Nitrogen and phosphorus loading in Lake Ohrid influenced by rivers Velgoshka and Koselska

The industrial growth, forced agriculture by application of agrotecnical means and modem lifestyle are more and more contributing to the change of quality of aquatic ecosystems. This impact, primarily induced by man, i.e. his impact on the environment has influenced Lake Ohrid as well, which qualities are well known. The antrophogenic impact, especially seen in certain areas of the shoreline, contributes to the presence of so called hot spots with an alarming condition of the water quality. Such segment is formed by River Velgoska and Koselska and, the littoral infront of its estuaries.The unsolved issues with wastewater, solid waste, low civilization of the residents migrating to that region has turned the once crystal-clear beach into one of the hottest spots of the shoreline. The issue imposed a more complex investigation which is a subject of this magistracy. Investigations of the nutrient loading with nitrogen and phosphorus were undertaken to realize the aim, and the following of certain physico-chemical and biological parameters gives a more complex insight into the overall state. The problem with continuing loading of these localities imposed investigation in two time periods: november 1997 - october 1998 and july 1999 - june 2000. One measuring point in the pelagial of the lake is taken as a comparative one, with a vertical profile of 13 depths. Investigations are done according to standard limnological methods, and the samples for analysis were collected with a monthly dynamics. According to the obtained results of the performed analyses for both investigation periods on the basis of the water act of RM and OECD regulations, a categorization of the water was made in the respective measuring points. River Velgoska water ranges from II to V class (mesotrophic to hypereutrophic), whereas River Koselska is from II to IV (mesotrophic - eutrophic). The littoral infront of River Velgoska (Grasnica) is categorited as II ti IV class. Apart from the previous measuring points, the littoral at the River Koselska estuary (Daljan) mainly holds the quality of I-II class of water. The results imply of the alarming condition from River Velgoska to littoral Grasnica, which are loaded with communal and industrial wastewater. The water, from a pelagial control point, implies of a stable oligotrophic system, where nutrient loading is not so present and which is more evident in the littoral zone. The application of the Carlson's method for deten-nination of the trophic state of water which refers to the followinf parameters: concentration of total phosphorus content of chlorophyl a and Secchi transparency; provides the numeric value of the trophic state index. According to the mean values of this index (on the basis of chlorophyll a content and total phosphorus concentation), a classification of the respective measuring points is made, which shows that littoral water in Grasnica is mesotrophic, whereas the littoral in Daljan is oligotrophic. The trophic state index for the control pelagic point, on the basis of the three parameters, belongs to the oligotrophic aquatic systems. The application of the mathematical model for the total nitrogen and total phosphorus (TN/TP) proportion proves that there is phosphorus limitation of the primary production in every investigation locality. (Author)