wAbstract
@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 Β15N of POM (Β15NPOM)
in the lake and draining rivers based on extensive sampling and
a synthesis of recent studies of N cycling. The Β15NPOM in the lake ranged from -4.7 to 7.6ρ and showed
a significant (p0.001) temporal variability in the surface waters
with relatively enriched Β15N during winter (mean }
SD ` 1.5 } 2.3ρ; n = 13) compared to summer (mean } SD ` -2.0
} 1.4ρ; n = 20). Temporal variability in Β15NPOM and data for Β15N 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 concentrations to plankton in summer than winter.
During a period of high riverine discharge, no significant difference
in coastal and open-lake Β15NPOM
was found, indicating limited effect of riverine POM on the lake.
Significant increase in Β15NPOM
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.
Keywords: Lake Superior; Nutrient; Nitrogen; Carbon; Isotopex
Introduction
Materials and methods
Results
@Temperature
@Β15NPOM and PON concentration
in the lake
@@Variation by time and depth
@@Spatial variation
@Relationships among nutrients concentration and Β15NPOM
@Isotopic composition of organic matter in rivers
@Isotopic composition of NH4+
in the lake
Discussion
@POM formation and role of nutrients
@Transformation of organic matter in deep layers
@Role of rivers
Conclusions
Acknowledgements
References