Variations and controls of nitrogen stable isotopes in particulate organic matter of lakes

Abstract

Nitrogen stable isotope (delta(15)N) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of delta(15)N(POM) in lakes at the global scale. The delta(15)N(POM) variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as delta(15)N(POM) maximum -- delta(15)N(POM) minimum) from 36 lakes with seasonal data did not change systematically with latitude, but was significantly lower in small lakes than in large lakes. The seasonal mean delta(15)N(POM) increased from oligotrophic lakes to eutrophic lakes despite large variations that are attributed to the occurrences of nitrogen fixation across the trophic gradient and the differences in delta(15)N of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean delta(15)N(POM) was significantly correlated with DIN concentration and delta(15)N(DIN) in two subsets of lakes. Seasonal minimum delta(15)N(POM) in individual lakes is influenced by nitrogen fixation and delta(15)N(DIN) while seasonal maximum delta(15)N(POM) is influenced by lake trophic state and delta(15)N(DIN). As a result of the dominance of non-living POM in the unproductive surface waters, seasonal delta(15)N(POM) amplitude was small (mean = 4.2 per thousand) in oligotrophic lakes of all latitudes. On the other hand, seasonal delta(15)N(POM) amplitude in eutrophic lakes was large (mean = 10.3 per thousand), and increased from low to high latitudes, suggesting that the seasonal variability of delta(15)N in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The delta(15)N(POM) from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2 per thousand depletion compared to the lakes with seasonal data. Along with the large seasonal variability of delta(15)N(POM) within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes.