Forest canopy cover determines invertebrate diversity and ecosystem process rates in depositional zones of headwater streams

1. Previous studies of the ecological linkages between forest and headwater streams have focused primarily on patterns and processes in erosional habitats, typically riffles. Depositional zones trap large amounts of sediments and particulate organic matter, suggesting that they may be important for forest–stream linkages. 2. We studied the invertebrate benthos and two key ecological processes, surface sediment reworking and leaf litter breakdown, in the depositional zones of streams bordered by contrasting riparian vegetation. We compared three stream reaches, draining open canopy forest regenerating after recent clearcut harvesting, with reaches on three different streams bordered by older forests with closed canopies. We also assessed whether, and to what extent, forest canopy cover determined abiotic factors at the reach scale (physicochemistry of stream water) and patch scale (sediment properties). 3. Depositional zones in both types of stream harboured a taxonomically and functionally diverse invertebrate community, including efficient sediment reworkers and specialised shredders. Higher diversity was found in open canopy than in closed canopy streams, despite similarities in habitat morphology and sediment properties. 4. Water temperature and sediment reworking rate were higher in open canopy forest than in closed canopy forest. As rates of sediment reworking, adjusted for temperature, did not differ between forest types, temperature was probably a key factor linking the forest canopy to stream depositional zones. The rates of leaf litter breakdown sometimes varied substantially between streams, but no consistent forest effect was detected for this process. 5. Temperature-adjusted rates of surface sediment reworking and litter breakdown were positively correlated with the density of invertebrates that rework sediments and shredders, respectively. A relationship between these two ecological processes was found across depositional zones in closed canopy forest, but not in open canopy forest. 6. This study on depositional zones provides new evidence of the strong linkage between forest and headwater streams. By moderating stream summer temperature, riparian canopy cover has the potential to affect invertebrate metabolic rates and, indirectly, the intensity of surface sediment reworking. However, other factors, such as the quality and diversity of basal trophic resources, may also account for invertebrate diversity pattern across streams and the positive relationship between litter breakdown and sediment reworking in closed canopy forest

A cost-effective method to quantify biological surface sediment reworking

We propose a simple and inexpensive method to determine the rate and pattern of surface sediment reworking by benthic organisms. Unlike many existing methods commonly used in bioturbation studies, which usually require sediment sampling, our approach is fully non-destructive and is well suited for investigating non-cohesive fine sediments in streams and rivers. Optical tracer (e.g., luminophores or coloured sand) disappearance or appearance is assessed through time based on optical quantification of surfaces occupied by tracers. Data are used to calculate surface sediment reworking (SSR) coefficients depicting bioturbation intensities. Using this method, we evaluated reworking activity of stream organisms (three benthic invertebrates and a fish) in laboratory microcosms mimicking pool habitats or directly in the field within arenas set in depositional zones. Our method was sensitive enough to measure SSR as low as 0.2 cm2.d-1, such as triggered by intermediate density (774 m-2) of Gammarus fossarum (Amphipoda) in microcosms. In contrast, complex invertebrate community in the field and a fish (Barbatula barabatula) in laboratory microcosms were found to yield to excessively high SSR (>60 cm2.d-1). Lastly, we suggest that images acquired during experiments can be used for qualitative evaluation of species-specific effects on sediment distribution