## 3 Results and Discussion
### Variation in Net Ecosystem Production  
In the seaward fringing forest, 16% of GPP and 27% of scrub forests GPP could not be accounted for and thus is assumed to be NEP in the mass balance budget (Table 2). This contrasted with the P fertilized scrub forest where the budget was balanced within 5% (inputs of fixed C was totally accounted for in the outputs measured or estimated). Correspondingly, production to respiration (P:R) ratios in the seaward fringing forest were 1.2, 1.4 and 0.95 for the fringing, scrub and P fertilized scrub, respectively. The presence of well-developed surface microbial mats is an important feature of mangrove habitats contributing both to GPP [38, 39] and to reducing diffusion of respired CO\({}_{2}\) across the soil surface thereby leading to under estimations of soil respiration [47, 48, 68]. Experiments where surface films were removed (scraped off) indicated that soil respiration increased by approximately 60% compared to when the surface was intact [47, 68, 73]. Possibly as a consequence of this reduced rate of diffusion, very high concentrations of dissolved inorganic C and low pH (6.5) are observed in the soil porewater (16 mmol in scrub forests compared to 1.5 mmol in the fringe soils [32, 38]). These high levels of dissolved inorganic carbon (DIC) could be due to root respiration as well as decomposition of microbial (and algal) detritus and also decomposition of soil organic matter derived from mangrove tissues (Table 1). [PERSON] _et al._[74] estimated that porewater contributed a large component (18%-87%) of Creek water. The interior ponds at Twin Cays are known to drain through creeks [75]. Photographic evidence indicates drainage of dissolved coloured organic material from the mangrove islands via creeks and subterranean channels [76]. Additionally, the C isotopic signature (\(\delta^{13}\)C) of seagrass on the edges of the mangrove islands is 7 per mil less (\(-14\)) compared to seagrass associated with the nearby coral reef [77]. The low value (\(\delta^{13}\)C of \(-\)14) indicates that seagrass on the edge of the mangrove are using an additional DIC source that is more negative than that in the seawater and may be mangrove in origin [78]. Mangroves have a \(\delta^{13}\)C of approximately \(-\)26 to \(-\)29, while algae and cyanobacteria are substantially higher [79]. Thus, mangrove root respiration or decomposition of mangrove derived soil organic matter is likely to contribute to DIC exported from the scrub forests and interior ponds. Despite the potential errors using mass balance approaches (because of errors in large fluxes e.g., GPP and respiration [11]), our estimates of NEP were similar to that observed in other mangrove forest C budgets [3, 10, 11, 13]. NEP is proposed to be exported from the ecosystem as DIC rather than dissolved organic carbon (DOC), which is known to be low [59]. Measurements of NEP of mangroves in Florida using eddy-covariance techniques gave a value of 97.5 mol m\({}^{-2}\)-year\({}^{-1}\) of which they estimate 25%-70% of NEP is likely to be exported [80]. This is similar in magnitude to our estimates of NEP for fringing mangroves in our study (70 mol C m\({}^{-2}\)-year\({}^{-1}\)) but higher than that measured for scrub mangroves (35 mol C m\({}^{-2}\)-year\({}^{-1}\)). Our values of NEP for Twin Cays are slightly lower than values reported for Australian and Asian mangrove forests which range from 101-178 mol C m\({}^{-2}\)-year\({}^{-1}\)[11, 16] and globally, 118 mol C m\({}^{-2}\)-year\({}^{-1}\)[10]. [PERSON] _et al._[81] observed that DIC export from mangroves in subterranean groundwater flow was driven by tidal pumping. Despite the very low tidal range at Twin Cays (\(<\)50 cm), photographic evidence and observed \(\delta^{13}\)C of adjacent seagrass suggest that export of DIC from porewater is a plausible fate for the unaccounted for carbon at Twin Cays.