Significance of Reference Observation in the Evaluation of Secchi Disk Visibility on the Example of Lakes in the Vicinity of Ełk

The method of measuring transparency of water environments using the Secchi disc is still applied, despite a certain level of subjectivity of obtained results. At present, it should be used as a supplement to more reliable methods of measuring the thickness of the euphotic zone. A serious drawback is the low informative value of isolated measurements of the transparency of water environments, representing different stages of cyclic annual variability. This is also confirmed by the summer visibility measurements in the waters of lakes near Ełk of 2003 and 2005. Reservoirs investigated twice were sometimes characterised by a significant diversity of the observed absolute values of the Secchi disc visibility in particular years. However, treating them as benchmark reservoirs makes it possible to determine relative relationships of optical properties among water environments of all the 21 lakes analysed then. They can be expressed in the form of an ordered sequence of reservoirs with increasingly better optical properties of water environments. The ordered sequence was verified on the basis of the research results of 2006. The observed stability of the position of the benchmark reservoirs with respect to each other makes it possible to abandon treating the observed isolated values of the Secchi disc visibility in other reservoirs as, out of necessity, representative ones of the investigated lakes. The knowledge of the values of scopes of the disc visibility in the waters of the benchmark lakes makes it possible to precisely estimate, against them, this value in less frequently investigated lakes.