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

Nitrogen stable isotope (delta N-15) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of delta N-15(POM) in lakes at the global scale. The delta N-15(POM) variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as delta N-15(POM) maximum - delta N-15(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 N-15(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 N-15 of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean delta N-15(POM) was significantly correlated with DIN concentration and delta N-15(DIN) in two subsets of lakes. Seasonal minimum delta N-15(POM) in individual lakes is influenced by nitrogen fixation and delta N-15(DIN) while seasonal maximum delta N-15(POM) is influenced by lake trophic state and delta N-15(DIN). As a result of the dominance of non-living POM in the unproductive surface waters, seasonal delta N-15(POM) amplitude was small (mean = 4.2aEuro degrees) in oligotrophic lakes of all latitudes. On the other hand, seasonal delta N-15(POM) amplitude in eutrophic lakes was large (mean = 10.3aEuro degrees), and increased from low to high latitudes, suggesting that the seasonal variability of delta N-15 in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The delta N-15(POM) from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2aEuro degrees depletion compared to the lakes with seasonal data. Along with the large seasonal variability of delta N-15(POM) within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes.