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

<p><span id="page2528"/>Hypersaline tidal flats (HTFs) are coastal ecosystems with freshwater deficits often occurring in arid or semi-arid 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 (<span class="inline-formula">±6</span>), 1.7 (<span class="inline-formula">±0.3</span>) and 1.4 (<span class="inline-formula">±0.3</span>) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">g</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="d56bb1a870d4b5859c1aa633400a5bee"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2527-2021-ie00001.svg" width="50pt" height="15pt" src="bg-18-2527-2021-ie00001.png"/></svg:svg></span></span>, 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 <span class="inline-formula">CO₂</span> influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal <span class="inline-formula">CO₂</span> flux measurements, the OC sedimentary profiles over several decades suggest efficient OC burial at all sites. Indeed, the stable isotopes of OC and TN (<span class="inline-formula"><i>δ</i>¹³C</span> and <span class="inline-formula"><i>δ</i>¹⁵N</span>) along with <span class="inline-formula">C:N</span> 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.</p>