The productivity of mangrove ecosystems is associated with litterfall production, which continuously contributes large quantities of organic matter, in the form of detritus, to the food web via adjacent ecosystems. However, the degree of deterioration of mangrove ecosystems worldwide has been increasing due to anthropogenic activities, leading to the loss of vegetation cover and changes in hydrological patterns, the chemical conditions of interstitial water and soil, and the litterfall degradation rate and, thus, the integration of organic matter into the ecosystem. In this study, we investigate the relationship between leaf degradation and interstitial water chemistry in two mangrove forests located in Oaxaca, Mexico, that are characterized by differences in environmental conditions, species, and anthropogenic activity. Forty-two 10 cm × 20 cm nylon mesh bags were installed in the Rhizophora mangle forest along two flood-associated lines (21 per line), and twenty-one bags were installed centrally in the Avicennia germinans forest because of the flood conditions in this area. Three bags per line were collected each month. This material was then dried and calcined for determination of the decomposition rate (k). The in situ redox potential and interstitial water salinity of mangrove forests were measured using a HACH HQ40d multiparametric probe and A&O refractometer, and the sulfate concentration was determined by ion chromatography. The results show that daily average decomposition rates were higher in the Salina lagoon (k = 0.01 g·day−1) than in the Chacahua lagoon (k = 0.004 g·day−1). The degradation model was Y = 66.054 × 10−0.010t, R2 = 0.89, p ≤ 0.05, for the Salina lagoon and Y = 67.75 × 10−0.004t, R2 = 0.76, p ≤ 0.05, for the Chacahua lagoon. Leaf decomposition rates differed between the Salina and Chacahua lagoons (F1,206 = 4.8, p < 0.03). In the Salina lagoon, dominated by A. germinans, an inverse relationship was established between the percentage of degraded biomass with respect to salinity concentration (R2 = 0.82, p < 0.013) and redox potential (R2 = 0.89, p < 0. 015), and for the Chacahua lagoon, dominated by R. mangle, the percentage of degraded litter biomass was found to be inversely correlated with redox potential (R2 = 0.94, p < 0.005) and sulfate concentration (R2 = 0.88, p < 0.017). Based on the results obtained in this study, we conclude that variations in the chemical conditions of interstitial water and hydrological patterns can affect the process of mangrove leaf degradation based on species and the integration of organic matter in the soil and in adjacent ecosystems. These findings are potentially useful for mangrove management because they advance understanding of the dynamics of organic matter in mangroves and the importance of maintaining the health of these ecosystems, which is necessary for the maintenance of coastal fishing production.
