Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
3
4
2006
10.5194/bg-3-439-2006
http://www.biogeosciences.net/3/439/2006/
http://www.biogeosciences.net/3/439/2006/bg-3-439-2006.html
http://www.biogeosciences.net/3/439/2006/bg-3-439-2006.pdf
439
449
2006-10-09
Photochemical production of ammonium in the oligotrophic Cyprus Gyre (Eastern Mediterranean)
V. Kitidis
vak@pml.ac.uk
G. Uher
R. C. Upstill-Goddard
R. F. C. Mantoura
G. Spyres
E. M. S. Woodward
Ocean Research Group, School of Marine Science and Technology, University of Newcastle upon Tyne, Newcastle upon Tyne, NE7 7RU, UK
Plymouth Marine Laboratory, Plymouth, PL1 3DH, UK
now at: Plymouth Marine Laboratory, Plymouth, PL1 3DH, UK
now at: International Atomic Energy Authority Marine Ecology Laboratory, Monaco
We investigated the photoproduction of ammonium (NH<sub>4</sub><sup>+</sup>) in surface
waters of the Cyprus gyre in the central Eastern Mediterranean in May 2002,
in 8 on deck irradiations with freshly collected, filtered samples.
NH<sub>4</sub><sup>+</sup> photoproduction (photoammonification) increased with
time-integrated irradiance during the course of irradiations.
Photoammonification rates around local noon were 0.4–2.9 nmol L<sup>−1</sup> h<sup>−1</sup>.
Normalised to time integrated irradiance, these rates were 0.9–3.8 pmol L<sup>−1</sup> h<sup>−1</sup>/(W m<sup>−2</sup>) and were significantly correlated with
Chromophoric Dissolved Organic Matter (CDOM) absorbance at 300 nm normalised
to Dissolved Organic Carbon (DOC). These results are consistent with the
notion that successive CDOM photobleaching in the surface mixed layer
results in decreased DOC-normalised light absorbance concurrent with
decreased dissolved organic matter reactivity with regard to photochemical
NH<sub>4</sub><sup>+</sup> release. Combining our experimental data with estimates of
annual solar irradiance and water column light attenuation yields an annual
photoammonification rate for the Cyprus Gyre of 40±17 mmol m<sup>−2</sup> a<sup>−1</sup>,
equivalent to ~12±5% of the previously estimated
annual nitrogen requirement of new production and in the same order of
magnitude as atmospheric N deposition in this region. Based on this
analysis, NH<sub>4</sub><sup>+</sup> photoproduction makes a small, but significant
contribution to the nitrogen budget of the euphotic zone in the oligotrophic
Cyprus Gyre.
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