The degree of urbanization across the globe is not reflected in the δ15N of seagrass leaves

Many studies show that seagrass δ15N ratios increase with the amount of urbanization in coastal watersheds. However, there is little information on the relationship between urbanization and seagrass δ15N ratios on a global scale. We performed a meta-analysis on seagrass samples from 79 independent locations to test if seagrass δ15N ratios correlate with patterns of population density and fertilizer use within a radius of 10-200 km around the sample locations. Our results show that seagrass δ15N ratios are more influenced by intergeneric and latitudinal differences than the degree of urbanization or the amount of fertilizer used in nearby watersheds. The positive correlation between seagrass δ15N ratios and latitude hints at an underlying pattern in discrimination or a latitudinal gradient in the 15N isotopic signature of nitrogen assimilated by the plants. The actual mechanisms responsible for the correlation between δ15N and latitude remain unknown.