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

Binhe Gu^1,2 and Claire L. Schelske³

¹ Fishery College, Guangdong Ocean University, 40 East Jiefang Road, Zhanjiang, Guangdong, 52488, China
² Southwest Research and Education Center, University of Florida, 2685 SR 29 N, Immokalee, FL 34142, USA
³ Department of Geological Sciences and Land Use and Environmental Change Institute, University of Florida, Gainesville, FL 32611, USA

Corresponding author: gubinhe@gmail.com

Received: 25 September 2009
Accepted: 29 December 2009

Abstract

Nitrogen stable isotope composition (δ¹⁵N) 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 δ¹⁵N_POM and selected environmental variables from 96 subtropical lakes to assess the patterns and controls of isotope variations. Results indicate that δ¹⁵N_POM values varied from −2.8 to 13.2‰ and were not significantly correlated with total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a). The ¹⁵N depletion in POM was found across the entire trophic gradient and likely reflected contributions from planktonic nitrogen fixation. The ¹⁵N 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 δ¹⁵N_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 δ¹⁵N_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 δ¹⁵N_POM and points to the complex interactions of nitrogen cycling processes in lakes with diverse biogeochemical features.