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Abstract:
Planktonic primary production was measured at monthly intervals between 1.11.1967 and 17.12.1968 in a bay of the lower Rio Negro, using the C14 method.
The net production per unit volume varied between 0,0191 and 0,2664 g C/m3/d and per surface area between 0,0112 and 0,181 g C/m2/d.The chlorophyll A content varied in the optimal depth zone
between 1 and 15 mg/m3, and between 3 and 40 mg/m2 per unit area of the trophogenic zone.
The depth of the trophogenic zone was relatively constant throughout the year at 2 m.
The gross production was determined as being approximately 0,03 to 0,43 g C/m2/d, and therefore on average about twice as great as the corresponding net production.
The dark fixation of carbon dioxide also fluctuated considerably, the values varying between 0,0011 and 0,0088 g C/m3/a, This corresponded to 2,7 and 11,8 % respectively (mean 5,6 %) of the
net production.
Since the sampling area has a tropical climate there is no interruption in the vegetation growth period for the phytoplankton in the course of a year, so the annual productivity could be calculated by
multiplying the mean daily value (0,063 g C/m3/d) by 365. With this conversion factor the value obtained is relatively high (230 kg C/ha/year), in view of the unfavourable nutrient and light conditions.
The extremely poor nutrient supply and the unfavourable light conditions in the black water of the lower Rio Negro must be considered as responsible for the characteristically low primary productivity here. These factors have been mentioned by various authors.
The fluctuations in the level of primary productivity and the characteristic inverse relationship between it and water level is a result of seasonality. There are several indications that the nutrient supply
is better during the low water period, although much work can still be done in this field. These points are dealt with in detail in the discussion.
The nutrient factor is, as far as can be seen from these results, the most important factor limiting primary production of phytoplankton in the black water of the lower Rio Negro.
