The role of fine sediment characteristics and body size on the vertical movement of a freshwater amphipod

This paper is closed access until 12 November 2019.1. Sedimentation and clogging (colmation) of interstitial pore spaces with fine sediment particles is widely considered to be one of the most significant threats to lotic ecosystem functioning. This paper presents the results of a running water mesocosm study examining the effect of benthic and hyporheic fine sediment loading and particle size on the vertical movement and distribution of the freshwater amphipod Gammarus pulex. 2. A gradient of fine sediment loading and different particle sizes were used to examine the ability of G. pulex from two body size classes to access and migrate vertically within subsurface sediments. 3. We tested three hypotheses: i) sediment loading would modify the distribution of G. pulex by limiting vertical movement; ii) the deposition of large particles and heterogenous sediments would limit the vertical movement of individuals more than homogeneous fine grained sediments; and iii) large bodied individuals would be prevented from migrating vertically with increasing sediment loading and particle size / heterogeneity. 4. Sediment loading, particle size and heterogeneity of deposited sediment had a significant effect on the vertical movement of individuals, with heterogeneous sand (0.125 - 4 mm) acting as the strongest barrier to the vertical movement of individuals through the infilling and clogging of interstitial spaces followed by coarse (1 - 4mm) and fine sand (0.125 - 4 mm). 5. Fine sediment loading and particle size acted as a filter on body size and limited the ability of large bodied individuals to migrate vertically to a greater extent than small bodied individuals. 6. This study demonstrates that the effects of fine sediment on habitat availability and faunal movement is dependent on both sedimentological characteristics and an individual’s body size. The results illustrate the importance of both abiotic and biotic factors when evaluating the ecological 66 effects of fine sediment deposition

Ecological assessment of a sediment by-pass tunnel on a receiving stream in Switzerland

Reservoir siltation is a major problem worldwide, decreasing reservoir storage capacity, trapping entrained sediment, and altering the natural sediment regime. Sediment By-pass Tunnels (SBT) are used to connect reservoirs with downstream receiving waters during high flows to reduce sediment accumulation in the reservoir. When operating, large volumes of sediment-laden waters are released into the receiving river for short periods of time (hours). The aim of this study was to assess the impact of SBT events on the downstream riverine ecosystem. We measured physico-chemical properties, sediment respiration, periphyton biomass and chlorophyll-a, and macroinvertebrate assemblages along a 5 km stretch of river during the first two years of SBT operation. During the study, 5 major SBT events occurred. Few changes were found in physico-chemical properties, mainly due to the input of tributaries entering the system. Results showed a clear reduction in sediment respiration, an indicator of ecosystem metabolism, especially after large SBT events. Periphyton levels and macroinvertebrate density/richness also decreased after SBT events. A non-metric dimensional scaling (NMDS) distinguished both temporal and spatial shifts in macroinvertebrate assemblages after SBT events, being related to downstream distance and SBT event magnitude. In summary, SBT events acted as a pulse disturbance, similarly to natural floods, followed by recovery of measured ecosystem indicators. SBT events can enhance sediment and flow connectivity, although the magnitude and frequency of operations should be controlled to prevent catastrophic disturbances.