## 9 Ecosystem Contributions to Coastal Blue Carbon Fluxes  
Within the global tropical coastal ocean, mangrove ecosystems contribute disproportionately to the blue carbon flow in relation to their small area (6% of all low-latitude habitats). Except for the global NPP, mangroves contribute between 4% and 28% to the total blue carbon flow (Table 4). The low contribution to the NPP is masked by the overwhelming global NPP flux of subtidal and intertidal macroalgae beds and floating macroalgae. Tropical and subtropical seagrass also contribute disproportionately to the low-latitude blue carbon flow due to their high rates of GPP and R\({}_{\rm E}\) (Table 4), whereas tropical salt marshes and tidal flats are small sources of blue carbon due mostly to their small area. Macroalgae and tidal flats have the highest P\({}_{\rm GPP}\)/R\({}_{\rm E}\) ratios, although all ecosystems are net autotrophic. Coral reefs are within the same order of magnitude as mangrove forests regarding their contribution to the global NEP (Table 4), but their inclusion as a blue carbon ecosystem is still debated [208]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Ecosystem** & **Area** & **R\({}_{\rm E}\)** & **Global R\({}_{\rm E}\)** & **GPP** & **Global GPP** & **Mean P\({}_{\rm GPP}\)/R\({}_{\rm E}\)** & **NEP** & **Global NEP** \\ \hline Mangroves & 14.5 & 1498 & 217 & 1971 & 286 & 1.3 & 473 & 69 \\ \hline Salt marshes & 0.8 & 192 & 10 & 234 & 16 & 1.5 & 42 & 6 \\ \hline Seagrasses & 18.1 & 2367 & 504 & 2853 & 608 & 1.2 & 486 & 103 \\ \hline Macroalgae & 158.7 & 368 & 583 & 1294 & 2054 & 3.5 & 926 & 1469 \\ \hline Tidal flats & 7.5 & 45 & 6 & 96 & 12 & 2.1 & 51 & 6 \\ \hline Coral reefs & 34.8 & 912 & 546 & 998 & 599 & 1.1 & 86 & 49 \\ \hline Totals & 234.4 & 5383 & 1866 & 7446 & 3575 & - & 2064 & 1702 \\ \hline Mangrove contribution & 6\% & 28\% & 12\% & 11\% & 8\% & - & 23\% & 4\% \\ \hline \end{tabular}
\end{table} 
Table 4: Contribution of mangroves to blue carbon fluxes compared with other coastal ecosystems in global low-latitude coastal oceans. Global areas of seagrass [209], macroalgae [210], mangroves [211], tidal flats [212], salt marshes [213], and coral reefs [214]. Percentage contributions of mangroves are based on total fluxes for each ecosystem. Macroalgae data are from [215; 216]. All other data are from [207; 214; 215; 216]. Abbreviations and units: R\({}_{\rm E}\) = (g C m\({}^{-2}\) y\({}^{-1}\)) = ecosystem respiration (respiration of canopy, sediment, tidal water, microalgae, and macroalgae); GPP (g C m\({}^{-2}\) y\({}^{-1}\)) = gross primary production; and NEP (g C m\({}^{-2}\) y\({}^{-1}\)) = net ecosystem production (=P\({}_{\rm GPP}\) - R\({}_{\rm E}\)). Units: area (10\({}^{10}\) m\({}^{2}\)), global R\({}_{\rm E}\), global GPP, and global NEP (Tg C y\({}^{-1}\)). \begin{table}
\begin{tabular}{c c c c c} \hline
**Component** & \multicolumn{2}{c}{**Inputs**} & \multirow{2}{*}{**Component**} & \multicolumn{2}{c}{**Outputs**} \\ \cline{2-3} \cline{5-5}  & **Original** & **Current** & & **Original** & **Current** \\ \hline Mangrove GPP & 20.0 & 19.7 & R\({}_{\rm CANOPY}\) & 8.0 & 14.8 \\ \hline Macroalgal GPP & 6.2 & 8.6 & R\({}_{\rm MACROALGE}\) & 2.9 & 4.0 \\ \hline Microalgal GPP & 4.4 & 5.5 & R\({}_{\rm MICROALGE}\) & 2.3 & 2.9 \\ \hline Sediment DIC production & 19.8 & 19.8 & R\({}_{\rm TIDALWATTER}\) & 3.3 & 3.3 \\ \hline Groundwater from terrestrial sources & 2.6 & 12.8 & R\({}_{\rm SEDIMENT}\) & 6.7 & 6.7 \\ \hline  & & & DIC\({}_{\rm CYORT}\) & 24.8 & 24.8 \\ \cline{3-5}  & & & DOC\({}_{\rm CYORT}\) & 3.8 & 3.8 \\ \cline{3-5}  & & & POC\({}_{\rm CYORT}\) & 1.9 & 1.9 \\ \cline{3-5}  & & & B\({}_{\rmerial}\) & 1.6 & 1.6 \\ \cline{3-5}  & & & CH\({}_{\rm A,AIR/SEA}\)\({}_{\rm CYORT}\) & 0.06 & 0.06 \\ \hline
**Total Net Flux** & 53.0 & 66.4 & 55.4 & 63.9 \\ \hline  & & & **Net Balance** & \(\bf{-2.4}\) & \(\bf{2.5}\) \\ \hline \end{tabular}
\end{table}