Seasonal variations in the trophic relationship between the scyphomedusa Aurelia aurita s.l. and mesozooplankton in a eutrophic brackish-water lake, Japan

Abstract

The seasonal variations in trophic relationship between the moon jelly Aurelia aurita s.l. and mesozooplankton were investigated in a brackish-water lake, Honjo District, Japan from June 2005 to August 2006. The medusae occurred abundantly (average abundance and biomass: 0.55 medusae m-3 and 58.8 mg Cm-3, respectively) during warm seasons (i.e. June-November, 2005), but were very scarce or absent during the remaining seasons. The mesozooplankton biomass fluctuated from 1.3 to 150 mg Cm-3 (overall average: 60.5 mg Cm-3) irrespective of the medusa biomass variation. Mesozooplankton were preyed upon by medusae almost non-selectively; the small copepod Oithona davisae and bivalve larvae were the predominant prey, comprising 52-99% (average: 85%) of the gastric pouch contents. The medusa population ingestion rate on mesozooplankton varied from 0.11 to 12.8 mg Cm -3d-1, which corresponded to 0.6 to 29% of the mesozooplankton biomass per day and to 1.6 to 47% of mesozooplankton daily production rate. A. aurita medusae were certainly a key component of the zooplankton community, but they did not exert any significant top-down control as to suppress mesozooplankton biomass in this eutrophic lake. © The Plankton Society of Japan.