Production of chromophoric dissolved organic matter from mangrove leaf litter and floating Sargassum colonies

Abstract

Chromophoric dissolved organic matter (CDOM) strongly absorbs solar radiation in the blue-green and serves as the primary attenuator of water column ultraviolet radiation (UV-R). CDOM interferes with remote sensing of ocean chlorophyll and can control UV-R-induced damage to light-sensitive organisms including corals. We used laboratory incubations to evaluate CDOM production from senescing Rhizophora mangle (red mangrove) leaf litter (yellow, orange, and brown) and floating Sargassum colonies. Mangroves exist at the land–ocean interface near coral reefs in sub-tropical and tropical regions while floating Sargassum colonies tend to congregate in sub-tropical ocean gyres. CDOM production (∼ 48 h) from mangrove leaves collected during a dry period in June 2004 (0.17 ± 0.11 m − 1 g − 1 l h − 1) was lower than production from leaves collected during a wet period in September 2003 (0.57 ± 0.42 m − 1 g − 1 l h − 1) suggesting that CDOM production from leaf litter fluctuates in response to environmental factors. CDOM production was greatest for the mid-senescence orange leaves and lowest for the severely senesced brown leaves in both experiments. Along the sub-tropical Florida Keys coral reef ecosystem, the primary source of CDOM is discharge from the shallow seagrass-dominated Florida Bay as evidenced by a strong correlation between field CDOM measurements and previously reported Florida Bay discharge volumes. However, field observations provide evidence that large expanses of red mangroves throughout the Keys could be important CDOM sources to the region's coral reefs during periods of reduced Florida Bay discharge. Floating Sargassum colonies also readily produced CDOM in laboratory incubations, but at much more variable rates than mangrove leaves. However, our calculations indicate that large mats of floating Sargassum could provide important CDOM quantities to oligotrophic oceanic waters including the Gulf of Mexico and North Atlantic.