Non‐native Chinook salmon add nutrient subsidies and functional novelty to Patagonian streams

Abstract

The impacts of non‐native species are hypothesised to be proportional to the functional distinctiveness of invaders in their invaded ecosystems. Throughout the Patagonia region of southern South America, Chinook salmon (Oncorhynchus tshawytscha) have recently established non‐native populations, and their anadromous, semelparous life cycle could be functionally unique such that marine‐derived nutrients are delivered to streams which have historically lacked such a resource linkage with the ocean. We tested the hypothesis that salmon subsidise biofilm‐associated algae in streams throughout the Aysén province of southern Chile. Using spatial and temporal variation in the presence of salmon among multiple streams and across two spawning seasons, we found strong evidence of salmon‐subsidised algae in three out of four streams examined that have spawning salmon populations. The biofilm of subsidised streams had enriched stable isotopic ratios of nitrogen and carbon, indicating that marine‐derived nutrients were incorporated by biofilms. This nutrient uptake translated into increased algal biomass and percent of total biofilm biomass composed of algae, indicating that the incorporation of marine‐derived nutrients stimulated autotrophic production of biomass. In one stream, the incorporation of marine‐derived nutrients by biofilm occurred in only one of the two studied spawning seasons. Incorporation occurred in a year with low flows of water throughout salmon spawning (4.59 m3/s) and did not occur in a year with much higher flows (41.6 m3/s), suggesting that inter‐annual variation in discharge can mediate the subsidising effect of salmon. These results indicate that Chinook salmon have bridged the historical gap between productive marine ecosystems and nutrient‐poor stream ecosystems in Patagonia. Anadromous salmon can be a significant source of nutrients in nutrient‐limited catchments, and their ongoing expansion in southern South America is likely to entail ecological impacts in stream and riparian food webs.

Multipath ecological influence of an iteroparous fish migration from Lake Biwa to an alluvial stream

“…This result clarifies that the difference between migratory and non‐migratory seasons may even be greater than observed. Such trends, by which the Chl‐ a density increases in the areas with high fish density, were observed from earlier studies of salmonid fish (Muñoz et al, 2021; Sanderson et al, 2009; Wipfli et al, 1998). Primary production in stream ecosystem is often N and/or P limited (Elser et al, 2007; Keck & Lepori, 2012).…”

Emergent trophic interactions following the Chinook salmon invasion of Patagonia

“…Our proposed framework of the role of Chinook salmon in Patagonian food webs reveals numerous interactions involving a diverse group of taxa and trophic levels (Figure 6). In streams, nutrients from adult salmon carcasses benefit stream algae (Muñoz et al, 2021), and salmon carcasses or spilt eggs are scavenged by trout and macroinvertebrates such as chironomid midges and freshwater crabs ( Aegla sp. ; Arismendi & Soto, 2012; Soto et al, 2007).…”

“…Moreover, the prevalence of these interactions likely varies spatially and temporally due to variation in physical and biological characteristics of ecosystems. For example, salmon‐derived nutrients increased algal biomass in Ríos El Toqui, Huemules, and Jaramillo, whereas there was no such subsidy effect detected in Ñirehuao, the study stream with the greatest discharge (Muñoz et al, 2021). Despite these caveats, this interactions framework can help guide future research on population‐ and community‐level impacts of non‐native salmon in streams and forests.…”

“…Black arrows represent direct interactions, the dashed arrow represents an indirect interaction, and the red arrow represents the transfer of carcasses from streams to riparian habitat scavenging salmon carcasses or spilt eggs (0/+, n = 7), predator-prey interactions (À/+ or +/À, n = 4), nutrient subsidies (0/+, n = 1), terrestrial prey subsidies (+/0, n = 1), and tri-trophic interactions (0/+/+, n = 1; Appendix S2: Table S1). Other than a nutrient subsidy effect on stream algae (Muñoz et al, 2021), none of these interactions have been studied at the population level (i.e., impacts on abundance or biomass).…”

Evaluación de las interacciones tróficas e impacto de salmónidos asilvestrados en la fauna nativa del Río Toltén.

“…Our second hypothesis is that the maternal signal from resident trout was enriched with marine-derived nutrients. This can be possible due to the presence of anadromous Chinook salmon in the ToltÈn River transporting marine nutrients from marine to freshwater environments when its reproductive migration occurs MuÒoz et al, 2021MuÒoz et al, , 2022. Rainbow trout could feed on marineenriched environments or directly predate Chinook salmonís juveniles or eggs, as was shown by Figueroa-MuÒoz et al (2022).…”