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Gu, Binhe, 1993

Natural abundance of 15N in a subarctic lake and biogeochemical implications to nitrogen cycling

Bibliographic Reference

Gu, Binhe, 1993, Natural abundance of 15N in a subarctic lake and biogeochemical implications to nitrogen cycling: University of Alaska Fairbanks, Ph.D. dissertation, 204 p., illust., maps.


Stable isotope ratios of nitrogen ([delta] 15N) were employed to track the origin and fate of nitrogen in a subarctic lake, Alaska. The annual planktonic nitrogen cycle was dominated by N2 fixation in spring and NH4+ assimilation in summer. In winter, microbial nitrification was the major sink for NH4+ and denitrification was accounted for most of the loss of NO3-. The small isotope fractionation in nitrification is proposed as a result of substrate (NH4+) limitation. The temporal and spatial homogeneity of the [delta] 15N of dissolved organic nitrogen may be related to its large pool size and refractory nature. A stable isotope mass balance suggests that the winter phytoplankton was only composed of 10 to 20% of the suspended organic matter in water column due to low primary productivity during the ice cover period. A close correlation between [delta] 15N of phytoplankton and [delta] 15N of dissolved pools indicates that NH4+ was the predominant nitrogen source for non-N2-fixing algae. The similarity of [delta] 15N between a spring blue-green bloom and N2 suggests an atmospheric origin for nitrogen. A mixing model estimated that the blue-green algal bloom derived approximately 70% of its nitrogen from molecular nitrogen. This fixed nitrogen was further transferred to higher trophic levels via the food chain and to other primary producers following mineralization. The [delta] 15N of aquatic macrophytes indicates that non-rooted species obtained their nitrogen from the water column while rooted species obtained their nitrogen largely from the sediment. Evidence from dual isotope tracers ([delta] 15N and [delta] 13C) suggests that the zooplankton were supported by phytoplankton throughout the growing season despite an apparent abundance of detritus in the water column. Benthic fauna relied on either phytoplankton detritus or other organic matter in the sediment. The [delta] 15N data exhibit only two to three trophic levels in both planktonic and the benthic communities in Smith Lake.

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