Phosphorus reference conditions in lowland streams: Survey in old forests and anaerobic sediment release

The reference (pristine) concentrations of total dissolved phosphorus (TDP) in surface waters are difficult to define in lowland regions because of their high population density. Here, we estimated reference conditions of TDP from headwaters and their riverbed sediments (n = 140) in old forests. Surface waters and sediments were collected in the lowland region of Belgium as transects starting in 20 different old forests (before 1775; pristine) to up to 1.5 km outside the forest edge (disturbed). Sediment P concentrations in disturbed samples readily increased with increasing distance from the borders of old forests. The P mobilized from the sediments to the overlying water was also measured in laboratory incubations to mimic P release during seasonal redox cycles. The sediment respiration was larger in disturbed than in the reference sediments. Disturbed sediments released more P during anaerobic than aerobic incubations, while reference sediments did not show such a difference. The mean TDP in the reference surface water samples is 57 μg TDP L^-1 [95 % confidence interval 41; 72], very close to the pre-industrial background previously estimated for the Scheldt river. The 90th percentile of the reference water is 137 [110; 180] μg TDP L^-1. That P90 is just under the prevailing TP limit of 140 μg P L^-1, illustrating that this limit is very close to the upper baseline of reference conditions. The P75, often proposed as the threshold for high ecological status, was 84 [55; 122] μg P L^-1. This study showed that soil or sediments in old forests might serve to identify reference conditions. The well-established summer peaks of TDP in lowland rivers are unlikely to occur in pristine areas.