Isotopic composition of nitrogen in suspended particulate matter of Lake Superior: implications for nutrient cycling and organic matter transformation
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Lake Superior, one of the largest lakes in the world, is an ecosystem where nitrate (NO3 −) concentration has risen almost fivefold in the last century. Recent studies suggest that this increase may be due to lack of a significant nitrogen (N) sink, such as sedimentary denitrification or burial, because of low organic matter supply to the lake bottom. In light of these new findings, it is imperative to examine the origin and transformation of suspended particulate organic matter (POM) in the lake as N biogeochemistry is inextricably linked to POM dynamics. We present an analysis of spatial and temporal variations in δ15Ν of POM (δ15ΝPOM) in the lake and draining rivers based on extensive sampling and a synthesis of recent studies of N cycling. The δ15ΝPOM in the lake ranged from −4.7 to 7.6‰ and showed a significant (p < 0.001) temporal variability in the surface waters with relatively enriched δ15Ν during winter (mean ± SD ~ 1.5 ± 2.3‰; n = 13) compared to summer (mean ± SD ~ −2.0 ± 1.4‰; n = 20). Temporal variability in δ15ΝPOM and data for δ15Ν of dissolved inorganic nitrogen (DIN) together suggest a seasonal shift in nutrient sources for plankton along with possible detrital and higher trophic level contributions to POM during winter. On an annual basis, ammonium (NH4 +) appears to be the dominant N source to plankton in the lake. NO3 − use was lower but seasonally variable with higher contributions to plankton in summer than winter. During a period of high riverine discharge, no significant difference in coastal and open-lake δ15ΝPOM was found, indicating limited effect of riverine POM on the lake. Significant increase in δ15ΝPOM and decreases in particulate N concentration with depth indicate transformations of organic matter settling to the lake bottom that are consistent with the hypothesized influence of low organic matter supply to the lake bottom leading to limited benthic denitrification.
KeywordsLake Superior Nutrient Nitrogen Carbon Isotope
We thank captain and crew of R/V Blue Heron for their excellent support. We also thank Sandy Brovold and Mike Mellesmoen for help during sample collection. Constructive suggestions about sampling were provided by R. Sherrell, G. Bullerjahn and R.M.L. McKay. This research was funded by a NSF-Chemical Oceanography grant (0352291). This material was also based on work supported by the National Science Foundation, while RWS was working at the Foundation.
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