The impacts of alder (alnus spp.) and salmon (oncorhynchus spp.) on aquatic nutrient dynamics and microbial communities in southwestern Alaska

Understanding the direction and impact of nutrient fluxes across ecosystem boundaries is fundamental to ecology. Nitrogen (N)-fixation by alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to freshwater systems. Southwestern (SW) Alaska supports some of the greatest salmon runs in the world. Alder is also a prevalent constituent of the regional vegetation. The importance of alder-derived nutrients (ADN) in the tundra is expected to increase as alder cover expands under climate warming, and as salmon harvesting reduces marine-derived nutrients (MDN) in salmon-spawning habitats. I evaluated the drivers and impacts of alder- and salmon-derived nutrients on aquatic systems by analyzing stream and lake water chemistry from a suite of 26 streams and 13 lakes, over a four-year period in SW Alaska. Additional water samples from all the study lakes were collected for analyses of microbial community structure and function. I statistically modeled relationships among aquatic nutrient concentrations, alder and salmon abundance, physiographic features within each watershed, and meteorological conditions to determine the ultimate controls on aquatic nutrient dynamics in this region. To assess the relative impacts of alder and salmon on aquatic microbial communities, I compared shifts in bacterial community composition and microbial function with catchment physical features and lake-water chemistry. I also measured microbial metabolic responses to N, P, and N+P enrichment to assess variation in nutrient limitation. My results reveal that watershed elevation is a key factor controlling the quantities of alder-derived N in streams. Elevation was inversely related to alder cover and N yield (ρ = -0.8 and -0.73 respectively, P < 0.05 for both). Alder cover had the largest influence on stream N (β estimate = 0.56, P < 0.05). In streams, higher P was associated with lower temperatures, possibly reflecting reduced P demand under low rates of metabolic activity. Stream N:P declined with elevation, suggesting that increased alder cover exacerbates aquatic P-limitation. In lakes, alder and salmon drive different nutrient cycles. Alder was the main driver of N (β estimate = 0.58, P < 0.05) in the spring, while relative catchment area and salmon drove P in the summer (β estimates = 0.42 and 0.32, respectively, P < 0.05 for both). However, elevation was inversely related to alder cover and lake N (ρ = -0.81 and -0.77 respectively, P < 0.05 for both). Also, accounting for relative catchment area weakened the relationship between salmon density and lake P (parameter estimate = 0.45, P = 0.08). Together, these results indicate that watershed physiography dictates lake nutrient dynamics in this region. Salmon had greater impacts on aquatic microbial community structure and function than alder. Seasonal shifts in bacterial community composition (F = 7.47, P < 0.01) were related to changes in lake N and phosphorus (P) concentrations (r2 = 0.19 and 0.16, both P < 0.05), and putrescine degradation (r2 = 0.13, P = 0.06), indicating the influx of, and microbial utilization of MDN. In contrast, alder cover was not related to microbial function, likely because alder-derived N provided less resource diversity than MDN. In response to nutrient additions, higher metabolic activity occurred among microbial communities from lakes with elevated Chl a concentrations (β estimates for +N, +P and +NP treatments = 0.78, 0.92, and 0.81, respectively, all P < 0.07) and larger relative catchment areas (β estimates for +N, +P and +NP treatments = 0.57, 0.54, and 0.53 respectively, all P < 0.05) in the spring. Thus, I infer that declining salmon abundance is likely to affect aquatic microbial community structure and function, but that watershed and lake features will potentially mediate these responses to nutrient loading. Overall, my research shows that aquatic nutrient regimes are ultimately driven by physiographic features that modulate the impacts of alder and salmon in freshwater systems. These results demonstrate that alder cannot buffer aquatic ecosystems from declining salmon, and that the watershed characteristics may have a greater influence on aquatic productivity than commonly assumed. Results from my microbial analyses also indicate that certain watershed features can make the metabolic response of lakes more vulnerable to increased allochthonous loading. Thus, it is likely watershed features will constrain aquatic system responses to future environmental change

Lake Pittsfield Illinois National Nonpoint Source Monitoring Program Project

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Risk Management of Cyanotoxins in Singapore

10.4236/jwarp.2020.126031Journal of Water Resource and Protection1206512-52