Transformation and fate of microphytobenthos carbon in subtropical, intertidal sediments: potential for long-term carbon retention revealed by 13C-labeling
Microphytobenthos (MPB) are ubiquitous in coastal sediments, but the fate of their production (carbon biomass) is poorly defined. The processing and fate of MPB-derived carbon in subtropical intertidal sediments was investigated through in situ labeling with 13C-bicarbonate. Of the added 13C, 100% was fixed within ~ 4 h, suggesting that MPB productivity was limited by inorganic carbon availability. Although there was rapid transfer of 13C to bacteria (within 12 h), a relatively small fraction of 13C was transferred to heterotrophs (up to 12.5% of total fixed 13C into bacteria and 0.01% into foraminifera). MPB was the major reservoir for 13C throughout the study, suggesting that production of extracellular polymeric substances was limited and/or MPB recycled 13C. This retention of 13C was reflected in remarkably slow estimated turnover times for the MPB community (66–100 d). Over 31 d, ~ 70% of the 13C was lost from sediments. This was primarily via resuspension (~ 55%), enhanced by elevated freshwater flow following rainfall. A further ~ 13% was lost via fluxes of dissolved inorganic carbon during inundation. However, 13C losses via dissolved organic carbon fluxes from inundated sediments (0.5%) and carbon dioxide fluxes from exposed sediments (<0.1%) were minimal. The retention of ~ 30% of the carbon fixed by MPB within one tidal exposure after > 30 d, despite high resuspension, demonstrates the potentially substantial longer term retention of MPB-derived carbon in unvegetated sediments and suggests that MPB may contribute to carbon burial ("blue carbon").