Seagrasses occur in coastal zones throughout the world, in the part of the marine habitat that is most heavily influenced by humans. Decisions about coastal management therefore often involve seagrasses, but despite a growing awareness of the importance of these plants, a full appreciation of their role in coastal ecosystems has yet to be reached. This book provides an entry point for those wishing to learn about their ecology, and gives a broad overview of the state of knowledge, including progress in research and research foci, complemented by extensive literature references to guide the reader to more detailed studies. It will be valuable to students of marine biology wishing to specialize in this area and also to established researchers wanting to enter the field. In addition, it will provide an excellent reference for those involved in the management and conservation of coastal areas that harbour seagrasses.

Carbon fluxes from a mangrove creek with adjacent seagrass meadows and coral reefs (at 4 km from the creek) were investigated in Gazi Bay (Kenya). Analysis of the stable isotope signature of sediment carbon in the seagrass zone and data on the sediment carbon content indicate that outwelling of particulate organic matter (POM) from the mangrove forest occurs, but that deposition of this POM rapidly decreases away from the forest. No evidence for any input of mangrove POM in the seagrass zone was found at a distance of 3 km from the mangrove creek, near the reefs. The gradient in sediment 6I3C values in the seagrass zone was paralleled by a similar gradient of 6I3C values in Thalassodendron ciliatum, the dominant subtidal seagrass. This gradient probably reflects the availability of respiratory CO2 denved from mangrove POM as a carbon source for the seagrass. Analysis of C:N ratios of particulate material (< 1 mm) collected with sediment traps in the seagrass zone yielded values ranging from 8.5 to 11.2. This range is remarkably low compared to C:N ratios of plant matenal produced in the mangrove forest, and suggests that some of the mangrove-derived organic particles deposited in the seagrass zone have gone through a phase of intensive processing. During flood tides conspicuous decreases were found in 6I3C values of seston flowing over the seagrass zone, coinciding with significant increases in the carbon content of the seston. These findings point to a reversed flux of organic particles from the seagrass zone to the mangrove forest. Our data indicate that, as far as POM fluxes are concerned, the mangrove forest and adjacent seagrass meadows are tightly coupled, but that the nearby coral reefs may exist in relative isolation.

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Seagrasses abound in the dynamic environment of shallow marine waters. From the often high annual biomass production it can be deduced that seagrass meadows have high requirements for inorganic nutrients, although the nutrient demands will be met to some extent by internal recycling. A series of processes lead to nutrient losses from the seagrass bed. Export of leaves and leaf fragments with currents, leaching losses from photosynthetically active leaves and from senescent and dead plant material, and nutrlent transfer by mobile foraging animals, are processes speclfic to seagrass meadows; in addition, the nutrient losses are aggravated by 2 processes con~monly occurring in marine sedirnents: denitrification and diffusion of nutrients from the sediments to the overlying water column. The persistence in time of most seagrass meadows points to an existing balance between nutrient losses and gains. Three processes may contribute to the replenishment of nutrients: nitrogen-fixation, sedimentation and nutrient uptake by the leaves. Nitrogen-fixation undoubtedly is important, but continued biomass production requires other nutrients as well. Crucial contributions, therefore, must come from sedimentation and/or leaf uptake. The concept of the seagrass meadow as an open system, with nutrient fluxes from and to the system varylng in time, allows for imbalances between nutrient losses and gains. It is suggested that these imbalances may contribute to fluctuations in annual seagrass biomass production.

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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