Effects of sunlight on decomposition of estuarine dissolved organic C, N and P and bacterial metabolism

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 NH4 or PO4 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 (r2 = 0.38, p < 0.01) with in situ NH4 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.