Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef

Nanne van Hoytema,Malik S. Naumann,Alessio Rovere,Vanessa N. Bednarz,Mamoon M. D. Al-Rshaidat,Christian Wild,Ulisse Cardini

doi:10.1007/s10021-016-9966-1

Abstract

Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m−2 reef d−1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m−2 reef d−1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.

Highlights

Nitrogen (N) is a fundamental component of all living organisms
We present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses
This limitation is of particular significance in coral reef ecosystems, as these are among the ecosystems displaying the highest rates of gross primary production (GPP), yet experiencing very low ambient concentrations of dissolved nutrients

Summary

Introduction

Nitrogen (N) is a fundamental component of all living organisms. In particular, N is needed by primary producers in capturing energy through photosynthesis and building biomass, leading to a tight coupling of the N and carbon (C) cycles (Gruber and Galloway 2008) and dictating constraints to the772 U. In unperturbed and oligotrophic marine systems, primary productivity is often limited by bioavailable forms of N, which are scarce due to low atmospheric inputs and N loss pathways (Vitousek and Howarth 1991). This limitation is of particular significance in coral reef ecosystems, as these are among the ecosystems displaying the highest rates of gross primary production (GPP), yet experiencing very low ambient concentrations of dissolved nutrients. Biological dinitrogen (N2) fixation (diazotrophy, BNF) is thought to play an important role in replenishing the N pool, maintaining the ecosystem productivity and its biological storage of C (D’Elia and Wiebe 1990; Capone 1996; O’Neil and Capone 2008)

Objectives

Methods

Results

Discussion

Conclusion