Monitoring the quality of the lake waters
Monitoring is normally carried out at different depths at a pelagic station where the lake bottom is 130 meters. The values measured refer to the whole body of water. Given that the phosphorus comes from the basin, its concentration along the coast is greater and brings additional pollution. If the phosphorus comes from agriculture it can be accompanied by chemicals such as pesticides and herbicides, if it comes from sewage leaks it’s always accompanied by faecal bacteria. In both cases, if the legal limits are exceeded, bathing should be prohibited. The body responsible for controls is ARPA.
The lake year begins at the end of February, when maximum remixing occurs, and ends at the beginning of the next. The “Associazione Lago di Bolsena” carries out monthly pelagic monitoring of temperature and oxygen with a multi-parameter probe. Samples for chemical analysis are taken twice a year, once at the beginning of March after the lake has reached maximum mixing and once in December when the lake has returned to stratification. Transparency is measured by lowering a white disc (called Secchi’s disk) into the water until it is visible. Transparency decreases with increased phytoplankton biomass.
The table shows the total phosphorus concentration in μg/l in the period of maximum mixing (February-March) and January 2017. The first column shows the seven depths at which the water samples were taken. The first line indicates the year. The highlighted years (2004, 2005, 2011, 2012, 2017) are those in which complete mixing had taken place so that the concentration of phosphorus was practically the same at all depths, and therefore can be considered accurate. In years when the concentration varies with depth, it is an evaluation that can result in approximations of the order of 10%. As can be seen, phosphorus, which was 8.1 μg/l in 2005, increased to 16.2 μg/l in 2017. The Management Plan (2009) recommended not exceeding 10 μg/l and indicated other measures to be taken, but was ignored.
The graphs show oxygen monitoring performed with the multi-parametric probe in the year 2012, the year in which there was the last complete remixing (before 2017 ). The vertical indicates depth. The first profile shows that in March at all depths oxygen was 11 mg/l. The second profile shows that in December of the same year the oxygen was exhausted. Much worse occurred when mixing had not happened. In fact in 2016 depths of 9 meters were registered without oxygen. During the first days of 2017 there were unexpectedly cold days with strong winds from the north: the lake had completely remixed and the anoxic layer had disappeared. In the immediate future the anoxic emergency is over but it will happen again even more seriously if the input of phosphorus from the basin is not reduced.
In summary, from 2005 to January 2017, phosphorus increased from 8 to 16.2 μg/l. From July 2016 an absence of oxygen was recorded at the bottom of the water which by December had reached a thickness of 9 meters. In January 2017, remixing cancelled the anoxic layer and the release of phosphorus from the lake bottom ceased. At the same time it distributed the phosphorus that had accumulated at the bottom, clearly demonstrating a continuous increase in concentration. In the future, if the current management of the lake continues, the phosphorus will continue to increase due to the addition of the internal load with the probable repetition of anoxia.
Source: Lago di Bolsena 2016, Associazione Lago di Bolsena e Comitato Bolsena Lago d'Europa, 2016