The Gross growth efficiency (GGE) is defined as the amount of consumer carbon produced relative to the amount of prey ingested in a given time interval.In population ecology,it can be considered as an index for the ability of a consumer to convert the biomass of food into its population production.In this study,it was firstly applied to estimate the role of amphipods grazing in the biomass dynamics of macroalgae (Ulva lactuca) in Yundang Lagoon (a subtropical eutrophic coastal lagoon,Xiamen City,China).It was found that amphipods were the dominant group (82.2%~98.7%) in the benthic assemblage,in which five amphipod species (Corophium uenoi,Ampithoe valida,Grandidierella japonica,Grammaropsis laevipalmata and Caprella panantis) were observed in the lagoon.However,only A.valida significantly consumed U.lactuca among these amphipod species.A series of experiments were conducted to estimate the consumption of U.lactuca by A.valida.It was suggested that the monthly secondary production (P) of A.valida using size frequency method ranged from 2.39 to 13.68 g DW/m2.GGE of A.valida exponentially reduced with body size and was closely associated with water temperature.Therefore,GGE equation for different months using the temperature recorded during the field investigation was fitted.The monthly consumption of U.lactuca was figured out to be 86.7-1549.0 g DW/m2 by the sum of consumption (P/GGE) by A.valida at different size,accounting for 2.0-37.5%of U.lactuca production.It indicates that the grazing effect of A.valida did play a role in reducing the accumulation of U.lactuca biomass when A.valida was abundant,but it finally fails to limit the U.lactuca growth in Yundang Lagoon.Low abundance of A.valida and high nutrient loading may be two major reasons for the outbreak of U.lactuca bloom in the lagoon.