Relative Influence of Salinity in the Flow and Accumulation of Organic Carbon in Open-Water Karstic Mangroves

Carbonat—open-water mangroves have high organic carbon (OC) content, apparently due to sediments’ biophysical characteristics. However, the role of key regulators such as salinity and hydroperiod, which modulate the forest structure and, therefore, carbon dynamics, has been little explored. This study evaluates the influence of salinity on the accumulation of aerial and underground OC (production of litter and roots), in open—water karstic forests. To this end, an experimental design was implemented on San Andrés Island, where an edaphic salinity gradient exists due to the water regime. Three physiographic types of mangroves, characterized by different saline regimes, were selected for the study. Two inland forests were selected, both of which exhibited a mesohaline regime (9.63 ± 6.26 and 11.54 ± 7.46 PSU), while a third site corresponded to a euhaline fringe forest (37.47 ± 5.76 PSU). The final location was characterized by a hyperhaline regime basin forest (62.36 ± 10.54 PSU). The fundamental hypothesis posited an inverse relationship between salinity and litter production, and a direct relationship between salinity and root production. To assess root production, the growth core implantation technique (108 soil cores) was employed, with live roots selected based on diameter (<2, 2–5, and 5–20 mm). The mean (±SD) OC content in dry litter (Mg C ha−1y−1) was 8.96 ± 0.28; 5.57 ± 0.15; 6.31 ± 0.27; and 4.54 ± 0.8; while The production of dry roots was 0.41 ± 0.08; 1.19 ± 0.46; 1.30 ± 0.5; and 0.24 ± 0.20, for the mesohaline forests, the euhaline forest, and the hyperhaline forest, respectively. The proposed hypotheses were confirmed when considering only the extreme salinity ranges. Upon incorporating all salinity ranges from the four forests into the analysis, it was observed that litter production exhibited a tendency to decrease with increasing salinity, while root production demonstrated a tendency to increase. However, this trend did not attain statistical significance, thereby suggesting that, in addition to salinity, other factors may also regulate production processes. These findings serve to affirm the high productivity of carbonate environments and the contribution of autochthonous production.