When &#8220;Blue Carbon&#8221; turns white: Isotopic evidence of calcification-driven CO2 emissions in a carbonate seagrass meadow
Authors: Zeller Mary , Van Dam Bryce , Lopes Christian , Smyth Ashley , Böttcher Michael , Osburn Christopher , Zimmerman Tristan , Pröfrock Daniel , Fourqurean James , Thomas Helmuth 
Published: March 2021

&lt;p&gt;Seagrasses are often considered important players in the global carbon cycle, due to their role in sequestering and protecting sedimentary organic matter as &amp;#8220;Blue Carbon&amp;#8221;.&amp;#160; However, in shallow calcifying systems the ultimate role of seagrass meadows as a sink or source of atmospheric CO&lt;sub&gt;2&lt;/sub&gt; is complicated by carbonate precipitation and dissolution processes, which produce and consume CO&lt;sub&gt;2&lt;/sub&gt;, respectively.&amp;#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&lt;sub&gt;x&lt;/sub&gt; or the escape of N&lt;sub&gt;2&lt;/sub&gt;.&amp;#160; Seagrasses also affect benthic biogeochemistry by pumping O2 into the rhizosphere, which for example may allow for direct H2S oxidation.&lt;/p&gt;&lt;p&gt;Our study investigated the role of these factors and processes (seagrass density, sediment biogeochemistry, carbonate precipitation/dissolution, and ultimately air-sea CO&lt;sub&gt;2&lt;/sub&gt; exchange), on CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt;, O&lt;sub&gt;2&lt;/sub&gt;, DI&lt;sup&gt;13&lt;/sup&gt;C, sulfide, DO&lt;sup&gt;13&lt;/sup&gt;C flux), solid phase chemistry (metals, PO&lt;sup&gt;13&lt;/sup&gt;C, Ca&lt;sup&gt;13&lt;/sup&gt;C&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;3&lt;/sub&gt;, AVS: FeS + H&lt;sub&gt;2&lt;/sub&gt;S, CRS: FeS&lt;sub&gt;2&lt;/sub&gt; + S&lt;sup&gt;0&lt;/sup&gt;), and porewater chemistry (major cations, DI&lt;sup&gt;13&lt;/sup&gt;C, sulfide, &lt;sup&gt;34&lt;/sup&gt;S&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;4&lt;/sub&gt;). An exciting aspect of this study is that it was conducted inside the footprint of an Eddy Covariance tower (air-sea CO&lt;sub&gt;2&lt;/sub&gt; exchange), allowing us to directly link benthic processes with CO&lt;sub&gt;2&lt;/sub&gt; sink-source dynamics.&lt;/p&gt;&lt;p&gt;During the course of our week long study, the seagrass meadow was a consistent source of CO&lt;sub&gt;2&lt;/sub&gt; to the atmosphere (610 &amp;#177; 990 &amp;#181;mol&amp;#183;m&lt;sup&gt;-2&lt;/sup&gt;&amp;#183;hr&lt;sup&gt;-1&lt;/sup&gt;).&amp;#160; Elevated porewater DIC near 15 cmbsf suggests rhizosphere O&lt;sub&gt;2&lt;/sub&gt; induced carbonate dissolution, while consumption of DIC in the top 5-10 cm suggests reprecipitation.&amp;#160; With high seagrass density, enriched &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;DIC &lt;/sub&gt;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.&amp;#160; We constructed a simple elemental budget, which suggests that net alkalinity consumption by ecosystem calcification explains &gt;95% of the observed CO&lt;sub&gt;2&lt;/sub&gt; emissions.&amp;#160; Net alkalinity production through net denitrification (and loss of N&lt;sub&gt;2&lt;/sub&gt;) and net sulfate reduction (and subsequent burial of FeS&lt;sub&gt;2&lt;/sub&gt; + S&lt;sup&gt;0&lt;/sup&gt;), as well as observed organic carbon burial, could only minimally offset ecosystem calcification. &amp;#160;&amp;#160;&lt;/p&gt;