Photobleaching kinetics of chromophoric dissolved organic matter derived from mangrove leaf litter and floating Sargassum colonies

Abstract

We examined the photoreactivity of chromophoric dissolved organic matter (CDOM) derived from Rhizophora mangle (red mangrove) leaf litter and floating Sargassum colonies as these marine plants can be important contributors to coastal and open ocean CDOM pools, respectively. Mangrove and Sargassum CDOM readily degraded when exposed to simulated solar irradiance (CPS SunTest solar simulator exposures). CDOM produced from brown mangrove leaves (representative of substantial senescence) exhibited shorter photobleaching half-lives ( a 305 t 1/2 < 50 h) than CDOM produced from yellow and orange (early and mid senescence) leaves ( a 305 t 1/2 ∼ 60–90 h). Mangrove CDOM photobleaching rates were higher in the mid-UVA ( a 350) than in the UVB ( a 305) spectral region. Photobleaching half-lives of Sargassum CDOM were mostly < 40 h and more consistent across UVB ( a 305) and UVA ( a 350) wavelengths. Sargassum CDOM photomineralized during simulated solar irradiation producing DIC at rates exceeding 2500 nmol m l – 1 h – 1 , indicating that regions of the surface ocean with large concentrations of this plant may provide a strong CO 2 source to the atmosphere. Sargassum CDOM photoreactions also produced CO more efficiently than terrestrial CDOM and much more efficiently than ambient CDOM in the open ocean. Thus, biological production of CDOM may be the rate-limiting step for photoproduction of DIC from Sargassum and other sources in the open ocean. An examination of CDOM photobleaching in Florida Keys coastal waters indicates that one month of summertime solar radiation may substantially increase UVB and UVA exposure to corals in shallow waters (< 4 m), especially along the offshore reef tract. But, our results also indicate that when ambient CDOM levels are high, the corals are well-buffered against increases in ultraviolet radiation (UV-R) exposure even after periods of extended CDOM photobleaching.