Nitrogen fixation and denitrification activity differ between coral- and algae-dominated Red Sea reefs

Area of Science:

• Marine Ecology
• Biogeochemistry
• Coral Reef Science

Background:

• Coral reefs undergo phase shifts from coral to algae dominance, impacting nitrogen cycling.
• The specific rates of nitrogen fixation and denitrification in these phase shifts are largely unknown.
• Understanding these processes is crucial for coral reef ecosystem management.

Purpose of the Study:

• To quantify dinitrogen (N2) fixation and denitrification rates across dominant benthic categories in a Red Sea coral reef.
• To assess how these nitrogen cycling processes differ between coral- and algae-dominated reef areas.
• To determine the role of benthic communities in nitrogen input and removal during phase shifts.

Main Methods:

• Measured N2 fixation and denitrification for six major benthic categories (hard corals, soft corals, turf algae, coral rubble, biogenic rock, reef sands).
• Extrapolated process rates based on the relative benthic cover in both coral- and algae-dominated zones.
• Analyzed the relationship between N2 fixation and denitrification activities within different benthic groups.

Main Results:

• Benthic categories with high N2 fixation generally showed low denitrification.
• Turf algae and coral rubble were the primary drivers of N2 fixation (>90%), while corals dominated denitrification (>50%).
• Algae-dominated areas exhibited double the N2 fixation rates compared to coral-dominated areas; denitrification levels were similar.

Conclusions:

• Algae-dominated reefs enhance nitrogen input through significantly higher N2 fixation.
• Increased nitrogen availability in algae-dominated systems can fuel productivity, favoring algae over corals.
• Phase shifts to algal dominance create a positive feedback loop, potentially impeding coral reef recovery.