Phosphorus in settling matter and bottom sediments in lakes loaded by agriculture

In shallow lakes, the cycling of P between water and bottom sediments is strongly influenced by wind-induced resuspension of particulate matter. The significance of this P flux as an algal nutrient source is unclear. We examined gross sedimentation in 3 open and shallow agriculturally loaded lakes. In addition, we estimated the potential P-release from settling and bottom matter by laboratory tests. The mean daily rate of gross sedimentation was 21–170 g m^–2 d^–1 of dry sediment, 0.04–0.18 g m^–2 d^–1 of P and 0.18–2.0 g m^–2 d^–1 of N; being the highest in the shallowest and most eutrophic lake. In Lake Karhijärvi, where the most intensive measurements were taken, wind explained the temporal variation in the gross sedimentation to some extent. The settling matter consisted of inorganic particles low in nutrients, especially during peak sedimentation periods. On average, 7.7 ± 3.1% (x ± 95% confidence interval) of the P in the settling matter in L. Karhijärvi was in an algal-available form according to 2–3 week bioassays. In the bottom matter of the three lakes, 3.0 ± 1.7% and 2.5 ± 3.6%, and 4.3 ± 3.7% of the P was utilized by the algae. In L. Karhijärvi, resuspension of the potentially available P exceeded 20 times the external loading during the open water season. According to sorption tests, P is released from the bottom matter only when the concentration of o-P is <2 g l^–1. Although such a low value cannot be determined with common analytical procedures, it seems probable that the P concentration allows P desorption during P-limited periods. However, the significance of resuspended matter as an algal nutrient source calls for further research.