The balance of nutrient losses and gains in seaccrass meadows

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.