Seasonal patterns in stream and lake nutrients as influenced by salmon and alder

Habitat loss and overfishing threaten the sustainability of salmon (Oncorhynchus spp.) populations, an important source of food and economic stability for many people around the world. Like most high-latitude freshwater ecosystems, lakes and streams that sustain salmon habitats are usually nutrient-limited, often exhibiting low nitrogen (N) and phosphorus (P) concentrations. These systems are dependent upon, and therefore strongly influenced by, the availability of nutrients from allochthonous or external sources. Several studies have established the importance of salmon on nutrient cycling in these ecosystems (Schmidt et al. 1998, Wipfli et al. 2003, Gregory-Eaves et al. 2003). Adult salmon are an important source of allochthonous nutrients because they deliver marine-derived nutrients (MDN) to stream systems via excrement, gametes, and carcasses (Gende et al. 2002). Salmon populations may also be affected by a positive feedback loop with these MDNs. Juvenile coho salmon derive approximately 30% of their N from spawning adult salmon (Bilby et al. 1996). Furthermore, increased nutrient levels are associated with increased growth rates of salmon in streams (Wipfli et al. 2003). The mechanism of increased MDNs and juvenile salmon growth is unclear, but it may be an indirect result of increased primary and secondary aquatic productivity. Because of the importance of MDN to juvenile salmon, a reduction of the historic levels of salmon escapement numbers, and thus reduced MDNs, may be responsible for the “downward spiral” of returning salmon in some areas (Gresh et al. 2000).

However, salmon-derived N subsidies may be less important to ecosystems that have additional nutrient sources such as alder. Alder (Alnus spp.) is a N-fixing shrub that is prevalent in many ecosystems where salmon are located. Bacteria that live in the root nodules of these plants convert atmospheric N into plant-accessible forms of N in the soil. Alder stands increase soil N pools (Mitchell and Ruess 2009), and there is a strong correlation between nitrogen levels in streams with alder coverage in watersheds (Compton et al. 2003) and alder leaf inputs (Volk et al. 2003a). Increased N availability also benefits other plants in riparian ecosystems, which derive a considerable proportion of total nitrogen from alder sources (Helfield and Naiman 2002). Alder presence in watersheds has also been linked to decreased soil total P (Mitchell and Ruess 2009) and increased P content of detritus exported from watersheds (Volk et al. 2003b) because of increased soil phosphatase activity (Giardina et al. 1995). Thus, watershed alder may result in increased P availability in lakes, although few studies have examined this relationship.

Like MDNs, alder-derived nutrient subsidies are associated with increased primary productivity (Goldman 1961). A pilot study by Devotta (2008) indicates that alder-derived nutrient inputs may be as much as 40 times greater than salmon-derived inputs in the Togiak Refuge in Alaska. In fact, the influence of alder-derived nutrients is so strong that the lack of clear correlations between primary productivity and MDN may be due to the confounding factor of alder presence in watersheds (Brock et al. 2007). I examine the complex relationship between the presence of salmon and alder on aquatic nitrogen and phosphorus availability and the related effects on aquatic productivity in streams and lakes within the Togiak National Wildlife Refuge (TNWR), Alaska. I hypothesize a positive relationship between the amount of alder present in watersheds and the amount of nitrogen and phosphorus present in stream and lake water. Furthermore, I expect alder coverage to be of greater importance than MDN in predicting lake chemistry and productivity.

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