Phosphorus fractions in discharges from artificially drained lowland catchments (Warnow River, Baltic Sea)

Understanding phosphorus (P) dynamics, from inland sources to the sea, is essential for developing strategies to reduce P loads. In this study, we examined concentrations, fractions, and association of P with other elements at a tile-drain outlet, the adjacent ditch and brook, further downstream from the brook, and in the river itself. The study was conducted in a sub-basin of the Warnow River catchment from 1 November 2013 to 30 April 2014 covering a mild and dry winter. Total phosphorus (TP) concentrations were lowest at the tile-drain effluent and increased in the ditch and brook, as a result of elevated dissolved reactive phosphorus (DRP), particulate reactive (PRP) and organic (POP) phosphorus. Dissolved organic phosphorus (DOP) concentrations remained constant. Further increase of TP along the brook and in the river reflected the increase in DRP + DOP along the first 2.5 km and the doubling of PRP along the 6.5 km thereafter. In the river, phytoplankton growth transformed P into POP in early spring. Total loads of DRP, PRP + POP, and DOP emitted during the study period were 4.3-5.6, 3-7, and 1-2 gha(-1) respectively, with an increasing tendency downstream. Despite their low P content (0.7-3.9%), clay minerals and Fe(hydr)oxides particles were the most important carriers because they formed 68-90% of all P-containing particles. A shift from Ca-phosphate to Fe-phosphate occurred from winter to spring and there was a variation in composition of P-containing particles along the flow course. Our results underline the importance of particulate P in discharge and show that the brook Zarnow following the drain outlet and the ditch is a location of P-enrichment and modification probably due to other inputs. The entire flow course has to be considered to assess nutrient inputs from agricultural land because P-composition and loads are changing in time and space. (C) 2017 Elsevier B.V. All rights reserved.