Rainbow trout (Oncorhynchus mykiss) have been repeatedly introduced into Ecuadorian streams since the 1920’s. The introduction of non-native trout can have profound effects on stream communities by means of nutrient enrichment and also by competing with native taxa. However, how non-native trout affect stream microbial communities and their associated function is largely unknown. To address this issue, we examined the association of the stream bacterial community structure with organic matter decay (function) and environmental variables among stream sites with (Trout) and without trout farms (NoTrout). We deployed cotton strips at 5 Trout stream sites and 5 NoTrout sites (20-29 day incubation). Bacterial communities were collected from cotton strips via standardized swabbing upon strip retrieval. We assessed bacterial community composition with Illumina™ (MiSeq) sequencing of the 16s rRNA gene region (V4) of bacterial DNA. Organic matter decay was assessed as percent loss of cotton strip tensile strength per degree day and utilized as a measure of stream bacterial community function. We compared bacterial community alpha diversity (richness and inverse Simpson index) and beta diversity between Trout and NoTrout stream sites. We used non-metric multidimensional scaling (NMDS) to explore bacterial community structure as related to environmental variables and organic matter decay.
Results/Conclusions
NMDS ordination displayed a pattern along NMDS axis 1 indicating a difference in bacterial community composition related to trout farming between sites. Furthermore, stream characteristics that are often altered by trout farming were correlated with NMDS axis 1 (pH: r = 0.69, p < 0.001; total dissolved solids: r = 0.59, p < 0.001; dissolved oxygen: r = 0.48, p = 0.004; and percent canopy cover: r = 0.83, p < 0.001). However, the rate of organic matter decay was not correlated with the NMDS. Bacterial taxonomic richness and diversity did not differ between Trout and NoTrout sites and the pattern observed in the NMDS ordination was not supported by permutation-based analysis of variance which indicated no significant effect of trout farming on bacterial community structure. Further research utilizing additional sites is needed to determine if and to what extent trout farming affects stream-associated bacterial community composition and function.