Composition, production, and loss of carbohydrates in subtropical shallow subtidal sandy sediments: Rapid processing and long-term retention revealed by 13C-labeling

The composition and production of carbohydrates (mannose, rhamnose, fucose, galactose, glucose, and xylose) and their transfer among sediment compartments (microphytobenthos [MPB], bacteria, and detritus) was investigated through in situ labeling with C-13-bicarbonate. After 60 h, C-13 was found in all sediment compartments, demonstrating rapid transfer of fixed carbon from autotrophs to heterotrophs. Carbohydrates were a major carbon reservoir, accounting for 30% (day 0) to 15% (day 30) of the C-13 within sediments, and probably played a role in this transfer. Carbohydrate fractions were highly reactive (65-87%), less reactive (7-18%), and nonreactive (6-23%) over the experimental period. The rate of loss of the less reactive fraction (0.01-0.05 d(-1)) was at least an order of magnitude lower than that for the highly reactive fraction (0.8-4.4 d(-1)). Patterns of diagenesis estimated from label uptake and loss matched the carbohydrate composition observed in the sediment (glucose > galactose > rhamnose > fucose > xylose > mannose) and were similar to patterns reported previously. C:N ratios and delta C-13 of sediment organic matter indicated an algal origin (MPB and phytoplankton). Although carbon was rapidly processed, loss from sediments was not immediate, and there was evidence of recycling into MPB and bacteria. Rapid transfer of carbon to and from carbohydrates has been found in various environments, including temperate, muddy, and intertidal sediments, and this study demonstrates the important role of carbohydrates in supporting heterotrophic production over extended periods (> 30 d) in subtropical shallow subtidal sands.