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The cycling of dissolved organic matter (DOM) and its significance to ecosystem metabolism was studled over a 16 mo period in a Thalassia testudjnum dominated meadow. The benthos was usually net autotrophic (annual gross primary production to respiration ratio [P:R] = 1.3) while water column respiration (R) exceeded gross primary production (annual P:R = 0.3). Net fluxes of dissolved organic carbon (DOC) from the benthos primarily occurred in the light (0 to 18 mmol C m-2 d-1) and from seagrass-dominated areas, suggesting that release of DOC was mainly due to seagrass exudation. Net benthic DOC fluxes measured in the light were significantly correlated (p < 0.0001, n = 61) with 'benthic net primary production (NPP). Average daily benthic NPP was significantly correlated to water column R (p < 0.002, n = 7) and appeared to explain about 88% of the variability in daily water column R. Estimates of bacterioplankton growth efficiencies ranged from 21 to 38 %, with peaks corresponding to maximal benthic DOC fluxes in spring and summer. Bacterioplankton were responsible for the remineralization of most (>50%) of the DOC released from the benthos on a daily basis. Annual estimates of bacterioplankton C demand, based on water column R (~8 mol C m-2 yr-1), represented >50% of the benthic NPP (~14 mol C m m-2 yr-1). These measurements indicate a stronger linkage between benthic and water column processes than previously believed, and it appears that water column heterotrophic processes are largely dependent upon seagrass exudation.

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