Nutrient sedimentation at the river inflow to a lake

Summary

Phosphorus content in suspension settled along the transect at the river inlet increased exponentially with decreasing water velocity. Main form of phosphorus in suspension was inorganic P and samples closest to the inlet were devoid of organic fraction. Phosphorus in surface layers of sediments followed the similar pattern, though the absolute concentrations were lower than in respective samples of suspension. Ratio of mineral: organic P was similar in settling matter and surficial sediments suggesting proportional liberation of phosphorus after the settlement. Contrary, organic P in littoral stand far from the river influence increased after sedimentation.

Organic carbon both in settling matter and in sediments showed maximum in the middle part of the transect. Organic carbon: Organic phosphorus ratio was high in that zone but decreased in samples farthest from the river inlet. It is then possible that organic matter load delivered by river (humic substances eroded from soils?) settled at a distance of up to 50–60 m while sedimentation in the last stands represented lake autochthonous matter.

Phosphorus and organic matter gradients produced under the influence of the river coincided with zonation of submerged macrophytes observed in estuary. Whether this coincidence is of causal character needs further explanation.