The results of the weekly SRP and TP measurements in an anastomosing (cross-veining) reach of Narew River (NE Poland) over one annual cycle as well as measurements of P in river sediments are presented here. Mass balance studies were used to examine net gains and losses of P along the river reach. The P concentrations are found to be very variable throughout the year with maximum levels, reaching 0.21 mg L-1, being observed during the low flows and minimum - during the flood events 0.04 +/- 0.008 mg L-1. Results showed that phosphorus was retained within the river system during the high flows in winter and spring, whereas net export of P occurred in summer period mainly due to internal loading. Under summer low-flow conditions, high temperature and low redox potential facilitate P release from river bed sediments into both sediment pore waters and the water column. At this time, the Narew Anastomosing River System (NAS) provides an important P source, which can affect the downstream habitats by additional load of highly bioavailable P. It appears that NAS, through its specific character, can transform the water chemistry and transport rate, compared with river fragments located upstream and downstream. It provides an example of a discontinuity analogical to the shallow lakes in lowland river systems. (c) 2005 Elsevier B.V. All rights reserved.
