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

Abstract

N2 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 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 (Pgross) 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 (Pnet) in coral rock and turf algae, respectively. By contrast there was no significant relationship between N2 fixation and Pgross in M. fistulifera, which displayed negative Pnet and heterotrophic metabolism (Pgross: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.