Stable nitrogen isotope composition in sedimentary organic matter as a potential proxy of nitrogen sources for primary producers at a fringing coral reef |
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| Authors: | Yu Umezawa Toshihiro Miyajima Isao Koike |
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| Institution: | (1) Marine Biogeochemistry Laboratory, Ocean Research Institute, the University of Tokyo, Minamidai, Nakano-ku, Tokyo 164-8639, Japan;(2) University of the Ryukyus, Senbaru, Nishihira, Okinawa 903-0123, Japan;(3) Present address: Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo, Nagasaki-city, Nagasaki 852-8521, Japan |
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| Abstract: | The stable nitrogen isotope ratio (δ
15N) in macroalgae is effectively used as a time-integrated bioindicator to record nitrogen sources for primary producers during
their growing periods in aquatic ecosystems. However, the utility of this tool is limited because the occurrence of these
organisms is often restricted in space and time. To investigate the potential of chemical composition in sedimentary organic
matter (SOM) as a proxy for time-integrated environmental conditions, nitrogen (N) and carbon (C) contents and their stable
isotope ratios (δ
15N and δ
13C) were determined, and systematically cross-checked against corresponding values in macroalgae at the Shiraho fringing reef
in Okinawa, Japan. Preliminary trials showed that δ
15N in SOM processed by the “wash-out method” for δ
13C analysis yielded similar δ
15N values to the bulk sediment, despite the loss of some SOM during the process. The amounts of organic matter and the ratio
of the HCl-insoluble portion were variable within the reef, probably reflecting local vegetation and subsequent decomposition.
The distribution of δ
15N and δ
13C in SOM showed similar trends to those of macroalgae, with mostly constant differences of 1.4‰ and −6.7‰, respectively. These
differences throughout the reef appeared to be explained in terms of mixed contributions from macrophyte and epibenthic microalgae
growing in different seasons and years, with their debris undergoing diagenetic alteration. Therefore, macroalgae and SOM
δ-values can be used in a complementary manner, over various time scales, as indicators of the integrated effect of dissolved
inorganic nitrogen (DIN) sources on coral reef ecosystems. |
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| Keywords: | Stable isotope signatures sedimentary organic matter (SOM) nutrient dynamics land-derived nitrogen macroalgae |
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