General biological features of the South Atlantic
The influence of the Congo river

The generally positive effects of river run-off on phytoplankton biomass and production should be expected during the Great Warm Season, which is also the period of maximum run-off. While some of the smaller rivers (Ogoue, Kouilou) have a clear localized influence (Dessier, 1981), the beneficial effects of the Congo river plume on marine coastal plankton are not evident and contradictory observations are probably related to sampling at different states of mixing between riverine and sea waters. The huge amount of water discharged overwhelm the region with approximately 45,000 mö3 sö-1 (Eisma and van Bennekom, 1978). Unlike the Amazon (see Gb.10 The western South Atlantic: Northern Brazil), the Congo has a deep canyon at its mouth which canalizes its very fast flow. The canyon’s depth, the low tidal range and high current velocities in the river lead to an extreme case of stratified estuary in which the residence time of the water in the mixed surface is very short. The influence of freshwater is thus confined to a thin surface layer (10 m at the mouth and 30 m at 700 km) and the dilution of river waters in the sea takes place very far from the mouth ( Gb25b).

November 1976 April-May 1978

( Gb25a) Salinity ( Gb25b) Salinity
( Gb25c) Temperature ( Gb25d) Temperature
( Gb25e) N03 ( Gb25f) N03
( Gb25g) Chl-a
( Gb25h) Primary production

Close to the river mouth, there is an increase in nitrate and phosphate which Van Bennekom et al. (1978) attribute to upwelling induced by estuarine circulation (nitrate) and desorption from suspended matter (phosphate). Silica concentrations decrease linearly with salinity, which suggests weak utilization by diatoms. The core of the plume is associated with surprisingly low chlorophyll (1 mg mö-3) and poor primary production (8 mg C mö-2 hö-1; Cadée, 1978). River phytoplankton die rapidly on arrival at the sea and owing to both high turbidity and the rapid change in salinity there is an inhibition of the growth of brackish water species. This results in a general decrease in chlorophyll biomass and production with increasing salinity to 20°/°°. Thereafter, an increase in phytoplankton biomass is observed at the edges of the plume to 30°/°°, after which nutrients become depleted (or imbalanced) and phytoplankton biomass and production become depressed (Cadée, 1978). The arrival of Congo waters off Pointe-Noire (4°50’ S) is always marked by a drop in phosphates, chlorophyll and production. Only silica enrichment persists (Dessier, 1981).

There are few studies of zooplankton in the vicinity of the Congo River mouth. Dessier (1981) reported high planktonic (both phyto- and zooplankton) concentrations between 4 and 6°S during May, which may have been related to the high runoff. He also reported high densities of opportunistic zooplankton species (the copepods Temora turbinata and Centropages chierchiae, the cladoceran Penilia avirostris) over the shelf between 4 and 13°S in November and low numbers of these species in March. These too could be related to periods of high and low runoff, respectively. The plankton content of these low salinity waters probably depends on the time elapsed since the mixing between continental and marine waters, and on the nutrients remaining after the first blooms.