The conditions in water systems change less over the seasons than the conditions in terrestial ecosystems.
The biggest change is the freezing of waters and the ecological changes caused by it. In small ponds and puddles, the changes between seasons are great; similar to those on land. In running waters, the changes are smaller.
Lakes usually freeze by the middle of October In Lapland. In Central Finland, small lakes freeze at the end of November and the middle parts of big lakes at the end of December. Lakes stay frozen in Lapland for about seven months (e.g. Inarinjärvi 216 days and Kilpisjärvi 228 days) and in Southern Finland for about five months (e.g. Päijänne/Tehi 130 days).