Hypersaline tidal flats as important blue carbon systems: a case study from three ecosystems

Hypersaline tidal flats (HTFs) are coastal ecosystems with freshwater deficits often occurring in arid or semiarid regions near mangrove supratidal zones with no major fluvial contributions. Here, we estimate that organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) were buried at rates averaging 21 (+/- 6), 1.7 (+/- 0.3) and 1.4 (+/- 0.3) gm(-2) yr(-1), respectively, during the previous century in three contrasting HTF systems, one in Brazil (eutrophic) and two in Australia (oligotrophic). Although these rates are lower than those from nearby mangrove, saltmarsh and seagrass systems, the importance of HTFs as sinks for OC, TN and TP may be significant given their extensive coverage. Despite the measured short-term variability between net air-saltpan CO2 influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal CO2 flux measurements, the OC sedimentary profiles over several decades suggest efficient OC burial at all sites. Indeed, the stable isotopes of OC and TN (delta C-13 and delta N-1(5)) along with C : N ratios show that microphytobenthos are the major source of the buried OC in these HTFs. Our findings highlight a previously unquantified carbon as well as a nutrient sink and suggest that coastal HTF ecosystems could be included in the emerging blue carbon framework.

Automated summary

Hypersaline tidal flats are important sinks for organic carbon, total nitrogen, and total phosphorus, with efficient organic carbon burial primarily sourced from microphytobenthos. Despite lower burial rates compared to other coastal ecosystems, their extensive coverage highlights their significance in the blue carbon framework.