Data on nutrient contents of 27 seagrass species at 30 locations were compiled from the literature. Mean ( f SE) concentrations of carbon, nitrogen and phosphorus in seagrass leaves were 33.6 2 0.31, 1.92 f 0.05, and 0.23 2 0.011 % dry wt, respectively. The median C:N:P ratio was 474 :24: 1, which represents a C:P ratio more than 4 times, and a N:P ratio more than 1.5 times that of oceanic seston. These ratios are, however, less than those previously reported for marine macrophytes (550 : 30 : 1) by Atkinson & Smith (1984). Nitrogen and phosphorus variability within species was large, but carbon contents exhibited little variability. Accordingly, carbon:nutrient (N and P) ratios were inversely related to changes in nutrient content, and the rate of change in C:N and C:P ratios with increasing nitrogen or phosphorus content in plant tissues should shift from high to small as nutrient supply meets the plant's demands. The median nitrogen and phosphorus contents reported here (1.8 % N and 0.20 % P as % DW) correctly discriminated between seagrass stands that did or did not respond to nutrient enrichment, thus offering a useful reference for comparisons of seagrass nutrient contents.

The implications of differences in plant size for seagrass productivlty were examined based on an extensive compilation of data on architecture and growth of seagrass species. The analysis revealed strong allometric relationships between the size of different components, particularly a close scaling of the size of leaves, shoots, and fruits to rhizome diameter, as well as strong relationships between shoot size and the dynamics (e.g turnover rate, plastochrone interval, and longevity) of seagrass leaves and rhizomes of different species. The decrease in rhizome elongation rates and leaf turnover rates with increasing seagrass size demonstrates the importance of architecture for seagrass productivlty, and also provides explanations for the different ecological roles of small, colonizing species, and large, climax seagrass species. In addition, these results demonstrate that while habitat conditions have important Influences on seagrass productivity, differences in size may explain the vast range of turnover tlmes, plastochrone intervals, and module longevities, encountered among seagrass species.

The spatial distribution of oxygen and the dynamics of the oxic mlcrozone around roots of Cymodocea rotundata were studied using oxygen microelectrodes under constant light conditions and during light-dark transitions. Under daylight steady state conditions, oxygen was present at concentrations up to 75 % of air saturation at the root surface, and the oxic microzone around the roots was 80 pm thick. Steady state oxygen concentrations were reached within 1.5 h after light-dark shifts. Under darkness, free oxygen, at about 20% of air saturation, was still present on the root surface at steady state, but the thickness of the oxic microzone shrank to 50 pm. The oxygen present in the rhizosphere during darkness was supplied from the water column to roots via, primarily, gas-phase diffusion in leaves and rhizomes. The oxic microzone around roots comprised about 0.5% of the total volume of the seagrass rhlzosphere, and the root-mediated oxygen supply was estimated to be insignificant for the whole sediment oxygen budget contributing about 1 % of total oxygen consumption only. However, the continuous oxygen supply may ensure a persistent oxic environment for belowground tissues of C. rotundata and, hence, protect the plant from reduced phytotoxins.

By directly measuring the size distribution of active (cells that took up and reduced the redox dye CTC, 5-cyano-2,3-ditolyl tetrazolium chloride) and inactive cells in a natural coastal bacterial community, we tested the hypothesis that the likelihood of a bacterium being active in marine plankton is a function of its size. The average size of an inactive bacterium was 0.055 pm3 while the average size of an active bacterium was 0.12 pm3. This average size was constant even after 3 d of incubation in dialysis bags placed in situ, which increased the percentage of active bacteria in the community from 6 to ca 43 %. The probability of a bacterium being active was a linear function of its size, from ca 5 % for cells of 0.01 l.1n13 to 100% for cells of the largest sizes. These results (1) support the hypothesis of Stevenson (1978, Microb Ecol 4:127-133) that very small bacteria are mainly dormant (inactive) while bigger bacteria are more likely to be active; (2) reconcile 2 apparently opposing observations, (a) commonly found higher specific activities in the larger size classes of bacterioplankton and (b) allometry regularities by which smaller unicellular organisms tend to have higher specific growth rates than larger organisms of similar metabolic mode; and (3) suggest that phagotrophic protists will preferentially crop the active portion of the bacterial community if they select their prey according to size.

All seagrasses are rhizomatous plants that grow by reiteration of a limited set of modules. Their past growth history can therefore be reconstructed from the scars left by abscised leaves and flowers on the long-lived rhizomes or the seasonal slgnals Imprinted in the frequency and size of their modules. We provide here the basic foundations and assumptions of these reconstruction techniques and the calculations involved in their application. We then show their reliability and potential to quantlfy an array of ecological processes, such as plant demography, leaf and rhlzome production, flowering ~ntensity, and seagrass responses to anthropogenic perturbations, based on our recent studies of Mediterranean, Caribbean and Indo-Pacific seagrass species. Reconstruction techniques have also proven useful in demonstrating the role of seagrasses as tracers of sedlment movement over seagrass beds and the rates of colonisation and expansion of seagrass patches. These reconstruction techniques should provide a powerful tool to improve our knowledge of seagrass species and populations from remote areas based on a single or just a few visits This should, therefore, allow us to sample many seagrass meadows using limited resources, thus generating a strong foundation for the study of comparatlve seagrass ecology and testing of theories previously applied to terrestrial plant populations.