In continuation of fixed anchor stations the behaviour of heavy metals was also studied in longitudinal sections in the bottom sediments (Calmano et al. 1982) and in the water (this paper). There are changes in concentrations of dissolved metals in water and changes in contents of metals in seston (suspended matter) in dependence on tides (Duinker et al. 1982 b). These changes can be related to the passing of different water bodies and different seston types, respectively, at the place of observation. In addtition, more changes were found at various hydrographically defined "stretches" along the whole length of the estuary. These changes were attributed to dilution of estuarine water by coastal seawater in the lower estuary; but in the mud reach in the central estuary they were attributed to the interchange with mud (alternating settling and resuspension of loose sediment particles through tides). In the central and the upper estuary they could be attributed to the influence of very low salinity, to the predominance of either organic matter or clay or quartz in seston, to the state of aggregation of seston particles which depends on salinity, the nearness to the mud reach and perhaps the amount of the particles in water; and the changes could further be attributed to emissions from industrial sites, sewage outfalls and the dumping of dredge spoil. Relation to organic water quality criteria were not found. The bioavailability of particulate metals is discussed: it is comparatively high in Cd, Zn and Mn (80-95%) and lower in other elements.