# Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis
## 3 Primary Production and Plant Respiration  
Rates of gross and net primary production in salt marshes and mangrove forests are among the highest for aquatic ecosystems and are within the range of rates for terrestrial grasslands and humid forests [52]. Median rates of gross primary production (GPP), aboveground net primary production (AGNPP) and canopy (plant) respiration (\(\mathrm{R_{C}}\)) are greater for mangroves than salt marshes (Figure 2), although there is significant variation in rates reflecting species-specific differences in production among salt marsh and mangrove plants; nutrient status; wetland age; and other factors such as soil salinity, location, hydrology, intertidal position and temperature.  
\begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Mangrove Forests** & **Salt Marshes** \\ \hline Aboveground biomass (AGB) & 109.3 \(\pm\) 5.0 (94.1) & 4.3 \(\pm\) 0.10 (2.4) \\ Belowground biomass (BGB) & 80.9 \(\pm\) 9.5 (34.2) & 12.9 \(\pm\) 1.2 (9.6) \\ Soil (0-1 m depth) & 565.4 \(\pm\) 25.7 (500.5) & 317.2 \(\pm\) 19.1 (282.2) \\ Total C stock (Mg \(\mathrm{C_{OR}}\) ha\({}^{-1}\)) & 738.9 \(\pm\) 27.9 (702.5) & 334.4 \(\pm\) 3.5 (294.2) \\ Global area (km\({}^{2}\)) & 86,495 & 54,951 \\ Global C stock (PG \(\mathrm{C_{org}}\)) & 6.17 & 1.84 \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of mean (\(\pm\)1 standard error, SE) carbon stocks between salt marsh and mangrove ecosystems. Units = Mg \(\mathrm{C_{ORG}}\) ha\({}^{-1}\). Median values are in parentheses and in Figure 1. Mangrove data from [6]. Salt marsh g dry weight (DW) biomass was converted to g \(\mathrm{C_{ORG}}\) using percent \(\mathrm{C_{ORG}}\) content in roots + rhizomes (41.5 \(\pm\) 1.2%) and shoots (38.5 \(\pm\) 0.7%) of various species [7; 8; 9; 10; 11; 12; 13; 14; 15]. Salt marsh aboveground biomass data (from [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47] and earlier references within). Global mangrove area (86,495 km\({}^{2}\)) from [48] and salt marsh global area (54,951 km\({}^{2}\)) from [49].  
Figure 2: Median rates of gross (GPP) and aboveground net (AGNPP) primary production and plant respiration for mangroves (green bars) and salt marshes (blue bars). Vertical line in each box denotes the median (values presented in each box), and the boxes encompass the 25 th and 75 th percentiles and the outer bars denote the 5 th and 95 th percentiles. Data from Table 2.  
Rates of belowground root production (Table 2) are significantly greater in salt marshes (one-way ANOVA on ranks; \(p<0.05\)), although mangrove root production has most likely been greatly underestimated due to problems with methodology and the lack of empirical measurements [53]. Rates of benthic microalgal productivity in both salt marshes and mangroves (Table 2) are not significantly different (one-way ANOVA on ranks; \(p>0.05\)). P\({}_{\rm{GFP}}\)/R\({}_{\rm{C}}\) ratios are significantly greater in mangrove forests than in salt marshes (one-way ANOVA on ranks; \(p<0.001\)) and are equivalent to those estimated for tropical terrestrial forests [52].