Natural and anthropogenic nitrogen uptake by bloom-forming macroalgae

Area of Science:

• Marine Ecology
• Biogeochemistry
• Coastal Eutrophication

Background:

• Macroalgal blooms are increasing in frequency and duration globally.
• Nutrient enrichment, particularly nitrogen (N), is a primary driver of these blooms.
• Understanding the sources and fate of N is crucial for coastal management.

Purpose of the Study:

• To investigate the nitrogen (N) content and isotopic signatures (delta(15)N) of bloom-forming algae in Narragansett Bay.
• To determine the extent to which anthropogenic N influences these algae across a gradient of human population density.
• To assess the reach of N inputs throughout the estuarine system.

Main Methods:

• Field surveys of macroalgal genera Ulva and Gracilaria in Narragansett Bay, RI.
• Laboratory culturing experiments using algae exposed to bay water.
• Analysis of nitrogen content and delta(15)N values in algal tissues.

Main Results:

• Field-collected Ulva exhibited high delta(15)N values (above 10 per thousand), indicating significant anthropogenic N.
• Algal delta(15)N values did not show a clear north-south gradient, suggesting widespread N influence.
• Cultured Ulva and Gracilaria also showed elevated delta(15)N signatures, confirming anthropogenic N uptake.

Conclusions:

• Anthropogenic N inputs have a substantial and far-reaching impact on macroalgal communities within estuaries.
• Estuarine systems can retain and be influenced by N pollution even in areas with lower direct inputs.
• The isotopic signature of algae serves as a valuable indicator of nutrient pollution in coastal waters.