The record of nitrate utilization and productivity limitation provided by d15N values in lake organic matterA study of sediment trap and core sediments from Baldeggersee, Switzerland
Limnol. Oceanogr., 45(4), 2000, 801-813 | DOI: 10.4319/lo.2000.45.4.0801
ABSTRACT: The response of nitrogen isotopic composition of organic matter to changes in nitrate utilization, N-limitation and lake productivity was studied in Baldeggersee, a small eutrophic lake in central Switzerland. Nitrogen isotope ratios were measured on organic matter accumulation in sediment traps, collected daily from March 1995 through October 1996, and on individually sampled light and dark annual laminae from a 108-yr sediment core sequence (1885-1993). Nitrogen accumulation in the sediment traps averaged 0.04 g N m-2 d-1. d15N values of the sediment trap material increased from 11‰ to 13‰ (atmospheric N2) as primary productivity decreased surface water [NO3-] from 1.4 to 0.7 mg N L-1 during the seasonal stratified periods. Very small amounts of isotopically enriched organic matter (d15N ~ 15-20‰) of heterotrophic and/or detrital origin accumulated in the winter months. Nitrogen accumulation in the core sediments average 4 g N m-2 per year. d15N values of the sediment core material increased up-core (from d15N ~ 6‰ to d15N ~ 11‰); several abrupt positive isotope shifts (>2‰) occur in the upper part of the record. Comparison between sediment d15N values and surface water [NO3-] measurements for the period of 1976-1993 reveals that the abrupt positive d15N shifts occurred in years where unusually large phytoplankton blooms depleted surface waters nitrate to concentrations of <0.7 mg N L-1. A 3‰ negative d15N 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 related to water column processes, and the similarity of core top d15N values to the weighted average N-isotopic composition of sediment trap material indicate that the sediment record reliably reflects the surface-generated d15N signal. Our results from sediment trap and uppermost core samples provide the first conclusive evidence that N-isotopes in lacustrine organic 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 effectively 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 influence on sediment d15N values. Instead, the up-core 6‰ increase in d15N values better corresponds to the documented history of external N-loading from agricultural runoff in the watershed over the last 100 years. These nitrogen sources have characteristically high d15N values (10-20‰) and could lead to progressive 15N-enrichment of the Baldeggersee dissolved inorganic nitrogen pool.