The University of Maine Maine In-situ Sound & Color Lab
School of Marine Sciences
The Role of Seagrasses as Sources of CDOM to Coastal Oligotrophic Waters

To examine CDOM sources to tropical oligotrophic coastal waters, we conducted a time series study using a moored AC-9, CTD and current meter placed at a site (7-8 m water depth) on the shallow water Bahama Banks. An examination of the data in this 30 hr time series indicated that there were large changes in CDOM absorbance over the tidal cycle, with high dissolved CDOM absorbance being associated with the high salinity waters that originate from the shallow banks. Combining these results with current measurements indicated that there was a net flux of CDOM off the shallow banks to the deeper waters of Exuma Sound. The CDOM spectral slope also varied with the tidal cycle implying that there were differences in the type of CDOM (as well as its absolute amount) being exchanged between deep waters and the shallow banks. Both CDOM absorbance and total DOC showed a positive relationship with salinity, which is opposite of that seen in most coastal systems, in which high DOC and CDOM concentrations are generally associated with low salinity waters of terrestrial origin. We believe that seagrasses and seagrass sediments represent the most likely sources of this CDOM. Seagrasses are important primary producers in oligotrophic tropical coastal waters and generate a significant amount of the particulate organic matter that settles into the sediments. They also promote the deposition of allocthonous organic matter to seagrass sediments as a result of the leaf canopy altering (dampening) water motion and tidal currents. Consistent with these suggestions, DOC and CDOM levels were significantly higher in the sediment pore waters of vegetated areas versus adjacent bare (ooid) sands. Therefore, we believe that processes associated with seagrass biogeochemistry and decomposition likely represent the predominant mechanisms by which CDOM is input to such oligotrophic tropical coastal waters that experience little or no terrestrial freshwater input.

Burdige, D., E. Boss, and J.R.V. Zaneveld, 2002. The Role of Seagrasses as Sources of CDOM to Coastal Oligotrophic Waters. Presented at AGU-ASLO, Hawaii, Feb.

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