ROOT PRODUCTION AND BELOW-GROUND SEAGRASS BIOMASS

The root and rhizome biomass of the seagrass species present in 3 mixe d and 2 monospecific meadows representative of different floras (Spani sh Mediterranean, Mexican Caribbean, Kenyan coast, and the South China Sea off The Philippines) was examined to test for the existence of ge neral patterns in the distribution of their biomass in the sediments, and to test a simple approach based on age determinations to estimate root production. The thickness of the roots was scaled to the thicknes s of the seagrass rhizomes (r = 0.92, p < 0.001). Root and rhizome bio mass were high (>100 and >200 g DW m(-2), respectively) for the mixed meadows examined; these belowground structures had a projected surface area often exceeding 1 m(2) m(-2) when roots and rhizomes were consid ered together, and they formed a dense web of root material comprising several hundred meters per square meter. Belowground biomass showed c onsiderable vertical stratification within the sediments, with a tende ncy for the larger species to extend deeper into the sediments than sm aller ones. This tendency for segregation should reduce the potential interspecific competition for sediment resources, which is likely to b e greater in the uppermost layers, where the belowground biomass is mo re evenly distributed among species. The rate of adventitious root pro duction on vertical shoots varied from species that produced a root on almost every node to species that produced 1 adventitious root for ev ery 10 nodes. Root production-both on horizontal rhizomes and Vertical shoots-was substantial, with the combined root production approaching , or exceeding, 1000 g DW m(-2) yr(-1). The resulting root turnover wa s quite high, with most Values ranging between 2 and 10 yr(-1), indica tive of a characteristic turnover time of months for the root compartm ent. The estimates of root production derived here often exceed those of rhizome production and reach Values comparable to leaf production, clearly demonstrating that root production is an important component ( up to 50%) of total seagrass production.