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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.

In laboratory decomposition expcrimcnts dead cclgrass lcavcs lost a maximum of 35% of the original dry weight in 104l days at 20°C. Whole leaves lost 0.5% of their organic content per day whereas particlcs smaller than 1 mm lost lo/O per clay. Sterilization of leaves by dry heat or potassium cyanide showed that leaching accounted for 82% of the total loss of organic matter from predried material and 65% of the loss from undried material. Bacteria acting alone incrcascd the nitrogen content of the detritus but only slowly degraded the leaf material.When protozoa wcrc introduced, they grazed on the bacteria, maintained the bacterial population in an active metabolic state, and hastcncd the rate of decay. The C : N ratio of incubated detritus decreased from over 20 : 1 to as low as 11 : 1, indicating an increase in its potential food value. The overall slow rate of decomposition could enable the eclgrass detritus system to continue functioning during periodic short term fluctuations in eelgrass primary production by ensuring that a rcscrvoir of slowly decomposing material is always p&Gent. --

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