We report on results of a long-term (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)

The effects of natural sunlight and microbial decomposition on DOC, DON, and DOP were investigated along the salinity gradient of a temperate coastal plain estuary. The impact of sunlight-irradiated DOM on bacterial properties (bacterial abundance, production, bacterial growth efficiency [BGE]) was also followed. Surface-water light levels resulted in no detectable abiotic production of NH 4 + or PO 4 3 -or loss of DOC. Bacterial decomposition of DOC was enhanced by 27 to 200% in irradiated relative to dark treatments. There was, however, no corresponding enhancement in DON and DOP remineralization. Significant differences in bacterial decomposition of light-exposed DOC were frequently observed following prolonged incubation (> 7 d), suggesting that enhanced reactivity may result from photochemical modification of higher molecular weight organic matter. BGE in light relative to dark treatments was positively correlated (r 2 = 0.38, p < 0.01) with in situ NH 4 + concentrations. In light treatments, significantly lower N and P remineralization in August 1999 corresponded with low in situ inorganic nutrient concentrations and bacterial growth efficiency (BGE) and with elevated bacterial DOC utilization. In contrast, enhanced DOC reactivity in April 2000 during nutrient-replete conditions corresponded with net immobilization of inorganic N and P by bacterial biomass production, but without a concomitant impact on BGE. These findings suggest that the combination of photochemical and microbial alteration of DOM may increase bacterial demand for inorganic nutrients, alter BGE, and influence the partitioning of C between bacterial biomass and respiration. KEY WORDS: DOC · DON · DOP · BGE · Phototransformation · Bacterial bioassays Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 40: [25][26][27][28][29][30][31][32][33][34][35] 2005 information available for estuarine DOM that is lower in vascular plant and soil influences and higher in contributions from younger (recently produced) or algalderived organic matter ' (Moran et al. 2000). The effects of sunlight-exposure on DOM cycling in estuaries are difficult to quantify. Variations in initial bioreactivity, age, sources and structural character (e.g. aromaticity) of DOM may affect its biological and photochemical fate during seaward transit (Mopper & Kieber 2002, Moran &Covert 2003 and references therein). In general, the balance between photoinhibition and photostimulation of DOM cycling may hinge on the initial reactivity of various subcomponents of DOM (Moran & Covert 2003). Thus, prediction of sunlight effects on DOM bioavailability and fate on a system scale is limited by the ability to characterize and predict the bioreactivity of the majority of organic compounds comprising the bulk DOM pool (Hedges et al. 2000). This has sometimes been circumvented by the use of microbial activity (e.g. bacterial abundance, growth and production) as a proxy for integrating both positive and negative photochemical imp...