| Over the past few decades, both human population and the global supply of agricultural goods have doubled. Most of the increase in crop yields is due to fertilizer use. Inevitably, some of the fertilizers applied run off croplands into aquatic ecosystems. These excess nutrients in coastal areas have caused detrimental changes in biodiversity and have had deleterious effects on many commercial fisheries. Today, both human population and the use of fertilizer continue to increase, especially in tropical regions where very little information has been collected on the potential effects of nutrient runoff to coastal ecosystems.; I studied eight small coastal catchments in tropical North Queensland, Australia, to determine the potential export of nutrients from sugarcane agricultural fields and to investigate whether the mangrove estuary in each catchment is being influenced by agricultural runoff. By comparing the land-use patterns in four catchments without sugarcane farming against four catchments with varying degrees of farming, I estimated that even moderately farmed catchments (5–37% land area as crops) release approximately 1.3 to 30 times the amount of total nitrogen (N) and 1.5 to 23 times the amount of total phosphorus (P) per hectare than catchments with no sugarcane.; In the mangrove sediments of farmed catchments, I found significantly elevated concentrations of extractable inorganic N, total P and ∂ 15 N. There were higher concentrations of N and P in the sun leaves of two common mangrove tree species and greater overall leaf [N] and canopy N content in the mangrove forests of farmed catchments. These results indicate that there are excess nutrients entering the mangrove forest from upstream and adjacent agriculture.; I developed a model to address whether the mangrove estuary can be a buffer against the export of N fertilizer to nearshore ecosystems. This demonstrated that even under low rates of N leaching from cane farms, a significant portion of a catchment would have to be mangrove estuary in order for the runoff to be fully buffered. Furthermore, the mangrove's buffering capacity is largely dependent on denitrification and sedimentation, on the hydrodynamics of the catchment-estuary system, and on the seasonality and intensity of precipitation. |
