Ecological Engineering 25 (2005) 429–441 Phosphorus dynamics and fluxes in a lowland river: The Narew Anastomosing River System, NE Poland Piotr Banaszuk ∗ , Agnieszka Wysocka-Czubaszek Institute of Environmental Engineering and Environmental Protection, Bialystok Technical University, Wiejska 45a Street, 15-351 Bialystok, Poland Received 30 January 2005; received in revised form 27 June 2005; accepted 30 June 2005 Abstract 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. © 2005 Elsevier B.V. All rights reserved. Keywords: Phosphorus; Bottom sediments; Anastomosing river system 1. Introduction Eutrophication is one of the most serious problems facing the ecology of freshwaters (Mainstone and Parr, 2002). Phosphorus is one of the major plant nutrients whose enrichment leads to eutrophication; therefore, ∗ Corresponding author. Tel.: +48 85 7469684; fax: +48 85 7469576. E-mail address: banaszuk@pb.bialystok.pl (P. Banaszuk). understanding its sources and transfers in catchments as well as knowledge about its fate in aquatic sys- tem, i.e. interaction with bottom sediments and biota are needed to help identify and reduce eutrophication risks. Research in this area has naturally grown con- siderably over the past 20 years. Phosphorus transport and dynamics has been studied widely at small and large catchment scales (e.g. Hill, 1982; Muscutt and Withers, 1996; House and Denison, 1998; Boar et al., 1995; May et al., 2001; Jarvie et al., 2002). 0925-8574/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2005.06.013