No abstract

ABSTRACT. We report here data on biomass and dynamics of shoots and rhizomes of Thalassia testudinum in the Mexican Caribbean based, primarily, on aging of plant material. We found 7 testudinum to be able to reach high biomass (> 1000 g dry wt m-') and develop highly productive populations (1500 to 4500 g dry wt m-' annually, of which 10% were allocated to the rhizomes) in the Mexican Caribbean. The plants appeared to grow slowly, as reflected in longer time intervals in between production of leaves and shoots, and slower rhizome elongation rates and leaf production rates than found in the past, possibly as a result of their low nutrient, particularly phosphorus, content. Calculated maximum shoot life spans ranged between 6 and 9 yr, shoot recruitment rates ranged from 0.02 to 0.07 In units per plastochrone interval (PI) and shoot mortality rate was substantial and averaged about 0.045 In units PI-' Shoots turned over at ca 0.6 yr-', which also represents the rhizome turnover rate.

Root production and belowground seagrass biomassCarlos M. ~u a r t e '~' , Martin ~e r i n o~, Nona S. R. ~g a w i n~, Janet uri3, Miguel D. ~o r t e s~, Margarita E. Gallegos4, Nuria Marba5, Marten A. Hemminga5 ' ABSTRACT: The root and rhizome biomass of the seagrass species present in 3 mixed and 2 nlonospecific meadows representative of different floras (Spanish Med~terranean, 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 D\,V m-2, respectively) for the mixed meadows examined; these belowground structures had a projected surface area often exceeding 1 m2 m-' 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 l~kely 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-' The resulting root turnover was quite high, with most values ranging between 2 and 10 yr-', 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 seayrass production.

A comparison of available data on iron concentration in seagrass tissues shows iron concentrations in seagrass leaves growing on carbonate sediments to be below critical levels for angiosperms [< 100 µg Fe (g DW)− 1]. Iron concentrations of leaves in Caribbean seagrasses declined by 5–10‐fold from terrigenous to carbonate sediments (inner Gulf of Mexico to the Caribbean coast off the Yucatan Peninsula). This observation provided evidence of possible iron deficiency in seagrasses, which was confirmed by an experimental demonstration that adding iron to the sediments stimulates growth of the Caribbean seagrass Thalassia testudinum and increases the chlorophyll a concentration of both T. testudinum and Syringodium filiforme growing above coralline carbonate sediments. We conclude that seagrasses growing above carbonate sediments are likely to experience iron deficiency.

The shoot demography and rhizome growth of Syringodium filiforme Kutz. and Halodule wrightii Aschers. were studied, based on plant dating techniques, to account for their role as pioneer in the succession sequence of Canbbean seagrasses. Results demonstrated that these species are able to develop dense meadows, supporting biomasses in excess of 500 g DW m" They produced more than 2000 g DW m -2 yr-' due to their high leaf (5.0 to 8 5 yr.') and rhizome (2 0 to 3.3 yr-') turnover. Rhizome growth and branch~ng rates were very high, allowing these seagrasses to rapidly occupy the space they colonise. The rapid rhizome turnover involved, however, a high shoot mortality rate and low 11fe expectancy (average shoot life expectancy 100 to 180 d). This implies that, while these pioneer species are able to rapidly occupy the space they colonise, their established shoots cannot occupy that space for a s long a s the more longlived species Thalassia testudinum. We suggest, therefore. that the role of seagrass species a s pioneer or climax species is independent of their capacity to support dense, productive populations, and is closely related to shoot longevity and rhizome turnover.