Effects of seagrass Thalassia testudinum on sediment redox

The redox conditions were compared in vegetated versus unvegetated sediments across a range of contrasting Thalassia testudinum (sometimes mixed with Syringodium filiforme) meadows at Puerto Morelos Mexico reef lagoon, Moreover, the role of seagrass photosynthetic activity in affecting the redox conditions was tested in one of the meadows by experimentally reducing seagrass photosynthesis through shading. The seagrass rhizosphere extended 26 to 40 cm into the sediment, and accounted for 23 to 504 g DW m(-2) of root material, mostly contributed by T testudinum. T testudinum placed 50% (i.e. centroidal depth) and 95% of its root biomass within 12.6 +/- 0.58 and 54.4 +/- 2.53 cm. of the sediment surface, respectively; while S. filiforme placed 50 and 95% of its root biomass within 8.0 +/- 0.87 and 343 +/- 3.8 cm of the sediment surface, respectively, Vegetated sediments presented 50% of positive redox potential anomaly (i.e, redox potential in vegetated sediments-redox potential in adjacent bare sediments), remarkably similar (t-test, p > 0.5) to depths to the centroidal depth of the seagrass roots in the sediments. The shading experiment conducted in situ for 5 d demonstrated that the positive redox anomaly found at depth in vegetated sediments was derived from seagrass photosynthetic activity. The sediments around seagrass rhizosphere in the shaded plots were progressively reduced to reach an average decline of the redox conditions by about 45 mV by Day 5. The results presented show that seagrasses contribute to modify sediment redox conditions around their rhizosphere.