Nitrogen removal processes in lakes of different trophic states from on-site measurements and historic data

Beat Müller,Carsten J Schubert,Raoul Thoma,Cameron M Callbeck,Kathrin B L Baumann

doi:10.1007/s00027-021-00795-7

Abstract

Freshwater lakes are essential hotspots for the removal of excessive anthropogenic nitrogen (N) loads transported from the land to coastal oceans. The biogeochemical processes responsible for N removal, the corresponding transformation rates and overall removal efficiencies differ between lakes, however, it is unclear what the main controlling factors are. Here, we investigated the factors that moderate the rates of N removal under contrasting trophic states in two lakes located in central Switzerland. In the eutrophic Lake Baldegg and the oligotrophic Lake Sarnen, we specifically examined seasonal sediment porewater chemistry, organic matter sedimentation rates, as well as 33-year of historic water column data. We find that the eutrophic Lake Baldegg, which contributed to the removal of 20 ± 6.6 gN m−2 year−1, effectively removed two-thirds of the total areal N load. In stark contrast, the more oligotrophic Lake Sarnen contributed to 3.2 ± 4.2 gN m−2 year−1, and had removed only one-third of the areal N load. The historic dataset of the eutrophic lake revealed a close linkage between annual loads of dissolved N (DN) and removal rates (NRR = 0.63 × DN load) and a significant correlation of the concentration of bottom water nitrate and removal rates. We further show that the seasonal increase in N removal rates of the eutrophic lake correlated significantly with seasonal oxygen fluxes measured across the water–sediment interface (R2 = 0.75). We suggest that increasing oxygen enhances sediment mineralization and stimulates nitrification, indirectly enhancing denitrification activity.

Highlights

Increasing anthropogenic nitrogen (N) loading in agriculturally dominated terrestrial systems has the potential to induce significant eutrophication in downstream aquatic environments and coastal marine ecosystems where N is often a key nutrient limiting primary productivity (Howarth and Marino 2006; Diaz and Rosenberg 2008; Howarth et al 2011; Erisman et al 2013)
While in Lake Baldegg, deposition rates of organic C and N were dominated by the large spring phytoplankton blooms (April–June, Fig. SI-1a), productivity in Lake Sarnen was low and its sedimentation regime was strongly shaped by the hydrology of its tributaries (Fig. SI-1b)
The elemental C:N ratio characterizes Lake Baldegg as autochthonously dominated, while Lake Sarnen was often exposed to stormwater events causing turbidity currents with allochthonous material permeating the epilimnion and dispersing in the hypolimnion

Summary

Introduction

Increasing anthropogenic nitrogen (N) loading in agriculturally dominated terrestrial systems has the potential to induce significant eutrophication in downstream aquatic environments and coastal marine ecosystems where N is often a key nutrient limiting primary productivity (Howarth and Marino 2006; Diaz and Rosenberg 2008; Howarth et al 2011; Erisman et al 2013). In the transport pathway from land to the ocean, lakes play an essential role with respect to the removal of N. In aquatic environments, including marine and freshwater sediments the N cycle comprises of N 2 fixation, denitrification, nitrification, anammox (Kuypers et al 2018; Crowe et al 2017), dissimilatory nitrate (NO3−) reduction to ammonium (DNRA, Giblin et al 2013; Burgin and Hamilton 2007), comammox (Daims et al 2015; van Kessel et al 2015), and organic nitrogen remineralization to ammonium. Studies in permeable marine sediments, where nitrification–denitrification are tightly coupled, posit that the availability of oxygen (O2) may play a role in moderating denitrification.

Methods

Results

Discussion

Conclusion