Stable nitrogen isotopes in coastal macroalgae: geographic and anthropogenic variability

Area of Science:

• Marine Ecology
• Biogeochemistry
• Stable Isotope Analysis

Background:

• Coastal waters receive nitrogen from both natural and anthropogenic sources.
• Stable nitrogen isotopes (δ(15)N) can trace these different nitrogen sources incorporated into biomass.
• Macroalgae are important indicators of nutrient dynamics in coastal ecosystems.

Purpose of the Study:

• To determine the relative importance of anthropogenic versus natural nitrogen sources along the NW Spain coast.
• To investigate the relationship between macroalgal δ(15)N, inorganic nitrogen, and human population density.
• To assess the influence of factors like upwelling and waste treatment on nitrogen cycling.

Main Methods:

• Measuring δ(15)N in two macroalgae species (Ascophyllum nodosum and Fucus vesiculosus).
• Analyzing δ(15)N in water samples (nitrate and ammonium).
• Correlating macroalgal δ(15)N with nitrogen concentrations, water isotopes, population density, and geographic location.

Main Results:

• Macroalgal δ(15)N showed a significant linear decrease along the coast, consistent with upwelling effects.
• Higher δ(15)N values were observed in macroalgae from estuaries and areas with larger human populations.
• Macroalgal δ(15)N was not directly correlated with inorganic nitrogen concentrations or water δ(15)N, suggesting complex nutrient inputs.

Conclusions:

• Macroalgal δ(15)N is a valuable tool for tracing nitrogen sources in coastal areas.
• Upwelling and local waste treatment efficiency significantly influence nitrogen cycling, more so than human population size alone.
• The study highlights the complexity of using macroalgal isotopes to assess anthropogenic impacts, emphasizing the need to consider multiple environmental factors.