The impact of accelerating land-use change on the N-cycle of tropical aquatic ecosystems: Current conditions and projected changes

Published data and analyses from temperate and tropical aquatic systems are used to summarize knowledge about the potential impact of land-use alterat ion on the nitrogen biogeochemistry of tropical aquatic ecosystems, identif y important patterns and recommend key needs for research. The tropical N-c ycle is traced from pre-disturbance conditions through the phases of distur bance, highlighting major differences between tropical and temperate system s that might influence development strategies in the tropics. Analyses sugg est that tropical freshwaters are more frequently N-limited than temperate zones, while tropical marine systems may show more frequent P limitation. T hese analyses indicate that disturbances to pristine tropical lands will le ad to greatly increased primary production in freshwaters and large changes in tropical freshwater communities. Increased freshwater nutrient flux wil l also lead to an expansion of the high production, N- and light-limited zo nes around river deltas, a switch from P- to N-limitation in calcareous mar ine systems, with large changes in the community composition of fragile man grove and reef systems. Key information gaps are highlighted, including dat a on mechanisms of nutrient transport and atmospheric deposition in the tro pics, nutrient and material retention capacities of tropical impoundments, and N/P coupling and stoichiometric impacts of nutrient supplies on tropica l aquatic communities. The current base of biogeochemical data suggests tha t alterations in the N-cycle will have greater impacts on tropical aquatic ecosystems than those already observed in the temperate zone.