bioassessment.
In this study, we assessed shredder- and microbial-mediated litter breakdown in forested runoff-dominated streams in geographically separate regions (British Columbia and Ontario, Canada), during three consecutive and hydrologically different autumns (2014-2016). We also determined litterbag-associated shredder abundance and fungal biomass on litter. Partial redundancy analyses (RDA) and regression analyses were conducted to partition the sources of variation of breakdown to compare the relative contributions of hydrology, shredder community composition and temporal factors. Lastly, the range of variability of breakdown was compared with the suggested range of natural variation in an assessment framework for stream functional integrity devised by Gessner and Chauvet (2002).
Overall, the ratio of shredder- and microbial-mediated breakdown rate at any given year to the three-year mean ranged from 0.78-1.39 and 0.75-1.30, respectively. The ratio of shredder- to microbial-mediated breakdown rate ranged from 1.04-2.85. This variation of breakdown rates was explained by inter-annual hydrologic changes. In the lower-flow years, litter breakdown was generally faster than the high-flow year. This was likely because under reduced flow, the increase in shredder abundance and hence breakdown approximated or surpassed the reduction in the physical fragmentation of litter. Hydrology and inter-annual heterogeneity were the major drivers of variation in litter breakdown.
We suggest that the utility of litter decomposition assays in stream bioassessments across hydrologically distinct years should deserve caution, especially in sites with poorly known flow-breakdown rate relationships. The range of inter-annual variability in forested, unimpacted sites slightly exceeded the range (0.75-1.33) considered to be indicative of ‘no impact’ on stream integrity in the bioassessment framework. The ratio of shredder- to microbial-mediated breakdown rate in our reference streams would have revealed ‘no’ to ‘severe impairment’ of ecosystem functioning, and is therefore a less robust assessment metric than the ratio of breakdown rate at impacted to reference site(s). The range of inter-annual variability of litter breakdown we established can serve as a useful baseline for evaluating the impacts on stream integrity by forest harvesting, as a major agent of land-use change in both study regions.