MEPS 533:79-92 (2015)  -  DOI: https://doi.org/10.3354/meps11383

ABSTRACT: N₂ fixation by coral reef benthic substrates may support primary productivity on oligotrophic coral reefs. However, little is known regarding the influence of environmental parameters on coral reef benthic N₂ fixation. This study quantified N₂ fixation and photosynthesis in 3 common reef framework substrates: turf algae, coral rock, and the abundant encrusting sponge Mycale fistulifera over 4 seasons in the northern Gulf of Aqaba. N₂ fixation activity was detected during day and night for all substrates, but on an annual average was significantly higher for turf algae (4.4 ± 3.9 nmol C₂H₄ cm^-2 h^-1) and coral rock (3.5 ± 2.8 nmol C₂H₄ cm^-2 h^-1) compared to M. fistulifera (0.2 ± 0.2 nmol C₂H₄ cm^-2 h^-1). There was strong seasonality in N₂ fixation, with rates one order of magnitude higher in summer when temperature and irradiance were highest but inorganic nutrient concentrations lowest. During summer and fall, when nutrients were low, we found a significant positive linear relationship between gross photosynthesis (P_gross) and N₂ fixation in turf algae and coral rock. Further, we estimate N₂ fixation can supply up to 20 and 27% of the N demand for net photosynthesis (P_net) in coral rock and turf algae, respectively. By contrast there was no significant relationship between N₂ fixation and P_gross in M. fistulifera, which displayed negative P_net and heterotrophic metabolism (P_gross:respiration <1). These findings highlight the role of environmental parameters in regulating benthic substrate-associated N₂ fixation and the potential importance of fixed N for supporting primary production, particularly during nutrient-depleted conditions.

KEY WORDS: Acetylene reduction · Photosynthesis · Turf algae · Sponges · Coral rock · Gulf of Aqaba