Root production and belowground seagrass biomass

Abstract

The root and rhizome biomass of the seagrass species present in 3 mixed and 2 monospecific meadows representative of different floras (Spanish Mediterranean, Mexican Caribbean, Kenyan coast, and the South China Sea off The Philippines) was examined to test for the existence of general 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 thickness of the seagrass rhizomes (r = 0.92, p < 0.001). Root and rhizome biomass 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 1m2 m-2 when roots and rhizomes were considered together, and they formed a dense web of root material comprising several hundred meters per square meter. Belowground biomass showed considerable vertical stratification within the sediments, with a tendency for the larger species to extend deeper into the sediments than smaller ones. This tendency for segregation should reduce the potential interspecific competition for sediment resources, which is likely to be greater in the uppermost layers, where the belowground biomass is more evenly distributed among species. The rate of adventitious root production on vertical shoots varied from species that produced a root on almost every node to species that produced 1 adventitious root for every 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 was quite high, with most values ranging between 2 and 10 yr-1, indicative of a characteristic turnover time of months for the root compartment. 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.