Variations and controls of nitrogen stable isotopes in particulate organic matter of lakes
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Nitrogen stable isotope (δ15N) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of δ15NPOM in lakes at the global scale. The δ15NPOM variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as δ15NPOM maximum − δ15NPOM 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 δ15NPOM 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 δ15N of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean δ15NPOM was significantly correlated with DIN concentration and δ15NDIN in two subsets of lakes. Seasonal minimum δ15NPOM in individual lakes is influenced by nitrogen fixation and δ15NDIN while seasonal maximum δ15NPOM is influenced by lake trophic state and δ15NDIN. As a result of the dominance of non-living POM in the unproductive surface waters, seasonal δ15NPOM amplitude was small (mean = 4.2‰) in oligotrophic lakes of all latitudes. On the other hand, seasonal δ15NPOM amplitude in eutrophic lakes was large (mean = 10.3‰), and increased from low to high latitudes, suggesting that the seasonal variability of δ15N in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The δ15NPOM from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2‰ depletion compared to the lakes with seasonal data. Along with the large seasonal variability of δ15NPOM within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes.