Exploring carbon dynamics in connected mangrove forests and seagrass beds: How important is it?

Abstract

Carbon sequestration is one of the most important ecosystem services provided by mangrove forests and seagrass beds that helps with climate change adaptation. Mangrove forests and seagrass beds are important ecosystems in tropical and subtropical locations that sequester significant greater amounts of carbon within their living biomass as well as in their sediments compared to terrestrial ecosystems. Ecosystems that exchange energy, organisms and materials are considered connected. This dissertation, however, will focus on connectivity as the specific exchange of particulate organic matter (POM). Connectivity across coastal vegetated ecosystems could have an important impact for ecosystem services across the seascape. Although mangrove forests and seagrass beds usually occur adjacent to each other they are frequently evaluated independently without taking into account how connectivity between these coastal vegetated ecosystems can influence carbon accumulation. Therefore, a comparison of connected with isolated mangrove forests and seagrass beds will help to understand the effect of connectivity on carbon accumulation at the seascape scale. Five field studies in Singapore (Asia), Adelaide (Oceania), Zanzibar (Africa), Florida and Bonaire (Americas), were chosen to evaluate the influence of connectivity. Sampling for sediment carbon content analysis was conducted across all places in different locations with connected mangrove forests and seagrass beds and/or isolated ecosystems. The general hypothesis was that mangrove forests and seagrass beds that are connected with each other would have greater quantities of carbon in the sediment, compared to those that are isolated from each other. Furthermore, other aspects such as connectivity with other ecosystems (i.e. salt marsh, macroalgal beds), community characteristics, sediment nutrient concentration and geomorphic settings were tested and compared separately in each place. Results of this dissertation in Singapore and Zanzibar showed that no significant differences were observed between mangrove forests connected with seagrasses beds and mangrove forests isolated from seagrass beds. Connectivity between mangrove forests and seagrass beds for carbon accumulation only increased sediment carbon accumulation in South Australia´s mangrove forests, where the sediment carbon content was higher in connected forests compared with an isolated mangrove patch. In general, sediment in seagrass beds have higher inorganic carbon indicating that mangrove forests are not the main allochthonous carbon source. Contrary to the general hypothesis, isolated seagrass beds had up to double sediment carbon content compared with connected beds. This was mainly driven by higher amounts of inorganic carbon in isolated beds since sediment organic carbon quantity was similar across the sampled beds. Thus, autochthonous production of inorganic carbon and connectivity with carbonate rich ecosystems (i.e. coral reefs) could be important for seagrass beds carbon accumulation. Although connected seagrass beds did not have higher sediment carbon content compared with isolated beds, results of this dissertation showed that mangrove forests are crucial donors of organic carbon to adjacent ecosystems such as seagrass beds and mud flats. The contributions to POM in the coastal water body of the main primary producers across all coastal vegetated ecosystems were measured in Singapore and South Australia. In both places, mangrove trees contributed between 10 to 70% and significantly more compared with other ecosystems with respect to the area occupied in the seascape by mangrove forests. Additionally, in Singapore and Bonaire it was observed that the export of dissolved organic carbon is an important pathway of exchanging carbon from mangroves forests to adjacent ecosystems. Besides mangrove trees, macroalgae thalli were with 10-50% the main contributors to the POM stocks in Singapore and South Australia. These results highlight the importance of mangrove forests as well as other coastal vegetated ecosystems, such as macroalgal beds, for carbon exchange and potential carbon accumulation across the seascape. Interactions between community composition and environmental parameters (i.e. water particle transportation or sediment nutrient concentration) could enhance the sediment carbon content. In Zanzibar, although functional diversity indices were not associated with higher sediment organic or inorganic carbon content, community species composition associated with prop roots in mangrove forests and large-leaved seagrass plants were correlated with higher sediment carbon content. In both ecosystems the area of tidal channels was also an important predictor of sediment carbon content although there were differences in the influence on sediment organic and inorganic carbon content. Additionally, in Florida the gradient of physical traits of mangrove roots and seagrass plants were compared against different carbon content type (organic and carbonate). A negative correlation was found between the sediment organic carbon content and the root complexity index, whilst higher sediment organic carbon levels were found in areas with higher shoot density and coverage. Besides connectivity other factors at the seascape level such us geomorphic settings or anthropogenic induced changes in the seascape settings could influence the connectivity and the sediment carbon content. In Zanzibar the area of tidal channels as well as the area covered by mangrove forest were positively correlated with sediment organic carbon content in mangrove forests. Anthropogenic induced changes such as changing freshwater fluxes and reduction of mangrove forest area in Singapore as well as reduction of area of healthy mangrove trees in Bonaire could affected carbon accumulation dynamics not only in mangrove forests but also in adjacent coastal ecosystems such as seagrass beds. In both places the same underlying mechanism was discussed, the reduction of mangrove forest area could reduce the exported amount of POM and decrease the amount of carbon accumulated in the adjacent ecosystems. I conclude that connectivity between mangrove forests and seagrass beds is affected by different localized factors and that degrees of connectedness between those two ecosystems and other adjacent ecosystems should be further study. Factors such as mangrove forests cover area, community composition and health state of mangrove communities influence both the sediment carbon accumulation in mangrove forests and their exportation of carbon to adjacent ecosystems. Additionally, including sediment inorganic carbon content in blue carbon studies concerning seagrass beds is critical, since sediment inorganic carbon content was an important component of their sediment carbon content. Geomorphic settings such as presence of low energy streams or rivers as well as presence of macroalgal beds can also enhance sediment carbon accumulation in mangrove forests and seagrass beds. Results of this dissertation in connectivity between different coastal vegetated ecosystems as well as terrestrial ecosystems highlight the importance of the conservation of all interconnected ecosystems (i.e. mangrove forests, seagrass beds, salt marches, macroalgae beds, coral reefs as well as connected terrestrial ecosystems) for carbon accumulation ecosystem service.