Effects of seagrass rhizospheres on sediment redox conditions in SE Asian Coastal ecosystems

We examined the rhizosphere structure of 14 seagrass meadows (seven mixed, three Enhalus acoroides, two Zostera japonica, one Thalassia hemprichii, and one Halophila ovalis) in the Philippines and Vietnam and tested their effect on sediment redox potential by comparing the redox potential in vegetated vs unvegetated sediments. The effect of seagrass photosynthesis on sediment redox potential was tested in an E. acoroides meadow during a short-term (2-day) clipping experiment. In all the meadows, the centroidal depth (i.e., depth comprising 50%) of seagrass belowground biomass was within the top 15 cm sediment layer. Redox potentials in vegetated sediments tended to be higher than those in adjacent unvegetated ones; sediment redox potential anomaly ranged from -61 to 133 mV across the meadows. The centroidal depths of positive redox potential anomaly and seagrass root biomass were significantly correlated across the meadows investigated (type II regression analysis, slope = 0.90, lower confidence limit [CL] = 0.42 upper CL = 1.82, R² = 0.59, p > 0.01). Experimental removal of E. acoroides leaves resulted in a decrease in rhizosphere redox potential by 20 mV, further confirming the positive effect of seagrass roots and rhizomes on sediment redox potential and, thus, the general conditions for microbial processes in the coastal zone.