Seasonality in dinitrogen fixation and primary productivity by coral reef framework substrates from the northern Red Sea

Abstract

N-2 fixation by coral reef benthic substrates may support primary productivity on oligo trophic coral reefs. However, little is known regarding the influence of environmental para meters on coral reef benthic N-2 fixation. This study quantified N-2 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. N-2 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 N-2 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 N-2 fixation in turf algae and coral rock. Further, we estimate N-2 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 N-2 fixation and Pgross in M. fistulifera, which displayed negative Pnet and heterotrophic metabolism (P-gross: respiration <1). These findings highlight the role of environmental parameters in regulating benthic substrate-associated N-2 fixation and the potential importance of fixed N for supporting primary production, particularly during nutrient-depleted conditions.