AME 26:295-304 (2002)  -  doi:10.3354/ame026295

ABSTRACT: Recent studies have shown that in a number of humic-rich surface waters in North America, NH₄⁺ is released when dissolved organic matter (DOM) is exposed to sunlight. However, photochemical NH₄⁺ production has not been observed in all surface waters, and factors that contribute to it are not well understood. We hypothesized that the presence or absence of NH₄⁺ photoproduction may be affected by the light exposure history of DOM. The present study was undertaken to determine whether DOM from surficial groundwaters, with minimal light exposure history, would produce labile nitrogen (N) photoproducts more consistently. In this study, estuarine surface waters and surficial groundwaters, collected just adjacent to estuaries in Georgia and South Carolina, USA, were exposed to sunlight to quantify the photochemical production of NH₄⁺, dissolved primary amines (DPA), and NO₂^-. The photoproduction of NH₄⁺ was observed in 4 of 5 irradiated estuarine surface water samples but in only 2 of 13 groundwater samples. In contrast, NH₄⁺ concentrations decreased in 5 of 13 groundwater samples when exposed to sunlight. The results indicate that a small amount of NH₄⁺ may be lost from waters in which groundwater-derived DOM is first exposed to sunlight. No consistent trends were observed in the photoproduction or loss of DPA and NO₂^-.

KEY WORDS: UV radiation · Dissolved organic nitrogen · Groundwater · Ammonium · Dissolved primary amines · Nitrite