<p>Seagrasses are often considered important players in the global carbon cycle, due to their role in sequestering and protecting sedimentary organic matter as &#8220;Blue Carbon&#8221;.&#160; However, in shallow calcifying systems the ultimate role of seagrass meadows as a sink or source of atmospheric CO<sub>2</sub> is complicated by carbonate precipitation and dissolution processes, which produce and consume CO<sub>2</sub>, respectively.&#160; In general, microbial sulfate, iron, and nitrate reduction produce total alkalinity (TA), and the reverse reaction, the re-oxidation of the reduced species, consumes TA. Therefore, net production of TA only occurs when these reduced species are protected from re-oxidation, for example through the burial of FeS<sub>x</sub> or the escape of N<sub>2</sub>.&#160; Seagrasses also affect benthic biogeochemistry by pumping O2 into the rhizosphere, which for example may allow for direct H2S oxidation.</p><p>Our study investigated the role of these factors and processes (seagrass density, sediment biogeochemistry, carbonate precipitation/dissolution, and ultimately air-sea CO<sub>2</sub> exchange), on CO<sub>2</sub> source-sink behavior in a shallow calcifying (carbonate content ~90%) seagrass meadow (Florida Bay, USA), dominated by Thalassia testudinum. We collected sediment cores from high and low seagrass density areas for flow through core incubations (N<sub>2</sub>, O<sub>2</sub>, DI<sup>13</sup>C, sulfide, DO<sup>13</sup>C flux), solid phase chemistry (metals, PO<sup>13</sup>C, Ca<sup>13</sup>C<sup>18</sup>O<sub>3</sub>, AVS: FeS + H<sub>2</sub>S, CRS: FeS<sub>2</sub> + S<sup>0</sup>), and porewater chemistry (major cations, DI<sup>13</sup>C, sulfide, <sup>34</sup>S<sup>18</sup>O<sub>4</sub>). An exciting aspect of this study is that it was conducted inside the footprint of an Eddy Covariance tower (air-sea CO<sub>2</sub> exchange), allowing us to directly link benthic processes with CO<sub>2</sub> sink-source dynamics.</p><p>During the course of our week long study, the seagrass meadow was a consistent source of CO<sub>2</sub> to the atmosphere (610 &#177; 990 &#181;mol&#183;m<sup>-2</sup>&#183;hr<sup>-1</sup>).&#160; Elevated porewater DIC near 15 cmbsf suggests rhizosphere O<sub>2</sub> induced carbonate dissolution, while consumption of DIC in the top 5-10 cm suggests reprecipitation.&#160; With high seagrass density, enriched &#948;<sup>13</sup>C<sub>DIC </sub>in the DIC maximum zone (10-25 cm) suggests continual reworking of the carbonates through dissolution/precipitation processes towards more stable PIC, indicating that seagrasses can promote long-term stability of PIC.&#160; We constructed a simple elemental budget, which suggests that net alkalinity consumption by ecosystem calcification explains >95% of the observed CO<sub>2</sub> emissions.&#160; Net alkalinity production through net denitrification (and loss of N<sub>2</sub>) and net sulfate reduction (and subsequent burial of FeS<sub>2</sub> + S<sup>0</sup>), as well as observed organic carbon burial, could only minimally offset ecosystem calcification. &#160;&#160;</p>
Total alkalinity production in a mangrove ecosystem reveals an overlooked Blue Carbon component