The origin and cycling of particulate and sedimentary organic matter and nitrate in Lake Superior

The elemental and isotopic composition of water column particulate and sedimentary organic matter and nitrate in Lake Superior was determined to assess the origin and cycling of these materials. The δ15N and δ13C of sedimentary organic matter and suspended particles at three stations were consistent with an origin primarily from autochthonous production. The δ15N of seston was controlled by a balance between the isotope effects associated with nitrate uptake and microbial degradation. The ratio of chlorophyll fluorescence to light hindrance (100-transmittance) was used in this study as an indication of the relative composition of recently produced photosynthetic vs. refractory and non-photosynthetic materials (such as bacteria or microzooplankton). Chlorophyll fluorescence to light hindrance (CF:LH) ratios were greatest within the region of the thermocline at the shallow station and lowest in the near bottom waters of the unstratified deep station. These changes in CF:LH indicated a predominance of recently produced photosynthetic material in surface stratified waters and an increase in refractory or non-photosynthetic material at the deepest station relative to the shallow station. Seston at the deepest station was characterized by the highest δ15N value of the three stations suggesting that degradation, bacterial growth, and/or an enrichment in microzooplankton resulted in an increase in the 15N content of seston. Seston at the shallowest station was characterized by high CF:LH ratios and low δ15N values suggesting a greater relative contribution of labile material and an influence of an isotope effect during nutrient assimilation. Suspended particles in the benthic nepheloid layer were characterized by marked depletions in 13C and 15N relative to seston and sedimentary organic matter and indicated a unique origin for this material, possibly from recent primary production. The δ15N and δ13C of particles within the sediment boundary layer were intermediate those of sediments and nepheloid layer particles and were suggestive of an origin from the mixing of these two materials. Nitrate in Lake Superior was characterized by the lowest δ15N reported for an aquatic environment (average of −4.1‰). These low δ15N values and large input of water from precipitation directly to the lake surface suggests that much of the nitrate in Lake Superior is derived from atmospheric deposition.