Coral reefs nitrogen cycling traced by stable isotopes-labeled compounds

Dr Thibodeau, Benoit   (Principal Investigator (PI))

Professor Baker David Michael   (Co-Investigator)

Dr Duprey Nicolas Noel   (Co-Investigator)

24

2015-11-20

2017-11-19

150000

Coral reefs nitrogen cycling traced by stable isotopes-labeled compounds

Biogeochemistry, Coral reef, Environmental Science, Oceanography, Stable Isotope

Environmental Studies and ScienceEarth Sciences

Physical Sciences (P)

201510159009

Seed Fund for Basic Research for New Staff

2015

Completed

Corals reefs are known for their high productivity despite the low concentrations of ambient inorganic nitrogen (Odum and Odum 1955; Johannes et al. 1972; Webb et al. 1975). Consequently, there is usually not enough nitrogen (N) relative to phosphorus (P) to satisfy the N: P uptake demand from the reef ecosystem (Atkinson, 2011). Unraveling nitrogen cycling is thus important for a better understanding of how coral reef ecosystems function in generally oligotrophic environments, but also in human-impacted coastal areas. A recent study revealed that anthropogenic N inputs can have a strong negative effect on corals and may lower the thermal threshold for bleaching (Wiedenmann et al., 2012), so a better understanding of such inputs is needed. Hong Kong is a perfect natural laboratory for such investigation since it offers a heavily human-impacted environment, with a wide range of inorganic nitrogen concentration. Studies about coral reefs have generally focused on the cycling of dissolved inorganic nitrogen (DIN = nitrate (NO3-) + nitrite (NO2-) + ammonium (NH4+)) with a typical water column concentration range of 0.1 to 1 µmol L-1 (2 to 9 µmol L-1 in Hong Kong). While, the concentrations of dissolved organic nitrogen ([DON]) can be high (~5 µmol L-1), and often exceed those of DIN in coral reefs, data on the sources, sinks and turnover of DON are scarce due to the technical challenges linked to their analysis. This paucity of data has prevented the identification of DON sources and sinks and thus limited the evaluation of the role of DON in sustaining the high productivity of coral reefs (Atkinson 2011; Suzuki and Casareto 2011). Recently, the first DON stable isotope measurements over a coral reef suggested that a significant amount of DON might be released from N-fixer organisms over relatively pristine reefs, thus reefs can act as a source of DON (Thibodeau et al., 2013). To our knowledge, no DON investigation has been carried in Hong Kong waters. The main goal of this study is to identify how nitrogen cycles between the different forms of dissolved nitrogen and how it is incorporated in coral reefs around Hong Kong. This project will provide basic knowledge about the N-cycle under Hong Kong human-impacted conditions and thus will lay down the premises for our future larger N-cycle-oriented projects. We aim to respectively quantify the importance of DIN and DON in the coral nutrient consumption. Moreover, our experimental set-up is also suitable to investigate the origins of the oxygen atom of nitrified nitrate, which has been a subject of debate for the last decades. This highly valuable information will help us build an accurate dynamic isotopic model including both 15N and 18O of dissolved nitrate. Bullet point Objectives: • Quantify the rate of release & uptake of nitrate, ammonia and DON • Quantify the rate of N-assimilation by the coral and identify which N-pool contributed the most • Identify the origin of the oxygen atom in nitrified nitrate (water vs atmosphere) • Build a model of the basic N-cycle in HK coral reefs (exploratory)