Wakasakanai Sand-Dune-Mire, northern Hokkaido, Japan hold more than a hundred of lakes. Analyses for aerial photographs taken in 2005 and 2009 revealed that some lakes have decreased in open water area. To clarify the relationship between decrease in open water area and vegetation, we investigated the relationship between submergence duration and species covers at three shrunk lakes and three stable lakes in terms of the open water areas.

Two distribution patterns along the gradient of submergence duration were observed. Pattern 1: occur in only constant-submergence site in both shrunk and stable lakes probably because of their physiological limitations (e.g. submerged Utricularia spp., floating-leaved Nuphar spp., and some emerged plants such as Schoenoplectus tabernaemontani). Pattern 2: frequent and abundant in rare/non-submerged site in shrunk lakes although can occur from non-submerged to constant-submerged site in nature.

If the lake shrinkages in aerial photographs indicated water level drawdown, increase in rare-submerged area would result in distribution pattern 2. And species of pattern 1 would have disappeared in the site where the submergence duration became shorter. As a result, distributions were differentiated between species of pattern 1 and 2 in shrunk lakes.