Growth and sediment space occupation by seagrass Cymodocea nodosa roots

The development, growth, and space occupation by the canopy and rhizosphere of a temperate Mediterranean seagrass meadow of Cymodocea nodosa, and the possible effects of meadow seasonal development on sediment redox conditions, were examined during the 1998 growth season. The meadow supported maximum biomass of leaves and rhizomes during July, and maximum root biomass in August. The meadow maintained 123.6 g dry wt m–2 of leaves and 94.4 g dry wt m–2 of rhizomes (July data), and 121.2 g dry wt m–2 of roots (August data) during peak biomass. On average, the root network contained 607 m of roots m–2, had 3.6 cm between neighbouring roots and comprised 1.7% of sediment volume. Half of the root biomass occupied the top 12.6 cm of sediment, although a few roots reached >35 cm sediment depth. The meadow developed 70, 62 and 50% of leaf, rhizome and root biomass during the growth season respectively, showing that the structure of the temperate seagrass rhizosphere is highly dynamic. C. nodosa produced leaves, rhizomes and roots at rates ranging between 1.17 and 3.98 g dry wt m–2 d–1, 0.01 and 0.75 g dry wt m–2 d–1, and 0.04 and 0.84 g dry wt m–2 d–1, respectively. C. nodosa meadow grew on sediments where redox conditions during the growth season varied from –74 to 396 mV, being between 21 and 112 mV more positive than adjacent unvegetated sediments from July to September. The magnitude of redox potential anomaly in seagrass rhizosphere tended to be coupled to the above and belowground meadow biomass, suggesting that C. nodosa metabolism alters sediment redox conditions. Structural change of seagrass meadows during the growth season, therefore, is expected to influence benthic biogeochemical processes.