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

Highlights: • Seagrass δ¹⁵N ratios are not correlated with urbanization in a 10–200 km radius. • Seagrass δ¹⁵N ratios are positively correlated with latitude. • Temperate seagrass species have higher mean δ¹⁵N ratios than tropical species. • The mechanism responsible for the global pattern in δ¹⁵N values remains unknown. - Abstract: 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