Isotopic composition of dissolved inorganic nitrogen in high mountain lakes: variation with altitude in the Pyrenees

摘要

Nitrogen deposition in remote areas has increased, but the effect onecosystems is still poorly understood. For aquatic systems, knowledge of themain processes driving the observed variation is limited, as is knowledge ofhow changes in nitrogen supply affect lake biogeochemical and food webprocesses. Differences in dissolved inorganic nitrogen (DIN) between lakescannot be understood without considering catchment characteristics. Inmountains, catchment features (e.g., thermal conditions, land cover) varyconsiderably with elevation. The isotopic composition of nitrogen (δ15N)is increasingly used to study aquatic ecosystem dynamics. Here weexplore the variability of δ15N in DIN in high mountain lakesand show that environmental conditions that change with altitude can affectthe isotopic ratio.We measured ammonium and nitrate δ15N values in atmosphericdeposition, epilimnetic water, deep chlorophyll maximum water (DCMW) andsediment pore water (SPW) from eight mountain lakes in the Pyrenees, bothabove and below the treeline. Lakes showed relatively uniform δ15N-NH₄+values in SPW (2.2±1.6‰), with no variationcorresponding to catchment or lake characteristics. We suggest that organicmatter diagenesis under similar sediment conditions is responsible for thelow variation between the lakes.In the water column, the range of δ15N values was larger forammonium (−9.4‰ to 7.4‰) than for nitrate (−11.4‰ to −3.4‰), as a result ofhigher variation both between and within lakes (epilimnetic vs. DCM water).For both compounds part of the difference correlated with altitude orcatchment features (e.g., scree proportion). Based on concentration,chemical and isotopic tendencies, we suggest that patterns arise from thedistinct relative contributions of two types of water flow paths to thelakes: one from snowpack melting, with little soil interaction; and anotherhighly influenced by soil conditions. The snow-type flow path contributeslow DIN concentrations depleted in 15N, whereas the soil-type flow pathcontributes high nitrate concentrations with higher δ15N. Theproportion of these two types of source correlates with average catchmentfeatures when there is extensive snow cover during spring and early summerand probably becomes more dependent on local characteristics around the lakeas summer advances. Lake depth and pore water ammonium concentrations, amongother features, introduce secondary variation. In the context of nitrogendeposition studies, lakes with higher snow-type influence will probablyregister changes in N deposition and pollution sources better, whereas lakeswith higher soil-type influence may reflect long-term effects of vegetationand soil dynamics.