[Nitrogen fixation and primary production in Western Mediterranean]

Area of Science:

• Marine biology
• Oceanography
• Biogeochemistry

Background:

• The Mediterranean Sea is an oligotrophic marine system with a high nitrate/phosphate ratio in deep waters.
• Understanding nitrogen inputs is crucial for marine ecosystem dynamics.

Purpose of the Study:

• To investigate nitrogen fixation, nitrate assimilation, and primary production in the northwestern Mediterranean Sea over 1.5 years.
• To quantify the contribution of nitrogen fixation to the marine nitrogen budget.

Main Methods:

• Utilized the (13)C/(15)N dual isotope method.
• Conducted year-round sampling and analysis in the northwestern Mediterranean Sea.

Main Results:

• Nitrogen fixation was detected throughout the year, with rates from 2 to 17 nmol N l(-1) d(-1).
• Peak nitrogen fixation rates coincided with spring phytoplankton blooms.
• Significant nitrogen fixation rates (4-8 nmol N l(-1) d(-1)) were observed during low primary productivity in summer.
• Diazotrophy provides substantial new nitrogen during oligotrophic summer periods.

Conclusions:

• Nitrogen fixation is a vital source of new nitrogen in the Mediterranean Sea, particularly during summer.
• This diazotrophy can balance the annual nitrogen biogeochemical budget.
• It may explain the elevated nitrate/phosphate ratios found in Mediterranean deep waters.