Trent M. Hoover

The role of stranding and inundation on leaf litterdecomposition in headwater streams

Abstract Discharge-driven shifts in the wetted area of streams can modify the amount of leaf litter resources available to stream consumers as well as the physical conditions to which leaf litter is exposed. The consequences of this continual movement from wet to dry storage for rates of organic matter processing and resource availability to benthic communities are poorly understood. We used a 30-day field experiment during the period of maximum stream contraction to examine the effects of stranding on black cottonwood (Populus trichocarpa) leaf litter decomposition rates and associated changes in microbial respiration in a forested stream in western Montana. Leaf litter was enclosed in both coarse and fine mesh bags and moved from the wetted area of the stream to the stream bank in six treatments designed to mimic a gradient of dry exposure due to stranding. We also measured existing accumulations of organic material in quadrats placed in both wet and dry areas of the stream. The total storage of litter resources (ash-free dry mass, g m2) retained on dry stream banks increased steadily as stream flow decreased, resulting from reductions in wetted width and continuous inputs from terrestrial zones. In contrast, total mass of stored litter submerged in the stream channel remained relatively constant. Leaf decomposition rates increased as a function of time inundated and were fastest in the presence of macroinvertebrates. Our results suggest that prolonged stranding can alter fundamental processes and energy pathways in stream food webs by shifting pools of resources from the active channel to dry storage on riverbanks where decomposition is driven primarily by microbial processes. Since the length of time that leaf litter is inundated prior to stranding alters decomposition rates, changes in stream hydrograph variability (as a consequence of land management practices or incipient climate change) has the potential to alter energy flow through stream systems. In particular, dry storage may function as a type of ‘temporal subsidy’ for stream organisms particularly if slowly decomposing stranded leaf litter is re-entrained during periods when in-stream detrital resources are otherwise scarce.

A model-based comparison of organic matter dynamics between riparian-forested and open-canopy streams

Abstract The food webs of forest streams are primarily based upon inputs of organic matter from adjacent terrestrial ecosystems. However, streams that run through open landscapes generally lack closed riparian canopies, and an increasing number of studies indicate that terrestrial organic matter may be an important resource in these systems as well. Combining key abiotically-controlled factors (stream discharge, water temperature, and litter input rate) with relevant biotic processes (e.g. macroinvertebrate CPOM consumption, microbial processing), we constructed a model to predict and contrast organic matter dynamics (including temporal variation in CPOM standing crop, CPOM processing rate, FPOM production, and detritivore biomass) in small riparian-forested and open-canopy streams. Our modeled results showed that the standing crop of CPOM was similar between riparian-forested and open-canopy streams, despite considerable differences in litter input rate. This unexpected result was partly due to linkages between CPOM supply and consumer abundance that produced higher detritivore biomass in the forest stream than the open-canopy stream. CPOM standing crop in the forest stream was mainly regulated by top-down consumer control, depressing it to a level similar to that of the open-canopy stream. In contrast, CPOM standing crop in the open-canopy stream was primarily controlled by physical factors (litter input rates and discharge), not consumption. This suggests that abiotic processes (e.g. discharge) may play a greater role in limiting detrital resource availability and consumer biomass in open-canopy streams than in forest streams. These model results give insight on functional differences that exists among streams and they can be used to predict effects of anthropogenic influences such as forestry, agriculture, urbanization, and climate change on streams and how riparian management and conservation tools can be employed to mitigate undesirable effects.