Movers and shakers: nutrient subsidies and benthic disturbance predict biofilm biomass and stable isotope signatures in coastal streams

Abstract

Summary Nutrient subsidies and physical disturbance from migrating species can have strong impacts on primary producers. In the north Pacific, adult salmon (Oncorhynchus spp.) transport marine‐derived nutrients back to freshwater streams and can also significantly disrupt the substratum during spawning events. We tested for effects of spawning pink (O. gorbuscha) and chum (O. keta) salmon on stream biofilm. Biofilm is a mix of algae, fungi and bacteria that provides food and habitat and forms the base of these aquatic food webs. We collected rock biofilm samples to compare stable isotopes and biomass prior to and following peak salmon spawning in 16 catchments on the central coast of British Columbia, Canada. We conducted two separate analyses. The first was a within‐stream comparison, which focused on 5 catchments that had a barrier to pink and chum salmon migration. The second was an among‐stream analysis that included all 16 catchments and explicitly considered biotic and abiotic factors, in addition to salmon density, known to influence biofilm growth and isotope ratios. Salmon density proved to be the best predictor of biofilm δ15N. Biofilm δ13C was best predicted by salmon density and catchment size. While spring chlorophyll a increased with mean salmon density, it was on average lower during spawning in the autumn, probably due to physical disturbance from spawning salmon. These results show that of the several variables considered to affect biofilm isotopes and biomass, salmon density and catchment size are among the most influential in coastal streams where salmon spawn.

Landscape Structure and Species Interactions Drive the Distribution of Salmon Carcasses in Coastal Watersheds

“…Salmon carcasses deposited in riparian forests by these consumers can increase soil organic content (Bartz and Naiman, 2005;Gende et al, 2007), elevate nutrient concentration, shift the diversity of riparian plant communities (Bilby et al, 2003;Hocking and Reynolds, 2011), and provide substantial resources to terrestrial invertebrate communities (Hocking et al, 2009(Hocking et al, , 2013. Although salmon play important roles in stream ecosystems as a source of nutrients and disturbance to stream beds through the action of digging redds (Janetski et al, 2009;Tiegs et al, 2009;Harding et al, 2014), considerable proportions of carcasses are also exported to estuaries (Gende et al, 2004b), which can elevate dissolved nutrient concentrations and provide substantial resource inputs into estuarine food webs (Cak et al, 2008;Harding and Reynolds, 2014a;Harding et al, 2015). However, differences in salmon nutrient input to estuaries likely vary as a function of spawner distributions and stream size as carcasses are usually transported limited distances downstream and are often retained within pools and organic debris within streams (Cederholm and Peterson, 1985;Minakawa and Gara, 2005;Strobel et al, 2009).…”

From earth and ocean: investigating the importance of cross‐ecosystem resource linkages to a mobile estuarine consumer

“…A considerable body of research has investigated the effects of the salmon's unique life history, which couples offshore marine productivity to coastal ecosystems (Gende et al 2002, Naiman et al 2002. However, the net effects of spawning salmon can vary (Harding et al 2014); salmon can subsidize freshwater and riparian habitats through excretion and egg and carcass deposition (Kline et al 1990, Cederholm et al 1999, Janetski et al 2009, and can export nutrients with juvenile emigration and disturbance as adults dig and defend nests (Moore et al 2007, Kohler et al 2013. Although considerable amounts of salmon-derived nutrients imported into coastal watersheds are retained, large proportions are also exported downstream to estuaries (Gende et al 2004, Mitchell and Lamberti 2005, Cak et al 2008.…”

The roles of extrinsic and intrinsic factors in the freshwater life‐history dynamics of a migratory salmonid

“…, Harding et al. ). We hypothesize that some of the increased growth in young steelhead due to high pink salmon returns could have manifested itself as shifted age structure toward a younger age at migration.…”

“…Mechanistically, pink salmon likely influenced steelhead life history by providing pulsed and energy-rich food resources. Steelhead could be subsidized through direct consumption of pink salmon eggs and carcass tissues (Bilby et al 1998, Moore et al 2008), or indirectly through increased invertebrate prey that have fed on pink salmon carcasses (Wipfli et al 2010) or have been dislodged during pink salmon spawning (Scheuerell et al 2007, Harding et al 2014. We hypothesize that some of the increased growth in young steelhead due to high pink salmon returns could have manifested itself as shifted age structure toward a younger age at migration.…”