Author Description

Seagrass meadows are coastal ecosystems occurring world-wide which have important functions for coastal stability and fisheries, but are under threat by human activities. Knowledge of the nitrogen cycle, an important nutrient for all organisms, is still scarce for seagrass meadows flourishing in nutrient-poor environments nearby offshore islands, like present in Indonesia. Seagrasses have a high leaf production. These leaves are quickly overgrown by epiphytes and become detached from the shoots before nitrogen resorption has occurred. These seagrass leaves form an important food source for other organisms living in the meadows, either through direct grazing, detritus collection or predation on primary consumers. Sea urchins consume a substantial amount of leaf material in the meadow. Burrowing shrimps collect much of the detached leaf material, comparable to half of the leaf production, in belowground structures before this material can be exported from the meadow through waves and currents. The high average water temperature stimulates microbial activity and decomposition of the leaves. Using 15N labelled material we showed that the released nitrogen is efficiently taken up again by seagrasses. They can take up the nitrogen from either dissolved inorganic of organic sources. Dense canopy meadows have a higher loss of nitrogen through leaf detachment, but also a higher efficiency to take up and conserve released nitrogen from this litter. I conclude that the interaction between seagrasses, macrofauna and microorganisms results in an efficient recycling of nitrogen outside the individual shoots, but within the seagrass meadows. This indicates the importance to preserve meadows to maintain the high productivity of these seagrass ecosystems.