Photochemical production of ammonium in the oligotrophic Cyprus Gyre (Eastern Mediterranean)

V. Kitidis, G. Uher, R. C. Upstill-Goddard, R. F. C. Mantoura, G. Spyres, E. M. S. Woodward
2006 Biogeosciences Discussions  
We investigated the photoproduction of ammonium (NH + 4 ) 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 + 4 photoproduction (photoammonification) increased with time-integrated irradiance during the course of irradiations. Photoam-5 monification rates around local noon were 0.4-2.9 nmol L −1 h −1 . Normalised to time integrated irradiance, these rates were 0.9-3.8 pmol L −1 h −1 /(W m
more » ... mol L −1 h −1 /(W m −2 ) 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 10 results in decreased DOC-normalised light absorbance concurrent with decreased dissolved organic matter reactivity with regard to photochemical NH + 4 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 −2 a −1 , equivalent to ∼12±5% of the previously estimated annual nitro-15 gen requirement of new production in this region. Based on this analysis, NH + 4 photoproduction makes a small, but significant contribution to the nitrogen budget of the euphotic zone in the oligotrophic Cyprus Gyre. to riverine inorganic nitrogen export. Similarly, Morell and Corredor (2001) estimated 450 BGD 3, 449-474, 2006 EGU the contribution of NH + 4 photoproduction to the phytoplankton nitrogen (N) demand of the eastern Caribbean Sea at ∼50%. In contrast, Buffam and McGlathery (2003) found that the contribution of photoammonification to the N sources of a coastal lagoon was small compared to other inputs. These observations seem to suggest that the potential impact of photoammonification is highest in shelf waters and marginal seas. However, 5 the role of photochemistry in open ocean nutrient budgets has received little attention, even though NH + 4 photoproduction may represent a significant source of N for primary producers in N-depleted surface waters of the open ocean. Recent work provides growing evidence for significant photoammonification rates particularly in inland, estuarine and coastal waters with elevated DOM levels. Photoam-BGD 3, 449-474, 2006 Abstract 25 Krom et al. (2005b) and is briefly reviewed here. A 200 m deep, nutrient depleted 453 BGD 3, 449-474, 2006 Abstract BGD 3, 449-474, 2006 Abstract 25 456 BGD 3, 449-474, 2006 Abstract 25 457 BGD 3, 449-474, 2006 Abstract 3, 449-474, 2006 Abstract rates from a range of aquatic systems. The reported rate data ( Table 2) span several 459 BGD 3, 449-474, 2006 Abstract 3, 449-474, 2006 EGU inversely proportional to initial NH + 4 concentration, consistent with the incorporation of NH + 4 -N into DON as observed by Thorn and Mikita (1992) and Kieber et al. (1997) . The contrast between these previous studies and consistent NH + 4 photoproduction in all our samples presumably reflected DOM compositional variability related to DOM source, transformation history or differences in DOM and NH + 4 concentrations. 5 3, 449-474, 2006 Abstract 3, 449-474, 2006 Abstract 3, 449-474, 2006 Abstract 464 BGD 3, 449-474, 2006
doi:10.5194/bgd-3-449-2006 fatcat:hccjo26wardifjyzkpvy2xdzla