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

Abstract

Abstract. We investigated the photoproduction of ammonium (NHPhotoammonification 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 −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 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 nitrogen requirement of new production and in the same order of magnitude as atmospheric N deposition 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.

Sensitivity analysis of CDOM spectral slope in artificial and natural samples: an application in the central eastern Mediterranean Basin

“…The minimum, maximum, average and standard deviation for all a 300 measurements were 0.17, 0.61, and 0.3 ± 0.2 m -1 , respectively. Values of a 300 were similar to those reported for the pelagic region of Atlantic Ocean and were slightly higher than values from the Cyprus gyre region (Kitidis et al 2006b). Absorption at 325 nm (minimum, maximum, average and standard deviation of 0.08, 0.40, and 0.20 ± 0.07 m -1 ) was significantly higher than values reported by Nelson et al (2007) Spectral slope values, calculated at wavelengths between 270 and 400 nm, were between 0.026 and 0.042 nm -1 with an average value of 0.033 ± 0.004 nm -1 .…”

“…In the Mediterranean Sea, only limited CDOM data are available (Seritti et al 1998;Babin et al 2003;Vignudelli et al 2004;Kitidis et al 2006b) and the bio-optical properties of CDOM are not well understood. The Eastern Mediterranean is dominated by ultra-oligotrophic waters and has a higher coastline/volume ratio than most ocean waters.…”

Global oceanic emission of ammonia: Constraints from seawater and atmospheric observations

“…Zhang and Anastasio [2003] also reported production of NH + 4 from the photolysis of aerosol organic nitrogen, which may involve deamination of amino acids [Milne and Zika, 1993]. In seawater, the production of NH x (sw) from the photolysis of DON has been demonstrated in many regions of the ocean [Bushaw et al, 1996;Morell and Corredor, 2001;Kitidis et al, 2006;Aarnos et al, 2012;Xie et al, 2012;Rain-Franco et al, 2014] [e.g., Norman and Leck, 2005] in different seasons are needed to further our understanding of the budget of atmospheric reduced nitrogen in the marine atmosphere.…”

Photochemical mineralization of dissolved organic nitrogen to ammonia in prairie lakes

“…For example, the greatest rates of photoammonification in a boreal pond were observed in irradiated water collected shortly after spring thaw (Bushaw et al, 1996). Kitidis et al (2006) observed that photoammonification rates were significantly correlated with CDOM concentrations (as measured by absorbance at 300 nm normalized to DOC), and that photochemical NH 3 production decreased as photobleaching progressed. These results suggest that photoammonification may be greatest for CDOM with little prior exposure to sunlight (Bushaw et al, 1996).…”