Bioavailability and sources of DOC and DON in macrophyte stands of a tropical coastal lake

Dissolved organic carbon (DOC) and nitrogen (DON) derived from aquatic and terrestrial vascular plants provide a major energy and nutrient source for freshwater and coastal marine biota. The bioavailability of this material m ay to a large extent depend on plant species. In this study, we have compar ed the bioavailability of DOC and DON sampled in two distinct stands of Typ ha domingensis and Eleocharis mutata in a coastal tropical lake and in the adjacent ocean in the state of Rio de Janeiro, Brazil. Bioavailability of o rganic matter was assessed by regrowth bioassays using natural bacterial in ocula. Nutrients were added to achieve carbon or nitrogen limitation. At al l sampling sites, DON comprised over 95% of the total bioavailable nitrogen , suggesting its dominant role as a nitrogen source. The bioavailability of lacustrine DON (22% in the Typha stand and 34% in the Eleocharis stand) ex ceeded the bioavailability of DOC (8 and 10%, respectively) and exhibited a larger difference between the stands. H-3-leucine incorporation studies sh owed that lake bacterioplankton had a well balanced supply of C, N and P. T herefore, an accumulation of labile DON due to an excess nitrogen supply is not probable. We propose that a substantial part of the lake DON was newly formed within the macrophyte stands, while DOC was predominantly of terres trial origin and more diagenetically changed.