Inter‐annual variation in responses of water chemistry and epilithon to Pacific salmon spawners in an Alaskan stream

Abstract

1. Pacific salmon (Oncorhynchus spp.) deliver salmon-derived nutrients (SDN) to the streams in which they spawn. However, many stream parameters, such as discharge and spawner abundance, can vary from year to year, which could alter the quantity and flux of SDN. 2. Over six consecutive years, we studied responses in streamwater chemistry and epilithon (i.e. the microbial community on submerged rocks) to salmon spawners in Fish Creek, southeastern Alaska, U.S.A. The lower reach of Fish Creek receives spawners of several salmon species, while the upper reach does not receive spawners because of an intervening waterfall. 3. We estimated salmon spawner biomass, analysed water chemistry [ammonium, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC)], and measured epilithon abundance [as chlorophyll a (chl a) and ash-free dry mass (AFDM)] in Fish Creek. Measurements were made in both the upper and lower reaches, before, during and after the major salmon runs. 4. Absolute values and relative differences indicated that the presence of salmon spawners consistently increased dissolved ammonium (by 58 lg L )1 on average, 41· over background), SRP (by 6 lg L )1 , 14·), epilithon chl a (by 35 mg m )2 , 16·), and epilithon AFDM (by 3 g m )2 , 8·). Salmon spawners did not increase nitrate or DOC in either absolute or relative amounts. The persistence and magnitude of spawner effects varied among years and appeared to reflect weather-driven hydrology as well as spawner biomass. 5. Salmon-derived nutrients can stimulate the growth of primary producers by increasing streamwater nutrient concentrations, but this positive influence may be modulated by other factors, such as water temperature and discharge. To better assess the ecological influence of SDN on stream biota, future studies should explicitly consider the role of key environmental factors and their temporal and spatial dynamics in stream ecosystems.

Nutrient Enrichment with Salmon Carcass Analogs in the Columbia River Basin, USA: A Stream Food Web Analysis

“…Differential response between inanimate SCA and live salmon spawner studies are more easily explained than differences observed following salmon carcass additions. Similar to live salmon spawners, carcass additions often elevate ammonium-nitrogen, soluble reactive phosphorus concentrations, or both (Claeson et al 2006;Chaloner et al 2007;Kiernan et al 2010;. Studies directly evaluating the dissolved nutrient response to inanimate salmon carcass and SCA additions under similar experimental settings are needed.…”

“…These results are similar to previous SCA evaluations described in Kohler et al (2008), but very different from studies that explored dissolved nutrient responses to spawning salmon or carcass additions. For example, significant increases in ammonium-nitrogen and soluble reactive phosphorus are commonly noted during and after salmon spawning (Minikawa and Gara 1999;Chaloner et al 2004Chaloner et al , 2007Mitchell and Lamberti 2005;Cak et al 2008;Janetski et al 2009), which is probably the result of direct sources of metabolic waste products (via excretion), the physical disturbance of the streambed during spawning (via bioturbation) (Moore et al 2004), and the decomposition of salmon organic matter following the spawning period. Differential response between inanimate SCA and live salmon spawner studies are more easily explained than differences observed following salmon carcass additions.…”

Nutrient additions to mitigate for loss of Pacific salmon: consequences for stream biofilm and nutrient dynamics

“…Returning adult salmon release organic and inorganic nutrients via excretion and metabolic wastes, spawning, redd building, and carcass decomposition (Moore et al 2007. Some of these nutrients are assimi-lated by stream autotrophs (algae and cyanobacteria; Johnston et al 2004, Chaloner et al 2007, Verspoor et al 2010), may stimulate rates of primary production and microbial respiration (Holtgrieve and Schindler 2011, Levi et al 2013, and may support secondary consumers such as aquatic invertebrates (Wipfli et al 1998, Lessard et al 2009) and fishes, including juvenile salmon (Bilby et al 1996, Larkin andSlaney 1997).…”

Persistent ecological effects of a salmon-derived nutrient pulse on stream invertebrate communities

“…Studies that describe how the ecological role of salmon in streams changes with salmon abundance across space and time (e.g., Chaloner et al 2007) both enhance our understanding of stream ecology and assist in ecosystem-based management of salmon populations.…”