Phosphorus Forms and Their Changes in Water of Three Subalpine Lakes in Italy

Abstract

AbstractTotal phosphorus and its main forms: dissolved mineral, dissolved organic, particulate organic and particulate mineral in the vertical water column of three subalpine lakes of various types in Italy, has been estimated during the winter‐vernal season. The range of variation in the phosphorus content in these waters was as follows: total phosphorus 16 ± 2860 μg/1 PO4, dissolved mineral phosphorus 4 ± 1040 μg/l PO4, dissolved organic phosphorus 1 ± 160μg/l PO4, particulate organic phosphorus 0 ± 290 μg/l PO4 and particulate mineral phosphorus 1 ± 100 μg/l PO4, Generally the content of total phosphorus and dissolved mineral phosphorus (phosphates) increased with the degree of eutrophy with the depth and with the progress of the vernal season towards the summer stagnation time.The amount of phosphates increased in water with the depletion of oxygen, both in the verical water column and with the progress of stagnation time.The amounts of dissolved organic phosphorus decressed with the depth of the vertical water column whereas the dissolved mineral phosphorus increased.The development of the particulate organic phosphorus stratification in the vertical water column was clearly visible in the eutrophic lake.The quantities of total phosphorus and its main component, dissolved mineral phosphorus, decreased evidently from January to May in all three lakes, mostly in the eutrophic lake. The reason of this decrease is sorption by lake sediments and to a certain degree sedimentation of phosphorus sorbed by ferric hydroxide.The increase of dissolved mineral phosphorus and that of total phosphorus in the vertical water column and with the progress of summer stagnation had as a reason the liberation of phosphorus from sediments, and not so much decomposition of sedimentating plankton or dissolved organic phosphorus.The share of single (mean) values of phosphorus forms in the total phosphorus was as follows: magnified image In the oligotrophic lake the share of particulate mineral phosphorus was extremely high in March (21% of the total), probably because of the inflow of the melting waters from the drainage area.The development of vertical stratification in waters of three subalpine Italian lakes at the end of the vernal season (May) indicates the quantitative prevailing of dissolved mineral phosphorus with its increase with the depth and domination of dissolved organic phosphorus in the trophogenic zone.