Primary production of phytoplankton in the three types of Amazonian waters. V. Some investigations on the phytoplankton and its primary productivity in the clear water of the lower Rio Tapajós (Pará, Brazil)

Abstract

Four limnological investigations were conducted in different seasons of the years 1968 and 1969 at the lower reaches of the Rio Tapajóz. The physical and chemical parameters generally correspond with earlier findings of other authors. Midriver areas show no stratification of the water body. In certain reaches of the river, distinct increases of pH-values temporarily occur due to strong phytoplankton activity. A few species of cyanophyceae and diatomeae make up the vast majority of phytoplankton in terms of nu,bers. Chlorophyceae, desmidiaceae, and other algae are characterized by high species variety but only occur in low or moderate densities. Despite of the low nutrient concentrations encountered, phytoplankton production is high, which can be attributed to the excellent light conditions in the river. The net production of 2.4 g C/m²/d exceeds by far production rates from várzea-lakes, although production densities are lower. A mighty productive zone in combination with extremely fast turn over rates male this unexpected high production per unit area possible. The shortest C turn over time of all samples was 0.3 days. Phytoplankton net production amounts to a calculated 5 t C hectare and year. Derived from primary production data, a theoretical fish production of 100 kg/ha/year can be estimated, which would allow yearly catches of at least 10 to 20 kg per hectare for human consumption without provocing ecological disturbances