The growth and population dynamics of Posidonia oceanica were examined in 29 meadows along 1000 km of the Spanish Mediterranean coast (from 36"46' to 42"22' N). oceanica developed the densest meadows (1141 shoots m-') and the highest aboveground biomass (1400 g DW m") between 38 and 39"N. P. oceanica shoots produced, on average, 1 leaf every 47 d , though leaf formation rates in the populations increased from north to south (range 5 7 to 8 . 9 leaves shoot-' yr-'). P oceanica is a long-living seagrass, with shoots able to live for at least 30 yr P, oceanica recruited shoots at low rates (0.02 to 0.5 In units yr-l) which did not balance the mortality rates (0.06 to 0.5 In unlts yr.') found in most (57 %) of the meadows. If the present disturbance and rate of decline are maintained, shoot density is predicted to decline by 50% over the coming 2 to 24 yr. Because P. oceanica rhizomes grow very slowly (1 to 6 cm yr-' apex-'), maintenance of existing meadows is essential, and our results suggest bad future prospects for P. oceanica in the Spanish Mediterranean Sea like most other seagrass species in the world oceans.

Valle de Bravo (VB) is a tropical reservoir located (19°21 0 30@ N, 100°11 0 00@ W) in the highlands of Mexico. The reservoir is daily swept by strong (7.4 m s À1 mean speed) diurnal (12:00-19:00 h) winds that blow along its two main arms. As expected from its fetch (6.9 km) and its depth (21.1 m mean), the reservoir behaves as a warm monomictic water body. During 2001, VB was stratified from February to October, and well mixed from November to January. Its mean temperature was 19.9°C; the maximum found was 23.8°C in the epilimnion, while a minimum of 17.8°C was registered during mixing. VB exhibited a thermal regime similar to other water bodies of the Mexican tropical highlands, except for a steady increase of its hypolimnetic temperature during stratification, which is attributed to entrainment of epilimnetic water into the hypolimnion. During stratification, the hypolimnion was anoxic, while the whole water column remained under-saturated (60%) during mixing. The flushing time is 2.2 years. Mineralization and total alkalinity are low, which allows strong changes in pH. Ammonia remained low (2.4 lmol l À1 mean) in the epilimnion, but reached up to 60 lmol l À1 in the hypolimnion. Soluble reactive phosphorous had a mean of 0.28 lmol l À1 in the epilimnion and a mean of 1.25 lmol l À1 in the hypolimnion. Nitrate exhibited maxima (up to 21 lmol l À1 ) during mixing, and also in the metalimnion (2 lmol l À1 ) during stratification. Low dissolved inorganic nitrogen indicated nitrogen limitation during stratification. Eutrophication is an emerging problem in VB, where cyanobacteria dominate during stratification. At VB chlorophyll a is low during mixing (mean of 9 lg l À1 ), and high during stratification (mean 21 lg l À1 ), when blooms (up to 88 lg l À1 ) are frequent. This pattern is similar to that found in other eutrophic tropical water bodies. We propose that in VB the wind regime causes vertical displacements of the thermocline (0.58-1.10 m hr À1 ) and boundary mixing, enhancing the productivity during the stratification period in this tropical reservoir.

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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.

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