Assimilation and nitrification in pelagic waters: insights using dual nitrate stable isotopes (delta N-15, delta O-18) in a shallow lake

作者：Botrel Morgan; Bristow Laura A; Altabet Mark A; Gregory Eaves Irene; Maranger Roxane

来源：Biogeochemistry, 2017, 135(3): 221-237.

DOI：10.1007/s10533-017-0369-y

• Nitrate
• Stable isotopes
• Nitrification
• Assimilation
• Nitrogen cycling
• Lake
• Cyanobacteria

Nitrate dual stable isotopes (delta N-15 and delta O-18 of NO3 (-)) have proven to be a powerful technique to elucidate nitrogen (N) cycling pathways in aquatic systems. We applied this technique for the first time in the pelagic zone of a small temperate meso-eutrophic lake to identify the dominant N cycling pathways, and their spatial and temporal variability. We measured the lake NO3 (-) delta N-15 and delta O-18 signatures over an annual cycle and compared them to that of the watershed. Both delta N-15 and delta O-18 of NO3 (-) in the lake increased during summer relative to the inputs. Relationships between lake NO3 (-) isotopic composition and concentrations were different across thermal strata with an apparent isotope effect in the epilimnion of (15)epsilon(epi) = 4.6aEuro degrees and (18)epsilon(epi) = 10.9aEuro degrees. We found a strong deviation of the lake NO3 (-) delta O-18 and delta N-15 from the expected 1:1 line for assimilation (slope = 1.73) suggesting that nitrification was co-occurring. We estimated that nitrification could support between 5 and 30% of nitrate-based production during the growing season, but was negligible in early spring and fall, and probably more dominant under ice. We showed that the technique is promising to study N processes at the ecosystem scale in shallow lakes, particularly during winter. Our results suggest that recycled NO3 (-) could support primary productivity and influence phytoplankton composition in the surface waters of small lakes.

• 出版日期2017-9