Primary Production in the Orinoco River

Autotrophic carbon fixation was studied from 1982 to 1985 along a 500—km reach of the Orinoco River, Venezuela, and in three major tributaries (Apure, Caura, Caroni). Among the tributaries, the annual mean gross primary production (measured as carbon) was highest for the Apure River (26 mg°m — 2 °d — 1 ), a whitewater river that drains from the Llanos region to the north of the Orinoco. Mean annual production was lower in the blackwater tributaries draining from the Guayana Shield to the south (Caura, 13 mg°m — 2 °d — 1 ; Caroni, 4 mg°m — 2 °d — 1 ). In the Orinoco main stem, mean production varied among stations from 19—43 mg°m — 2 °d — 1 . Both in the main stem and the tributaries, production per unit volume and per unit area peaked during the period of low water. Seasonal variations at a given station were almost entirely explained by variations in the phytoplankton biomass per unit volume rather than by seasonal variations in transparency. However, differences in mean transparency among the tributaries partly explained differences in mean production per unit area: suppression of potential production by dissolved and suspended substances was 85% in whitewaters (Apure), 40% in blackwaters (Caura, Caroni), and intermediate in the Orinoco main stem. Autotroph biomass (as chlorophyll a) was highest for the Apure River (mean, 0.36 mg/L), lowest for the blackwater tributaries (0.01—0.04 mg/L), and intermediate for the Orinoco River (0.11—0.19 mg/L). Transport of biomass had a different seasonal pattern than either production per unit area or biomass per unit volume. All stations along the main stem of the Orinoco River and all tributaries except the Caroni, which is impounded by a large dam, showed a significant rise in transport of algal biomass at high discharge, particularly on the declining limb of the hydrograph. Significant transport also occurred during the period of low water, well after the period of contact between the ricer and the floodplain. A mass—balance method was used in estimating the yield of phytoplankton biomass from the floodplain to the river. The annual carbon yield per unit floodplain area from 3660 km 2 of floodplain was only 75 mg°m — 2 °yr — 1 , or < 1% of estimated floodplain phytoplankton production. The data indicate that non—floodplain sources, including within—channel and near—channel stagnant or slow—flowing areas, account for 63% of the annual transport of phytoplankton carbon (2.4 x 10 6 kg/yr), while the floodplain accounts for only 37%. The floodplain, although productive, conserves algal production, and in this respect functions as a closed system, even though it is physically an open system.