The effect of decreasing artificial feeding on waterfowl was investigated in the artificial pond in Lake Izunuma-Uchinuma by comparing data between 2007/08 and 2008/09. The amount of monthly artificial feeding decreased 80% from 826kg to 42kg. Pintail Anas acuta, the most abundant species, and Whooper Swan Cygnus Cygnus decreased 79% and 74%, respectively. Moreover, the percentage (feeding/Field metabolic rate×100) of energetic dependence on artificial feeding by waterfowl declined 35-102% to 6-58%. Decrease of artificial feeding results in a more severe foraging situation for waterfowl.

The genus Azolla (Azollaceae) is a floating water fern and two species, A. imbricata and A. japonica, are known from Japan. Recently, introduced species of the genus have been naturalized in Japan, but the difficulty of morphological characterization has made them overlooked. Here we report Azolla plants occurring in Lake Izunuma for the first time. Azolla plants were found on the southern coast of Lake Izunuma and covered an approximate 50×5 m area in high density. Morphological examination showed that the plants had both one-cell and two-cell leaf surface trichomes, and seemed to be a hybrid between A. microphyllum and A. filiculoides. Nucleotide sequence data of internal transcribed spacer (ITS) regions in nuclear rDNA showed a composite pattern of A. microphyllum and A. filiculoides sequences, also suggesting the hybrid nature of Azolla plants in Lake Izunuma. Although the hybrid is sterile, the fern’s ability of active vegetative reproduction may have some impacts on native organisms and environments in Lake Izunuma-Uchinuma if stable populations are established. Further studies are warranted.

Submerged plant communities in Lake Izunuma-Uchinuma have declined drastically in the past 30 years and only isolated populations are now observed. We investigated the present status of submerged plant populations in Lake Izunuma-uchinuma and surrounding environments for 2008 and 2009. We found five species of submerged plants (Myriophyllum spicatum L., Ceratophyllum demersum L., Najas oguraensis Miki, Potamogeton octandrus Poir., and Hydrilla verticillata (L.f.) Royle). Ceratophyllum demersum and Myriophyllum spicatum were dominant in 2008 and 2009, respectively. Hydrilla verticillata, a dominant submerged plant in the past, was only found sporadically, and growth density of the species has declined greatly. Two stable populations of submerged plants were found in Izunuma, but no submerged plants were found in Uchinuma in 2009. There were no obvious pattern in underwater visibility of points with submerged plants, except these points have sandy floors. Based on the comparisons with previous investigations, submerged plant communities in Lake Izunuma-Uchinuma experienced both decline and changes of species composition, and further investigations about the causes should be considered.