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 (δ15NPOM) in the lake and draining rivers based on extensive sampling and a synthesis of recent studies of N cycling. The δ15NPOM 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 δ15NPOM 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 δ15NPOM was found, indicating limited effect of riverine POM on the lake. Significant increase in δ15NPOM 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.
- Lake Superior