Abstract
The deposition of excess fine sediment and clogging of benthic substrates is recognised as a global threat to ecosystem functioning and community dynamics. Legacy effects of previous sedimentation create a habitat template on which subsequent ecological responses occur, and therefore, may have a long-lasting influence on community structure. Our experimental study examined the effects of streambed colmation (representing a legacy effect of fine sediment deposition) and a suspended fine sediment pulse on macroinvertebrate drift and community dynamics. We used 12 outdoor stream mesocosms that were split into two sections of 6.2 m in length (24 mesocosm sections in total). Each mesocosm section contained a coarse bed substrate with clear bed interstices or a fine bed substrate representing a colmated streambed. After 69 days, a fine sediment pulse with three differing fine sediment treatments was applied to the stream mesocosms. Added fine sediment influenced macroinvertebrate movements by lowering benthic density and taxonomic richness and increasing drift density, taxonomic richness, and altering drift assemblages. Our study found the highest dose of sediment addition (an estimated suspended sediment concentration of 1112 mg l−1) caused significant differences in benthic and drift community metrics and drift assemblages compared with the control treatment (30 l of water, no added sediment). Our results indicate a rapid response in drifting macroinvertebrates after stressor application, where ecological impairment varies with the concentration of suspended sediment. Contrary to expectations, bed substrate characteristics had no effect on macroinvertebrate behavioural responses to the fine sediment pulse.
Highlights
At a global scale, many freshwater ecosystems experience increased fine sediment loadings that impact their ecological functioning and biodiversity (Ormerod et al 2010)
This study aimed to identify the effect of differing doses of suspended fine sediment on the propensity of invertebrates to drift whilst accounting for the legacy effects of previous streambed colmation
Before the suspended fine sediment pulse, there was a significant difference in the size of bed sediment particles between the coarse and the fine bed substrate (ANOSIM; r = 0.907, p < 0.001; Fig. 3)
Summary
Many freshwater ecosystems experience increased fine sediment loadings that impact their ecological functioning and biodiversity (Ormerod et al 2010). Fine sediments (generally defined as inorganic and organic particles < 2 mm in size: Wood and Armitage 1997; Jones et al 2012) can infiltrate into bed substrates and cause streambed colmation/clogging (Mathers et al 2017a; Wilkes et al 2019; McKenzie et al 2020), which can alter macroinvertebrate community structure and functioning (Jones et al 2012; Descloux et al 2013; Wood et al 2016; Mathers et al 2017b). Colmation may have a long-lasting influence on macroinvertebrate communities and effect their responses to future disturbances. Identifying the legacy effects of colmation and other stresses on river ecosystems is important for water managers and conservation efforts in order to understand the responses of macroinvertebrate communities to future disturbances
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