Disturbance of freshwater habitats by anadromous salmon in Alaska

Abstract

High densities of habitat modifiers can dramatically alter the structure of ecosystems. Whereas spawning sockeye salmon ( Oncorhynchus nerka) dig nests that cover over 2 m(2) and are at least 20 cm deep, and can spawn at high densities, relatively little attention has been devoted to investigating the impacts of this disturbance. We hypothesized that this temporally and spatially predictable bioturbation has large impacts on the coastal aquatic habitats used by sockeye. We experimentally investigated the impacts of disturbance caused by spawning sockeye in two streams and two lakes in Alaska by excluding salmon from 2.25 m(2) plots where they traditionally spawn. We sampled exclusions and control plots before, during, and after spawning. During sockeye spawning, fine sediment accumulated in areas where sockeye were excluded from spawning. In addition, sockeye spawning significantly decreased algal biomass by 80% compared to exclusion plots. We found mixed effects of spawning on the invertebrate assemblage. Tricladida and Chironomidae densities increased by 3x in exclusion plots relative to control plots in one creek site. However, for most taxa and sites, invertebrate densities declined substantially as spawning progressed, regardless of experimental treatment. Habitat modification by spawning salmon alters both community organization and ecosystem processes.

Opposing forces: Evaluating multiple ecological roles of Pacific salmon in coastal stream ecosystems

“…We found that streams with elevated salmon densities had fewer invertebrates in the spring and fall and that invertebrate biomass was higher upstream of salmon spawning barriers compared to downstream, regardless of season. These findings support the disturbance effect (Moore et al 2004), whereby invertebrate biomass downstream of the barriers might not have had time to recover from spawning the previous fall. Moore and Schindler (2008) found that invertebrate dry mass did not fully recover shortly after spawning but returned to near normal levels the following year prior to spawning.…”

“…This scenario would also explain the negative correlation of invertebrate biomass density with algae and biofilm; depressed invertebrate grazer populations could release algae in the biofilm from grazing pressure . In combination or isolation, a salmon subsidy, benthic disturbance with slow invertebrate recovery or a trophic cascade could explain the decline in spring invertebrate biomass density, while substrate disturbance during spawning, as shown by others, would explain the decline observed in the fall (Moore et al 2004, Holtgrieve and Schindler 2011.…”

“…Some studies have found that invertebrates from streams with higher salmon density have a more enriched nitrogen and carbon isotope signature (Bilby et al 1996, Hicks et al 2005 and are found in greater numbers (Wipfli et al 1999, Verspoor et al 2011, while other studies have found fewer invertebrates in the presence of spawning salmon (Moore et al 2004, Moore andSchindler 2008). These studies seem to contradict each other.…”

“…They therefore provide a good case study to investigate ecosystem-level responses to subsidies and disturbance at a convenient scale. Invertebrates occupy several functional feeding groups across a wide range of freshwater habitats, with community dynamics affected by stream temperature and pH (Verspoor et al 2011), chemistry (Allan and Castillo 2007), flow (Minshall and Minshall 1977), watershed size (Vannote et al 1980), substratum size and stability Minshall 1977, Verspoor et al 2011), biofilm biomass (Cummins and Klug 1979), riparian vegetation (Fisher andLikens 1973, Vannote et al 1980), predators (Gilinsky 1984) and salmon density (Wipfli et al 1999, Chaloner et al 2004, Moore et al 2004.…”

Stabilization of fine gravels by net‐spinning caddisfly larvae

“…have been found to modify significantly instream hydraulic conditions, reducing flow velocity within individual stands and leading to fine sediment deposition of up to 0.085 m 3 m -2 in a single month (Wharton et al, 2006). The creation of spawning redds by salmonid fish removes fine sediment from the surface layers of the substrate, changing the local bed material grain-size distribution (Kondolf et al, 1993;Moore et al, 2004) and, at popular spawning sites, redd construction can substantially affect vertical sediment sorting, armour development and bedload flux (Gottesfeld et al, 2004). The foraging activities of other fish species may have significant cumulative impacts on the presence of macrophyte and local substratum composition (Power, 1990;Flecker, 1996;Statzner et al, 2003b).…”