Nitrogen sedimentation in a lake affected by massive nitrogen inputs: autochthonous versus allochthonous effects

Abstract

1. Monthly changes to N loadings, in-lake particulate organic nitrogen (PON), planktonic PON, and PON sedimentary flux were studied in a Spanish flowthrough, seepage lake subject to massive nitrogen inputs from June 2003 to December 2004 when water renewal was very rapid (0.09–0.17 year). 2. The distribution of in-lake PON did not show a seasonal trend. Total nitrogen input flux ranged from 1.23 to 4.83 g N m)2 day)1, 71–76% of which is nitrate while PON represents 6–10%. PON sedimentation rates ranged from 9 to 90 mg N m)2 day)1 and fluctuated on a seasonal basis, reaching a minimum in winter and early spring and a maximum after thermal turnover had occurred. 3. This fluctuation was not related to either autochthonous planktonic production or allochthonous inputs. Since charophyte populations in Colgada Lake underwent a seasonal pattern of growth and decomposition, and 15dN values of settling material peaked at the end of that decomposition process, we suggest that PON sedimentary flux could be partly driven by decomposed charophyte particles. 4. However, the picture of PON sedimentation in this lake was more complex than anticipated because water residence time partly explained PON variability, albeit with a 1 month lag. Water residence time explained 40% of the overall variance of yearly averaged PON sedimentary flux in a meta-analysis of 13 lakes worldwide. However, the factors such as phytoplankton composition, trophic structure, bottom communities, nutrient loading or productivity levels may also be influential on PON settling dynamics.