The record of nitrate utilization and productivity limitation provided by δ¹⁵N values in lake organic matter—A study of sediment trap and core sediments from Baldeggersee, Switzerland

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 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 Ϫ1 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 ␦ 15 N values to the weighted average N-isotopic composition of sediment trap material indicate that the sediment record reliably reflects the surface-generated ␦ 15 N 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 NO 3 Ϫ 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 ␦ 15 N values. Instead, the up-core 6‰ increase in ␦ 15 N 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 ␦ 15 N values (10-20‰) and could lead to progressive 15 N-enrichment of the Baldeggersee dissolved inorganic nitrogen pool.

Climatic imprint of the mid-latitude Westerlies in the Central Tian Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years

“…yr BP (Figure 5), which are paralleled by lowered δ 13 C org values. High δ 15 N values in the sediments could be attributed to (1) denitrification processes in an anoxic hypolimnion during periods of pronounced meromixis (Hodell and Schelske, 1998;Talbot and Laerdal, 2000), (2) increased aquatic productivity (Gu et al, 1996;Talbot and Laerdal, 2000;Teranes and Bernasconi, 2000), (3) evaporative loss of isotopically light ammonia during dry climate conditions (Talbot and Johannessen, 1992), or (4) increased input of terrestrial OM (Haberzettl et al, 2005;Mayr et al, 2009).…”

“…Facilitated by the characteristic geochemical/isotopic signatures of the different aquatic and terrestrial organic components involved in lake sediment formation, these parameters allow to track for instance changes in environmental conditions, productivity, nutrient availability, or the origin of OM (e.g. Hodell and Schelske, 1998;Hollander et al, 1992;Talbot, 2001;Talbot and Laerdal, 2000;Teranes and Bernasconi, 2000). While δ 15 N is in this regard a partly ambiguous proxy because of the complexity of the nitrogen cycle and the various in-lake and catchment processes and nitrogen sources involved in lake sediment OM formation (Talbot, 2001), the δ 13 C org and C/N signatures of different organic materials are relatively well known.…”

Allochthonous contribution to seasonal and spatial variability of organic matter sedimentation in a deep oligotrophic lake (Lake Constance)

“…2003) which previously had undergone passages through food chains and/or was metabolised by microbial communities (Hodell and Schelske. 1998;Teranes and Bernasconi. 2000).…”

A geochemical record of environmental changes in sediments from Sishili Bay, northern Yellow Sea, China: Anthropogenic influence on organic matter sources and composition over the last 100years

“…However, the fractionation of d 15 N during the absorption and assimilation of phytoplankton depends on the concentration and species of inorganic nitrogen (Owens, 1985;Mishima et al, 1999;Cohen and Fong, 2005). Normally, d 15 N is used as a subsidiary factor for special events; for example, an increase in organic matter d 15 N values can record deliveries of isotopically heavy nitrate (d 15 N = 10-25‰) from farm runoff and human sewage (Teranes and Bernasconi, 2000;Ramaswamy et al, 2008;Sampaio et al, 2010).…”