Suspended matter in the Zaire estuary and the adjacent Atlantic ocean
Eisma, D.; Kalf, J.; Van der Gaast, S.J. (1978). Suspended matter in the Zaire estuary and the adjacent Atlantic ocean. Neth. J. Sea Res. 12(3-4): 382-406 In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579; e-ISSN 1873-1406, more |  |
| Authors | | Top | - Eisma, D., more
- Kalf, J.
- Van der Gaast, S.J.
| | |
| Abstract | The Zaire (or Congo) river has a relatively low suspended load mainly because of the predominance of chemical erosion and low gradients in the river basin. The suspended load largely consists of koalinite (26%), quartz (22%), organic matter (32%), ironhydroxydes (10%) and minor amounts of other clay minerals and feldspar. Organic material is also carried seaward mainly in the form of floating vegetation and plant fibres. In the estuary the outflow of river water is concentrated in a narrow surface layer at the centre of the stream. The distribution of suspended matter in the estuary is in agreement with the outward flow of surface water and a slow resultant inward movement of the deeper water in the canyon. About half the suspended load carried seaward by the river is deposted in the estuary in the head of the canyon, and possibly in the mangrove swamps north and south of the main river channel. The larger and heavier grains settle out, while large amounts of fine grains are removed by coagulation and aggregation. Oceanward there is a relative increase of fine material and organic matter in the surface water. In the open ocean the suspended material consists almost entirely of particles smaller than 10 um. Offshore a large river plume is directed towards the NNW, forming a thin surface layer of low salinity. The relation between content of suspended matter and salinity is far less regular than would be expected from a combination of gradual mixing with ocean water and continuous settling out of the larger particles. This may be accounted for by irregularities in river discharge and suspended load, by mixing with different kinds of ocean water, by variable hydrodynamic conditions influencing the degree of settling (and probably aggregation), and by variable supply of particles of organic origin. Offshore the water at the surface generally has a higher content of suspended matter than the water at 1 m depth and also the organic content is higher. Particle size distributions in the estuary reflect the original distribution in the river water as well as the effects of coagulation caused by the content of the river water with sea water. Through settling of the larger particles the size distributions gradually pass into the log-normal distributions usually found in the ocean water. Just off the river mouth, in an area characterized by strong mixing of river water and ocean water, irregular size distributions are found; probably suspended material is being supplied from the coast, the shelf bottom or the mangrove swamps. Only a small part (<5%) of the material in suspension in the river actually reaches the deep sea, mainly as particles < 18 um. From this fraction the particles of 9.5 to 18 µm and to a lesser extent those of 5 to 9.5 µm settle out on the continental slope and on the adjacent ocean floor whereas the particles <5 µm largely remain in suspension. Most suspended material is already removed from the surface water at salinities below 20‰, the coarser size fractions settling out, the fine size fractions chiefly being removed by coagulation and/or scavenging. The relations between total weight and total volume of the suspended material agrees with the mixed organic and inorganic composition. Apparent densities mainly lie between 1 and 2,66 but the data are not sufficiently accurate to calculate an average density. |
|
