The formation of small particles and aggregates in the rhine estuary

Abstract

Particulate matter in suspension in the Southern Bight of the North Sea consists mainly of more or less round, often loose aggregates (particles glued together with organic matter) and further of single mineral grains, some small (<5 μm) chain-like aggregates, flocs sometimes containing small mineral grains and very small (<5 μm) organic particles. The smallest particles observed were ca 0.02 μm. The average density of the particles is 1.97 and their water content 27% but there is a large variation (density 1.14 to 2.65, water content 0 to 90%) due to the mixed character of the particles and the uncertainties in estimating organic matter content. Aggregates can be formed by: 1. (1) flocculation on the contact of fresh and more saline (estuarine) water, 2. (2) erosion of soils and consolidated fine-grained deposits, 3. (3) increased interparticle contact near the bottom at high concentrations of suspended matter, 4. (4) still larger interparticle contact during temporary deposition (during calm weather or slack tide, or accumulation in more quiet areas during stomrs), 5. (5) activity of organisms gluing particles together (by forming faeces, pseudo-faeces, or secreting mucus). In the Rhine estuary aggregate formation on the contact of the river water and more saline estuarine water is a limited process; probably aggregates of 1 to 3 μm are formed. Aggregation by electrostatic or Van der Waals forces may occur along the entire length of the estuary but is not very important. Also there are strong indications that at low salinities oval-round particles are formed containing Ca and S and probably being a complex of Ca ++, SO 4 −−, CO 3 −− and OH −. They were not found at salinities above 20% S and probably are not stable in sea water. The fact that the aggregates are glued together with structureless, often greenish or brownish substance that can be removed with H 2O 2 and therefore is assumed to be organic matter, points to aggregation by organisms or to a fresh water origin (erosion of soil aggregates). At high particle concentrations in the fluid mud that is present in the Rhine estuary large loose flocs up to several mm diameter are formed, and small particles as well as plankton cells are trapped, but on resuspension they break up into aggreagates and mineral particles of predominantly 2 to 10 μm with a strong admixture of aggregates of 20 to 70 μm. The larger aggregates are not normally found in the Southern Bight in such high (relative) concentrations and probably are not stable, but the smaller aggregates are of the type normally found in suspension. It can therefore not be excluded that such aggregates are formed on resuspension of fluid mud or loose bottom deposits, although their organic content points to aggregation by organisms or in soils. The fact that flocculation on the contact of river water and estuarine water was found to be a very limited process may be caused by the pollutionn of the Rhine with salts, which has raised the salinity of the river to an average of 0.61% S (maximum 0.91% S). It follows from this discussion that the aggregates in suspension in the Southern Bight of the North Sea are likely to be of multiple origin but predominantly formed by organisms. A multiple origin is also implied by the multiple origin of the material in suspension as a whole which is chiefly supplied from the Channel, from rivers and from seafloor erosion with additional amounts from coastal erosion, primary and secondary production and the atmosphere.