Abstract: Based on the MODIS data and field measurement, the net primary production (NPP) in 2000 and 2006 in the middle reaches of Yarlung Zangbo River and its two tributaries in Tibet were estimated by using a mathematic model. The results showed that in study area, the NPP decreased gradually from valley to ridge, which was accorded with the gradients of precipitation and temperature. The annul NPP per unit area was averagely 86.8 g C·m^-2·a^-1, and was 2.15 g C·m^-2·a^-1higher in 2006 than in 2000. Farmland ecosystem had the highest annual NPP per unit area (243.1 g C·m^-2·a^-1), while desert ecosystem had the lowest one (36.5 g C·m^-2·a^-1). The average total NPP in the two years was 512.8×10¹⁰ g C·a^-1, with the value in 2006 being 12.7×10¹⁰ g C·a^-1 higher than that in 2000. Meadow ecosystem had the highest annual total NPP (194.4×10¹⁰ g C·a^-1), while desert ecosystem had the lowest one (30.3×10¹⁰ g C·a^-1). In 2000-2006, the NPP value in the areas with strong human disturbance (0-4 km away from road) had a decreasing trend, while that in the areas difficult for human to reach was in adverse.

Key words: chlorophyll a, nutrient, Aureococcus anophagefferens