Biomass dynamics of seagrasses and the role of mangrove and seagrass vegetation as different nutrient sources for an intertidal ecosystem

The input of organic matter and nutrients produced by mangrove and seagrass vegetation in the intertidal bay on Inhaca island, Mozambique, was estimated. Mean mangrove tree height was 2.20 m, diameter at breast height was 6.4 cm and density was 6047 trees per hectare. Above-ground biomass of mangrove was calculated at 107 t ha¿1 and annual litterfall at 6 t ha¿1. Above- and below-ground biomass, shoot density, leaf size, number of leaves per shoot, production and production/biomass (P/B) ratio of three different seagrass species, Cymodocea serrulata, Halodule wrightii, and Zostera capensis, was measured. Shoot densities were low and plants small. Total dry weight (DW) biomass was 18¿198 g m¿2, with higher values in summer, except for Z. capensis. Production was 0.08¿0.62 g DW per day, 2.2 times higher in summer, compared with winter, with highest values recorded for C. serrulata. The P/B ratio varied from 3.2 to 5.8. The decomposition decay constant K of mangrove and seagrass leaves was 0.010¿0.017; t50 values were 69 and 41 days, respectively. Decomposition was faster at Saco, compared with the Banco area. Mean litter biomass was higher at Saco, than at Banco (0.24 and 0.05 g ash-free DW per square metre, respectively), with significant differences for litter type, season and substrate. The ratio of mangrove to seagrass litter stock was equal to expected values, based on production and decomposition. Local differences in litter stock could be explained by tidal current patterns. Litter production was estimated at 0.08 and 0.10 g ash-free DW per square metre, respectively, for the mangroves and seagrasses. The annual input of mangroves into the system was estimated at 330 t C, 6 t N and 0.6 t P, whilst seagrass produced more nutrients, 490 t C, 30 t N and 2 t P. This emphasised the importance of seagrass vegetation as the dominant nutrient source of the syste