Patterns and controls of nitrogen stable isotopes of particulate organic matter in subtropical lakes

Abstract

-Nitrogen stable isotope composition (d 15 N) of particulate organic matter (POM) has been used to infer dominant nitrogen cycling processes in lakes. However, very few studies have compared the isotope variations in lakes along trophic state and other biogeochemical gradients. Here we report an analysis of d 15 N POM and selected environmental variables from 96 subtropical lakes to assess the patterns and controls of isotope variations. Results indicate that d 15 N POM values varied from x 2.8 to 13.2‰ and were not significantly correlated with total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a). The 15 N depletion in POM was found across the entire trophic gradient and likely reflected contributions from planktonic nitrogen fixation. The 15 N enrichment was attributed to high primary production and the contributions of anthropogenic wastes in eutrophic lakes, and the presence of microzooplankton in the water samples of the oligotrophic lakes. The d 15 N POM was negatively correlated with water color and positively correlated with pH. Because water color is indicative of light availability which affects phytoplankton growth and pH is significantly correlated with Chl a concentration in the study lakes, the close relationship between water color, pH and d 15 N POM therefore suggests primary productivity-driven isotope fractionation. Results from this study did not reveal the importance of trophic state, nitrogen concentration and surface area to the variations of d 15 N POM and points to the complex interactions of nitrogen cycling processes in lakes with diverse biogeochemical features.

Evidence for the Primary Role of Phytoplankton on Nitrogen Cycle in a Subtropical Reservoir: Reflected by the Stable Isotope Ratios of Particulate Nitrogen and Total Dissolved Nitrogen

“…There are different δ 15 N compositions in rivers, lakes, reservoirs and marine regions, because of characteristic living organisms and inanimate matter. Many studies have suggested that variations in δ 15 N PN and δ 15 N TDN are associated with thermal and hydrological characteristics, trophic states, nitrogen sources, N 2 fixation, and phytoplankton abundance (Altabet, 2006; Gu et al, 2006, Gu, 2009; Gu and Schelske, 2010; Hou et al, 2013). However, the key factors that control variations in δ 15 N PN and δ 15 N TDN are not well understood.…”