Quantitative evaluation of water-level effects on “regeneration safe-sites” for lakeshore plants in Lake Kasumigaura, Japan

Abstract

Nishihiro, J. and I. Washitani. 2009. Quantitative evaluation of water-level effects on “regeneration safe-sites” for lakeshore plants in Lake Kasumigaura, Japan. Lake Reserv. Manage. 25:217–223. We evaluated the effects of water-level regime on the area of “regeneration safe-sites” for seed germination and initial establishment of seedlings of helophytes occurring in the emergent vegetation zone of Lake Kasumigaura, Japan. Based on the results of previous studies and an original experiment, potential regeneration safe-sites (PRS) for helophytes were defined as sites in which the ground surface was inundated for less than 3 consecutive days from 1 April to 15 May. We estimated and compared the areas of PRS both before and after the implementation of an artificial water-level regime in the mid-1970s. Comparisons were made using data on daily water levels during 1960–1969 and 1996–2005, topographic profiles of the lakeshore in 1967 and 2005, and areas of the emergent vegetation in 1972 and 1997. We estimated that the current area of PRS in the lake has been reduced to 24% of past levels. This reduction was accompanied by both a complete loss of a spring drawdown and a decrease in the area of emergent vegetation. The areas of PRS under various water management regimes indicated that a relatively slight lowering of the spring water level could cause a marked increase in the area of PRS (e.g., a 10-cm decrease in the managed target water level could result in a doubling of the area of PRS). Our results suggest a water-level management policy that includes a spring drawdown can be effective in the restoration of helophyte species diversity through the recovery of PRS.