Transformation and fate of microphytobenthos carbon in subtropical, intertidal sediments: potential for long-term carbon retention revealed by 13C-labeling
- Centre for Coastal Biogeochemistry, Southern Cross University, Lismore, Australia
Abstract. 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").