Ecological connectivity of seagrasses with mangroves increases the carbon storage of tropical seagrass meadows of an island ecosystem


 Ecologically connected ecosystems are considered more resilient to climate change mitigation by storing increased amounts of carbon than individual ecosystems. This study quantified the carbon storage capacity of seagrass (Thalassia hemprichii) meadows that are adjacent to mangroves (MG; Rhizophora apiculate) and without mangroves (WMG) at three locations in tropical Andaman and Nicobar Islands (ANI) of India. The sediment organic matter (OM) carbon (Corg) content was 2-fold higher at the MG sites than WMG sites of all three locations within the top 10 cm. The Corg in the total biomass was higher at MG sites than the biomass at WMG sites. The sediment grain size positively influenced the sediment OM and Corg content. The canopy height of T. hemprichii showed a better relationship with sediment OM and Corg at MG sites. In contrast, the shoot density of T. hemprichii showed a better relationship with sediment OM and Corg at WMG sites. The total carbon in 144 ha of T. hemprichii meadows of all three MG sites was 11031± 5223 Mg C, whereas the carbon in 148 ha of WMG sites was 4921±3725 Mg C. These T. hemprichii meadows of ANI store around 40487±19171 ton of CO2 in the MG sites and 18036 ±13672 ton of CO2 at WMG sites. The social cost of these carbon stored in these T. hemprichii meadows is around US$ 34.82 and 1.5 million at the MG and WMG sites, respectively. This study points out the efficiency of seagrass ecosystems of ANI as carbon sinks and the potential of these connected seascapes in increasing the efficiency of seagrass carbon storage. Therefore, this connectivity approach should be further explored to include these connected ecosystems of India as a nature-based solution for climate change mitigation and adaptation plans.


Introduction
Seagrasses are keystone coastal ecosystems that act as natural carbon sinks by sequestering 37 atmospheric CO2 and storing it in their sediments. Together with saltmarsh and mangroves, 38 seagrass ecosystems account for the storage of 50% of oceanic organic carbon in their 39 sediments (Duarte et al. 2005). Blue carbon refers to the organic carbon (Corg) stored in these 40 ecosystems that has a high potential for offsetting global carbon emissions derived from  Considering the vast area of seagrass ecosystems in India, the present study aims to quantify 68 the carbon storage capacity of the Andaman and Nicobar Islands (ANI). The ANI in the 69 Andaman Sea is home to 13 of the 16 seagrass species found in India, covering 29.3 km2 70 (Jagtap et al., 2003;Ragavan et al., 2016). Thalassia hemprichii is one of the keystone seagrass 71 species of ANI found in the sandy intertidal habitats and amidst coral rubbles up to a depth of 72 15 m (Jagtap et al., 2003). Thalassia hemprichii is also found associated with other seagrass 73 species such as H. ovalis and C. rotundata or with mangrove ecosystems ( (Fig.1). These islands are rich in seagrass and mangroves due to their tropical settings.   Burmanallah is situated in the southeast region of ANI (Fig.1c). This location has rocky 123 intertidal beaches and human-made coastal concrete walls. The study sites with mangroves 124 have an outlet that discharges land run-off into the mangrove area. The T. hemprichii meadows   Table 2). The separated and dried biomass of 0.2 mg was combusted 134 using a CHNS analyser (Elementar, UNICUBE) to determine the total organic carbon (Corg).

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The biomass (g DWm -2 ) was converted to Mg C DW ha -1 after multiplying by the carbon 136 content (%C) in the above-ground and below-ground biomass. . We used the regional approach to estimate SCC, than the global approach, 175 as country level (regional) estimates allow better understanding of regional impacts and are

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The sediment dry bulk density of T. hemprichii meadows was significantly different (p<0.001) 200 between the three locations than among the MG and WMG sites of each location (Table 1). In 201 general, the dry bulk density was 1-fold higher among the T. hemprichii meadows associated 202 without mangroves (0.84 ± 0.13 g cm -3 ) than the meadows with mangroves (0.75±0.23 g cm -3 )

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In general, the carbon content in the sediment was lower than that of carbon in the biomass 230 of T. hemprichii meadows at both MG and WMG sites (  (Fig.4d). Among the locations, the MG sites of Neil Island stored 263 the highest CO2 equivalent, followed by Havelock and Burmanallah (Fig.4) (Table 1).

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The relationship between sediment silt content with organic matter/Corg was significant for 268 both MG and WMG sites, with higher significance at MG sites (Fig.5). The morphometrics of 269 T. hemprichii, such as canopy height and shoot density, positively influenced the sediment 270 organic matter/Corg content at both MG and WMG sites (Fig.6). The canopy height of T. 271 hemprichii showed a better relationship with sediment organic matter/Corg at MG sites of all 272 three locations (Fig.6a & b). In contrast, the shoot density of T. hemprichii showed better 273 relationships at WMG sites of all three locations (Fig.6c & d).    Barcelona et al. 2021). This mechanism is evident in our study, as 308 there is a better correlation between the canopy height of T. hemprichii with organic matter 309 13 content at MG sites (Fig.6). Similar correlations are observed for shoot density and organic 310 matter content in MG sites (Fig.6). However, it is essential to notice here that the T. 311 hemprichii meadows at the WMG sites exhibited better correlations of organic matter with 312 shoot density than the MG sites. This is probably because the WMG sites in our study locations 313 are exposed to high wave dynamics that make them prone to leaf breakage resulting in reduced 314 canopy height (Fig.7b, Mishra and Apte, 2020). As a result, the plant invests in better below 315 ground structures to withstand the wave dynamics, resulting in higher organic matter trapping.

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Secondly, the ecosystem matrix of Neil and Havelock islands are such that the T. 317 hemprichii meadows at WMG sites are exposed to high intertidal temperatures resulting in leaf 318 damage and leaf reddening (Fig.7a). Similar mechanisms of hydrodynamic setting and its

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The mean AB-biomass and BG-biomass at the MG and WMG sites of ANI were within the  Table 2). The differences in Corg in AB-and BG-biomass was more evident at the WMG sites

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(2.8-fold) than at the MG sites (1.5-fold) because the AB-biomass of the WMG sites were more 360 prone to breakage and loss than at MG sites. As compensation for this breakage of leaves, T. The total carbon stored in the seagrass ecosystem (sediment + biomass) at the MG and WMG 375 sites showed a high standard deviation (Table 1)  WMG sites at all three locations were different. For example, the total biomass of T. 378 hemprichii meadows was higher at the MG site of Neil Island than the other two locations, but 379 due to the small area, the total carbon in the ecosystem was on the lower side (Fig.4). Similarly, 380 with a smaller size at the WMG site, the carbon in the ecosystem was the lowest at Neil Island.

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Consequently, even though the biomass of T. hemprichii at Burmanallah MG and WMG sites 382 were lower than the other two locations, due to the higher meadow area, the meadows of T. 383 hemprichii had the second-highest carbon in the entire ecosystem (Fig.4)