The composition and settling behaviour of suspended particulate matter (SPM) from the limnic to the polyhaline zone of the Elbe Estuary of northern Germany was studied from 1992 to 1994 to receive substantial data, important for management proposals for sediment dredging in the harbour and the river Elbe. Special attention was given to organic content, phytoplankton pigments, and species composition in suspended matter of different settling velocities. SPM was separated into three fractions of different settling velocities using an Owen tube.

The main part of SPM settled slower than 0.3 millimeters per second (mm/s). This type of SPM was distributed nearly homogeneously over the water column. SPM having a settling velocity higher than 1.9 mm/s was mainly present near the bottom. Downstream of Hamburg harbour phytoplankton contributed only a very small part to the total content of SPM. Chlorophyll-A, taken as a parameter of algal biomass, made up only 0.3% of the total suspended matter. Contrary to the total organic content which was always highest in the slow settling fraction (about 80-90%), up to 30% of chlorophyll-A associated with SPM was found in the mean and fast-settling fraction.Degradation products of chlorophyll-A associated with SPM even had more often highest values in both faster-settling fractions than native chlorophyll-A. Bacillariophyceae and Dinophyceae were represented preferentially in the fast-settling fraction, whereas the number of Chlorophyceae and Cyanobacteria was higher in the slow-settling fraction. The most important abiotic factors correlated with the amount of chlorophyll-A of total SPM contents were water temperature and light or turbidity, respectively. Phytoplankton species composition was mainly influenced by salinity. Positive correlations between the amount of algal pigments and exoenzymatic activity of heterotrophic bacteria were observed.