Natural and anthropogenic nitrogen uptake by bloom-forming macroalgae

The frequency and duration of macroalgal blooms have increased in many coastal waters over the past several decades. We used field surveys and laboratory culturing experiments to examine the nitrogen content and δ15N values of Ulva and Gracilaria, two bloom-forming algal genera in Narragansett Bay, RI (USA). The northern end of this bay is densely populated with large sewage treatment plant nitrogen inputs; the southern end is more lightly populated and opens to the Atlantic Ocean. Field-collected Ulva varied in δ15N among sites, but with two exceptions had δ15N above 10‰, reflecting a significant component of heavy anthropogenic N. This variation was not correlated with a north–south gradient. Both Ulva and Gracilaria cultured in water from across Narragansett Bay also had high signals (δ15N = ∼14–17‰ and 8–12‰, respectively). These results indicate that inputs of anthropogenic N can have far-reaching impacts throughout estuaries.