Salmon influences on dissolved organic matter in a coastal temperate brown-water stream: An application of fluorescence spectroscopy

We examined how spawning Pacific salmon (genus Oncorhynchus) affect streamwater concentrations of inorganic nitrogen and phosphorus and dissolved organic matter in Peterson Creek, a stream in southeast Alaska. When spawning salmon were present, concentrations of ammonium (NH4-N) increased by more than 100 times over prespawning levels and concentrations of soluble reactive phosphorus increased by more than an order of magnitude. In contrast, concentrations of nitrate (NO3-N) increased by only two to three times during spawning and were not significantly higher than at an upstream reference site with no salmon. During spawning, concentrations of dissolved organic carbon and dissolved organic nitrogen were significantly higher in the spawning reach compared with the upstream reference site. The influx of salmon-derived dissolved organic matter (DOM) altered the fluorescence index (FI), which has previously been used to distinguish between terrestrial and aquatic sources of DOM, with the FI increasing significantly during the salmon run. Salmon DOM was rich in protein compared with the DOM derived from the terrestrial portion of the watershed. Spawning salmon may be an important source of labile DOM in Peterson Creek. Southeastern Alaska contains over 4,000 salmon streams (Halupka et al. 2002). These streams serve as conduits through which nutrients derived from marine ecosystems are returned to terrestrial and freshwater aquatic ecosystems. Salmon returning to spawn in their natal streams release nutrients to aquatic ecosystems through several pathways including excretion across gill membranes, leaching from gametes, and leaching from decaying carcasses (Gende et al. 2002). These salmon-derived nutrients (SDN) have previously been shown to increase chlorophyll standing stocks (Johnston et al. 2004; Mitchell and Lamberti 2005) and the density and growth rates of invertebrates and fish in streams (Bilby et al. 1998; Chaloner and Wipfli 2002). Many previous studies of SDN in aquatic ecosystems have focused on levels of inorganic nutrients (N and P), because N and P often limit primary productivity in freshwater ecosystems. However, SDN occur predominantly in organic forms (Compton et al. 2006) and salmon carcasses are an important source of organic matter in coastal streams (Bilby et al. 1996; Cederholm et al. 1999). As a result, salmon have the potential to deliver large quantities of dissolved organic matter (DOM) to aquatic ecosystems in spawning streams. DOM affects both the physical and chemical characteristics of aquatic ecosystems and is often the dominant source of carbon and energy for heterotrophic production (Wetzel 1995). Most DOM in freshwater ecosystems is derived from vegetation and soil organic matter in the terrestrial system. This terrestrial DOM is typically dominated by humic and fulvic acids, which are relatively recalcitrant and have a low N content. Because salmon carcasses have a much lower C : N ratio (,4 : 1; Johnston et al. 2004) than terrestrial vegetation in temperate coniferous forests (.60 : 1; Prescott et al. 2000), carcasses have the potential to contribute a pulse of relatively nitrogen-rich, labile DOM to streams during the spawning season. The chemical composition of DOM determines its optical properties; therefore spectroscopic measurements can be used to identify different fractions of DOM in natural waters. Fluorescence spectroscopy has been used in a variety of watershed-scale studies to trace changes in the DOM pool and to fingerprint DOM source or precursor material (Hood et al. 2003; Stedmon et al. 2003; Baker and Spencer 2004). For example, Stedmon et al. (2003) demonstrated that the fluorescence characteristics of DOM derived from forested and agricultural streams were 1 Corresponding author ([email protected]).