The record of nitrate utilization and productivity limitation provided by delta N-15 values in lake organic matter - A study of sediment trap and core sediments from Baldeggersee, Switzerland

The response of nitrogen isotopic composition of organic matter to changes in nitrate utilization, N-limitation and lake productivity was studied in B aldeggersee, a small eutrophic lake in central Switzerland. Nitrogen isotop e ratios were measured on organic matter accumulation in sediment traps, co llected daily from March 1995 through October 1996, and on individually sam pled light and dark annual laminae from a 108-yr sediment core sequence (18 85-1993). Nitrogen accumulation in the sediment traps averaged 0.04 g N m(- 2) d(-1). delta(15)N values of the sediment trap material increased from 11 parts per thousand to 13 parts per thousand (atmospheric N-2) as primary p roductivity decreased surface water [NO3-] from 1.4 to 0.7 mg N L-1 during the seasonal stratified periods. Very small amounts of isotopically enriche d organic matter (delta(15)N similar to 15-20 parts per thousand) of hetero trophic and/or detrital origin accumulated in the winter months. Nitrogen accumulation in the core sediments average 4 g N m(-2) per year. d elta(15)N values of the sediment core material increased up-core (from delt a(15)N similar to 6 parts per thousand to delta(15)N similar to 11 parts pe r thousand); several abrupt positive isotope shifts (>2 parts per thousand) occur in the upper Dart of the record. Comparison between sediment delta(1 5)N values and surface water [NO3-] measurements for the period of 1976-199 3 reveals that the abrupt positive delta(15)N shifts occurred in years wher e unusually large phytoplankton blooms depleted surface waters nitrate to c oncentrations of <0.7 mg N L-1. A 3 parts per thousand negative delta(15)N shift, observed between 1973-1975 at the time of maximum anoxic conditions and meromixis, can be attributed to phytoplankton incorporation of ammonia, which was present in concentrations of up to 0.7 mg L-1 in the epilimnion. Preservation of isotopic shifts in the sediment core which are clearly rela ted to water column processes, and the similarity of core top delta(15)N va lues to the weighted average N-isotopic composition of sediment trap materi al indicate that the sediment record reliably reflects the surface-generate d delta(15)N signal. Our results from sediment trap and uppermost core samp les provide the first conclusive evidence that N-isotopes in lacustrine org anic matter record the increasing isotopic enrichment of surface water NO3- due to its utilization by phytoplankton. Artificial aeration of the Baldeggersee bottom water since 1982 has effecti vely reduced water column anoxia. Thus, water column denitrification, which could considerably increase the isotopic composition of residual nitrate, does not occur in the present lake. Even so, present day nitrogen isotopic values in the sediment core data are higher than most all previous periods suggesting that water column denitrification has never been a dominant infl uence on sediment delta(15)N values. Instead, the up-core 6 parts per thous and increase in delta(15)N values better corresponds to the documented hist ory of external N-loading from agricultural runoff in the watershed over th e last 100 years. These nitrogen sources have characteristically high delta (15)N values (10-20 parts per thousand) and could lead to progressive N-15- enrichment of the Baldeggersee dissolved inorganic nitrogen pool.