Fine sediment effects in trout - new insights from laboratory and field studies

Worldwide native salmonid populations are reported to be in decline. Habitat degradation, and increased anthropogenic fine sediment input in aquatic ecosystems is an important contributing factor. Suspended fine sediment can directly impair the health of free swimming juvenile salmonid fish, either by causing direct physical damage and/or via turbidity. Deposited fine sediment can hinder oxygen supply to salmonid embryos and hence impair their survival. Numerous studies have been published so far. Despite this, we still lack an integrated understanding of how fine sediment affects salmonid fish at different stages of their life-cycle. My thesis contributes to such an integrated understanding in juvenile salmonid fish (Part 1), and for salmonid embryo survival (Part 2).
In part 1, we could demonstrate with an in vivo exposure that pulses of suspended mica clay particles affect rainbow trout (Oncorhynchus mykiss) mostly via turbidity. Moreover, we found that (i.) rainbow trout could adapt within days when they cannot avoid sediment plumes, and (ii.) that exposure over 24 days can cause cellular changes in inner organs and metabolic stress, even when gill damage is absent and effects on condition are moderate. Altogether, this experiment provides the first systemic investigation of suspended mineral particle pulse effects in a salmonid fish. In an in vitro exposure with the epithelial gill cell-line RTgill-W1 we could, to my knowledge, demonstrate for the first time (i.) that natural mineral particles can cause cytotoxic effects in gill epithelial cells, and (ii.) that clay particles are more cytotoxic than framework silicates. Further, the clay particles differed in the kind of cytotoxic effects induced, causing either oxidative stress or cell membrane damage. Most interesting, our results are the first empirical evidence that clay particles could induce comparable cytotoxic effects in gill epithelial cells than synthetic nano-particles
In part 2, a field experiment provides an integrated perspective on the factors affecting brown trout embryo survival in a channelized river. The introduced steps created a repetitive step-pool-glide morphology (“terraces”). Up to hatch salmonid embryos incubate in distinct gravel nest (“redds”), and during this phase they depend on sufficient oxygen supply. Both fine sediment and a modified river structure can affect hyporheic exchange, and hence embryo survival. For such modified rivers, our experiment is, to my knowledge, the first (i.) to characterize the factors affecting fine sediment deposition, water exchange and oxygen concentrations in salmonid redds, and (ii.) to explicitly identify important predictors for salmonid embryo survival. We found that river morphology and flow dynamics causing fine sediment re-suspension, rather than fine sediment transport per se, were important for maintaining redd gravel permeability over most of the incubation season. Similarly, water exchange and oxygen concentrations in the redds were mostly controlled by processes driven on the intermediate (i.e. individual terraces) and/or regional scale (i.e. river channel). Gravel permeability and the distance of the redd to the next upstream step, which created constant upwelling of short-residence hyporheic water, were the most important predictors for brown trout embryo survival. Altogether, we demonstrated (i.) that artificial steps could benefit salmonid embryo survival in channelized and (ii.) that appropriate flow dynamics and river structure are essential for sustaining salmonid incubation success.
My thesis clearly illustrates that it is essential to apply a holistic perspective to understand how fine sediment can affect salmonid fish. My results show that numerous factors can contribute. Fine sediment effects in juvenile salmonid fish could be modulated by exposure concentration and duration, the geochemical composition of the particles, but also adaptive responses of the organism. For fine sediment effects on salmonid embryo survival, the particular river system, with its hydrological and geomorphological setting, as well as the developmental stage of the embryo has to be considered. I believe we ultimately need to integrate fine sediment as one aspect of environmental change, and from there to develop strategies to sustain salmonid populations in the 21st century.