In shallow lakes, macrophytes may improve water quality by keeping phytoplankton at low densities. In the southern part of Lake Biwa, a rare occurrence of severe drought in 1994 dropped the water level maximally down to -123 cm below the standard level. This drought event improved transparency and light condition of lake water, resulting in the excessive flourish of macrophytes. Massive amount of floating macropyhtes drifted against the lake shore and spoiled, giving off a bad smell and entangled boat screws. Since 2011 the local government started a macrophyte reaping project. To examine the efficacy of the project our research group tracked concentration of macrophytes using a fish finder in 2010 (before the reaping), 2012 and 2013 (after the reaping). We set 10 census lines, separated by 1,600 m across the southern part of Lake Biwa. We recorded the macrophyte community height, depth and type (Hydrilla verticillata, Potamogeton maackianus and mixed community) every 40 meters on the fish finder charts. We estimated the biomass for each point and calculated the biomass of an 800 meter area based on the average biomass of three types of dominant species and made a biomass map GIS. In addition, we calculated the PVI (percent volume infested by macrophytes) in the vertical water column by factoring in the water depth and community height. The estimated total biomass and the mean PVI were 11,850 t and 32.8%, 4,236 t and 17.1%, and 7,873 t and 25.7% in 2010, 2012 and 2013, respectively. These results showed that both biomass and PVI dropped greatly from 2010 to 2012, but recovered to some extent by 2013.The big drop in biomass observed in 2012 and the resultant recovery in 2013 may be explained by the reaping treatment and phytoplankton density, the latter of which was high in 2012 and low in 2013 in terms of chlorophyll A concentration. This suggests that reaping is effective to suppress excessive macrophyte growth specifically when phytoplankton density is higher than a threshold value.
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