Higher biodiversity stabilizes ecosystem functions in fluctuating environments although the relative importance of consumer and prey diversity remained unknown. We tested the effects of salinity decline and community compositions on the consumption of red algae Chondrus yendoi and green algae Ulva sp. by gastropod Lacuna decorata and isopod Idotea ochotensis to examine the roles of grazer and macroalgae diversity under the low salinity condition. We designed an indoor mesocosm experiment with two salinity levels (low at 10 and control at 31), three grazer treatments (Lacuna, Idotea, Lacuna+Idotea), and three macroalgae treatments (Ulva, Chondrus, Ulva+Chondrus) and measured consumption rates, defecation rates, and absorption efficiency. Salinity did not affect the consumption and defecation rates, but both rates declined in Lacuna treatments. Absorption efficiency increased when salinity declined but decreased in Lacuna treatments. It was significantly lower when Idotea fed on Ulva+Chondrus than when Idotea, Lacuna, and Lacuna+Idotea fed on Ulva. Our results illustrate how the presence of multiple grazer species stabilizes the consumption and defecation rates when salinity declines. Significant grazer-macroalgae interaction and salinity effects suggest the importance of both consumer and prey diversities on the absorption efficiency, but biodiversity does not always affect the responses to the low salinity condition.