# Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests
## 3 Climate Change Impacts
### Rising Atmospheric CO\({}_{2}\)  
Mean atmospheric CO\({}_{2}\) concentrations have increased from 326 ppm to 416 mm over the past 50 years and are still rising [49]. Elevated CO\({}_{2}\) levels enhance mangrove photosynthesis, growth, respiration, and leaf chlorophyll concentrations with responses being species-specific and complex due to interactive effects with other drivers, namely salinity, temperature, nutrient availability, and water-use efficiency [9; 52]. Most mangrove species will respond positively to increasing CO\({}_{2}\) concentrations, but some species will respond negatively or will exhibit no or little change. For instance, growth of _Avicemnia germinans_ seedlings in greenhouse experiments was enhanced only under elevated CO\({}_{2}\) and high nutrient conditions, but root growth doubled under low nutrient and elevated CO\({}_{2}\) conditions [76]. Other species similarly exhibit complex responses when subjected to elevated CO\({}_{2}\) concentrations with different amounts or rates of other physicochemical factors. For example, when subjected to ambient CO\({}_{2}\) and a temperature of 38 \({}^{\circ}\)C, _Rhi-zophora apiculata_ seedlings responded positively to the higher temperature but elevated CO\({}_{2}\) enhanced growth only at a lower temperature; under high temperature and elevated CO\({}_{2}\), the seedlings nearly died [77].  
Confounding responses to elevated CO\({}_{2}\) when subjected to co-drivers have been observed in other species such as _A. marina_, _R. stylosa_, _Kandelia obovata_, and _Aegiceras comriculatum_[78; 79; 80]. Net photosynthetic rates and water-use efficiency of _A. marina_ and _R. stylosa_ increased when grown under elevated (800 ppm) CO\({}_{2}\) and increasing tidal flooding to simulate sea-level rise [78], but these gains were minimal under prolonged flooding conditions. Growth of _A. comriculatum_ and _A. marina_ seedlings within a model salt marsh community under increasing CO\({}_{2}\) and low salinity resulted in both mangrove species outcompeting the salt marsh plants, with _A. marina_ producing 48% more biomass (and thus more carbon) under elevated CO\({}_{2}\)[79]. However, _A. comriculatum_ was unaffected by higher CO\({}_{2}\) but had 36% more growth than the salt marsh plants when grown under full seawater. Increased CO\({}_{2}\) can alter species composition and carbon metabolism of the root microbiome [80]. Leaf chlorophyll levels of _K. obovata_ and microbial root biomass increased over 20 weeks with elevated CO\({}_{2}\) with a shift in composition of ammonia-oxidizing archaea. There was also a shift in carbon metabolism with a shift in carbon utilization from sugars, amino acids, and carboxylic acids under ambient conditions to use of additional substrates in the order of amino acids > carbohydrates > polymers > carboxylic acids > amines > phenolic acids [80]. In summary, while elevated CO\({}_{2}\) in isolation results in increased carbon stocks and cycling in mangroves, the future reality is that other changes in climate must be considered important co-factors (e.g., SLR, rising temperatures) that are likely to alter the outcome of a positive mangrove response and make predictions concerning blue carbon more difficult; a case-by-case basis may be considered a viable solution.