PHOSPHORUS LOADS FROM PEATY POLDERS IN THE SW FRISIAN LAKE DISTRICT, THE NETHERLANDS

The purpose of this study was to quantify discharges and P-loads from peaty polders (total surface area 11560 ha) surrounding lakes in south-western Friesland, and to examine whether processes inside the polders contribute to the P-loads to the lakes. A detailed study in the Echtener Veenpolder (2850 ha), the largest polder in the study area, showed annual gross P-loads of 1.3 to 1.83 kg P ha-1 polder area and net P-loads of 0.63 to 1.48 kg P ha-1. Processes inside this polder, such as leaching, surface runoff, resuspension, spreading manure and fertilizers, can result in high total P concentrations (TP) in discharged polder water (up to 0.9 mg L-1) and therefore contribute to the P-load from the polder to lake Tjeukemeer. During 1984-1987 the averaged TP concentrations in polder water were higher than in lake Tjeukemeer water: 0.37 mg L-1 and 0.28 mg L-1, respectively. The annual gross P-loads from 11 other polders ranged from 1.01 to 4.13 kg P ha-1 polder area, while TP concentrations reached values up to 3.0 mg L-1. The P-loads from five polders to lake Tjeukemeer were high compared with the loads to the other lakes in the district. Particularly, during the winter period (October-April) P-loads were highest: up to 3.13 kg P ha-1. This paper shows that hydrological measures and the reduction of the concentrations are important to combat the eutrophication problem in the lakes. However, for an optimally integrated water management of the polders, further research is needed to develop, for example, dynamical water and P balances in order to gain a better insight into the role of agriculture.