Isotopic composition of nitrogen in suspended particulate matter of Lake Superior: Implications for nutrient cycling and organic matter transformation

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 δ¹⁵N of POM (δ¹⁵N_POM) in the lake and draining rivers based on extensive sampling and a synthesis of recent studies of N cycling. The δ¹⁵N_POM 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 δ¹⁵N during winter (mean ± SD ~ 1.5 ± 2.3‰; n = 13) compared to summer (mean ± SD ~ -2.0 ± 1.4‰; n = 20). Temporal variability in δ¹⁵N_POM and data for δ¹⁵N 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 δ¹⁵N_POM was found, indicating limited effect of riverine POM on the lake. Significant increase in δ¹⁵N_POM 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.