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 determine d to assess the origin and cycling of these materials. The delta(15)N and delta(13)C of sedimentary organic matter and suspended particles a t three stations were consistent with an origin primarily from autocht honous production. The delta(15)N of seston was controlled by a balanc e between the isotope effects associated with nitrate uptake and micro bial degradation. The ratio of chlorophyll fluorescence to light hindr ance (100-transmittance) was used in this study as an indication of th e relative composition of recently produced photosynthetic vs, refract ory and non-photosynthetic materials (such as bacteria or microzooplan kton). 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 i n refractory or non-photosynthetic material at the deepest station rel ative to the shallow station. Seston at the deepest station was charac terized by the highest delta(15)N value of the three stations suggesti ng that degradation, bacterial growth, and/or an enrichment in microzo oplankton resulted in an increase in the N-15 content of seston. Sesto n at the shallowest station was characterized by high CF:LH ratios and low delta(15)N values suggesting a greater relative contribution of l abile material and an influence of an isotope effect during nutrient a ssimilation. Suspended particles in the benthic nepheloid layer were c haracterized by marked depletions in C-13 and N-15 relative to seston and sedimentary organic matter and indicated a unique origin for this material, possibly from recent primary production. The delta(15)N and delta(13)C of particles within the sediment boundary layer were interm ediate those of sediments and nepheloid layer particles and were sugge stive of an origin from the mixing of these two materials. Nitrate in Lake Superior was characterized by the lowest delta(15)N reported for an aquatic environment (average of -4.1 parts per thousand). These low delta(15)N values and large input of water from precipitation directl y to the lake surface suggests that much of the nitrate in Lake Superi or is derived from atmospheric deposition. (C) 1998 Elsevier Science B .V. All rights reserved.