Photochemical and microbial alterations of DOM spectroscopic properties in the estuarine system Ria de Aveiro

Abstract

The influence of photochemical transformations of chromophoric dissolved organic matter (CDOM) on microbial communities was evaluated in the estuarine system Ria de Aveiro. Two sites, representative of the marine and brackish water zones of the estuary, were surveyed regularly in order to determine seasonal and vertical profiles of variation of CDOM properties. Optical parameters of CDOM indicative of aromaticity and molecular weight were used to establish CDOM sources, and microbial abundance and activity was characterized. Additionally, microcosm experiments were performed in order to simulate photochemical reactions of CDOM and to evaluate microbial responses to light-induced changes in CDOM composition. The CDOM of the two estuarine zones showed different spectral characteristics, with significantly higher values of the specific ultra-violet absorbance at 254 nm (SUVA₂₅₄) (5.5 times) and of the absorption coefficient at 350 nm (a₃₅₀) (12 times) and lower S_R (S_275–295/S_350–400) ratio at brackish water compared with the marine zone, reflecting the different amounts and prevailing sources of organic matter, as well as distinct riverine and oceanic influences. At the marine zone, the abundance of bacteria and the activity of Leu-AMPase correlated with a₃₅₀ and a₂₅₄, suggesting a microbial contribution to the HMW CDOM pool. The irradiation of DOM resulted in a decrease of the values of a₂₅₄ and a₃₅₀ and an increase of the slope S_275–295 and of the ratios E₂ : E₃ (a₂₅₀/a₃₆₅) and S_R, which in turn increase its bioavailability. However, the extent of photoinduced transformations and microbial responses was dependent on the initial optical characteristics of CDOM. In Ria de Aveiro both photochemical and microbial processes yielded optical changes in CDOM and the overall results of these combined processes determine the fate of CDOM in the estuarine system and have an influence on local productivity and in adjacent coastal areas.