According to the Ministry of the Environment:

(our translation translation from the French only web site)

"The transparency of the water varies according to the climatic conditions and plankton abundance, which is an indication of the productivity of the lake. These variations are reflected in the measurements that are made both within a season and from one year to the next. For example, a measure of transparency in calm weather may differ from that obtained after a period where theh water is stirred caused by high winds, especially in shallow lakes. It is therefore normal for the transparency of the water to fluctuate from the beginning to the end of the summer."

"The set of measurements taken over the years makes it possible to document the variability of the transparency of the water of a lake. As long as the data accumulated over the years remain within the limits of normal interannual variability, we speak of stable conditions. Outside these limits, there is significant change or an upward or downward trend. However, it takes several years of data collection to determine the normal variability of a lake."