COS 1-2
Limitation and landcover: Nutrient status and limitation within tributaries of two major Southeast Asian Rivers
Paucity of nitrogen (N) or phosphorus (P) in ecosystems can have bottom-up limiting effects on primary production that cascade upward, ultimately limiting biomass of higher trophic levels. Due to anthropogenic alteration of biogeochemical cycles, global biologically active pools of N and P have increased substantially in the past six decades. While much of this increase has contributed to meeting daily caloric requirements of an expanding human population, these same nutrients in excess are increasingly implicated in the degradation of ecosystems worldwide. However, the effects of cultural eutrophication are relatively unknown from rapidly developing countries in the tropics.
Today nearly half of the Southeast Asian landmass is agricultural. Expanded agricultural production and availability of synthetic fertilizers has resulted in a 1900% increase in fertilizer application between 1961 and 2002 across Asia. Potential consequences of rapid agricultural expansion on aquatic ecosystems are substantial. However, nutrient status of most rivers in Southeast Asia is unknown and no published studies have investigated nutrient limitation of algal accrual within Southeast Asian rivers. We analyzed nutrient status and limitation in 6 tributaries of the Salween and Chao Phraya River basins. We related nutrient status to land-use types and human population density within upstream tributary basins.
Results/Conclusions
Nutrient status differed between basins. Nitrate values were an order of magnitude lower in Salween tributaries than Chao Phraya tributaries; however, ammonium and soluble reactive phosphorus values did not differ. Elevated nitrate values resulted in significantly different N:P ratios, with Salween sites having values suggesting potential N limitation (<17:1) and Chao Phraya sites suggesting P limitation of algal accrual (>24:1). In Salween tributaries, nutrient diffusing substrate (NDS) assays revealed N limitation of algal biomass, as estimated by chlorophyll a. NDS assays in Chao Phraya tributaries, however, strongly suggested co-limitation of N and P, despite stoichiometry of ambient water chemistry suggesting P limitation. Spatial analysis of land-use and human population densities indicated strong correlation between percentage agricultural land-use and nitrate concentrations. However, agriculture was not correlated with ammonium or phosphate. Population density was not correlated with concentrations of any measured nutrients. These results suggest differential limitation status of these two major Southeast Asian river basins. Taking these results together, we suggest that historically both Chao Phraya and Salween tributaries may have been N limited, but that increased application of synthetic fertilizers may push nutrient concentration and limitation status within agricultural basins towards co- or P limitation, with potentially cascading ecological effects.