Fate of macroalgae in benthic systems: Carbon and nitrogen cycling within the microbial community

High nutrient loading to coastal bays is often accompanied by the presence of bloom-forming macroalgae, which take up and sequester large amounts of C and N while growing. This pool is temporary, however, as nuisance macroalgae exhibit a bloom and die-off cycle, influencing the biogeochemical functioning of these systems in unknown ways. The objective of this study was to trace the C and N from senescing macroalgae into relevant sediment pools. A macroalgal die-off event was simulated by the addition of freeze-dried macroalgae (Gracilaria spp.), pre-labeled with stable isotopes (13C and 15N), to sediment mesocosms. The isotopes were traced into bulk sediments and partitioned into benthic microalgal (BMA) and bacterial biomass using microbial biomarkers to quantify the uptake and retention of macroalgal C and N. Bulk sediments took up label immediately following the die-off, and macroalgal C and N were retained in the sediments for at least 2 wk. Approximately 6 to 50% and 2 to 9% of macroalgal N and C, respectively, were incorporated into the sediments. Label from the macroalgae appeared in both bacterial and BMA biomarkers, suggesting that efficient shuttling of macroalgal C and N between these communities may serve as a mechanism for retention of macroalgal nutrients within the sediments