Functional diversity of mesograzers in an eelgrass–epiphyte system

Abstract

Historically, small invertebrate grazers in marine plant communities have been considered to be a relatively homogeneous group in their impact on ecosystem processes. However, recent studies propose that species composition is an important agent in determining grazer effects. We used four mesocosm experiments to test the biomass-specific and density-dependent effects of common mesograzers in temperate regions (Littorina littorea, Rissoa membranacea, Idotea baltica and Gammarus oceanicus) on epiphyte and eelgrass biomass and productivity. Mesograzer species identity strongly influenced epiphyte accumulation and eelgrass growth, where Rissoa was the most efficient mesograzer (per biomass) and Gammarus had the weakest impact. Density-dependent effects varied considerably among species. Both gastropod species reduced epiphyte accumulation in direct proportion to their density, and Littorina had the strongest negative effect on epiphyte biomass. The impact of Idotea seemed to level off to a threshold value and Gammarus had no density-dependent effect on epiphyte accumulation at all. Rissoa and Idotea increased eelgrass productivity in accordance with their effect on epiphyte accumulation, whereas Littorina showed a less positive effect than could be expected by its strong impact on epiphyte biomass. Gammarus had no significant impact on eelgrass growth. Our results show that the different functional traits of superficially similar mesograzers can have important consequences for ecosystem processes in macrophyte systems.

Seasonal variation in carbon sources of ­mesograzers and small predators in an eelgrass community: stable isotope and fatty acid analyses

“…The seasonal pattern was most pronounced in the small gastropod Rissoa membranacea, in which a reduction from total dependence on epiphytes in spring and autumn to about 45% in summer was observed and agreed with the model of Phillips & Gregg (2003). The decrease in epiphyte abundance caused this herbivorous species to switch to sand microflora as an alternative food source, a behaviour Phillips & Gregg (2003), which could only be calculated from April to October that we also observed in laboratory experiments (Jaschinski & Sommer 2008a). The fatty acid composition of R. membranacea confirmed the conclusions determined from the stable carbon signatures.…”

“…This indicates the capacity of mesograzers to control epiphyte biomass on eelgrass (Hughes et al 2004, Jaschinski & Sommer 2008a. The seasonal pattern was most pronounced in the small gastropod Rissoa membranacea, in which a reduction from total dependence on epiphytes in spring and autumn to about 45% in summer was observed and agreed with the model of Phillips & Gregg (2003).…”

“…By feeding preferentially on fast-growing epiphytic algae, and thus facilitating growth and survival of the seagrass, small invertebrate grazers play an important role in these communities , Jaschinski & Sommer 2008a. Additionally, large parts of the energy flux to higher trophic levels including commercially important fish species are channelled through these consumers (Klumpp et al 1992, Edgar & Shaw 1995.…”

A summer heat wave decreases the immunocompetence of the mesograzer, Idotea baltica

“…Isopods of the genus Idotea are key herbivores within macrophyte meadows (seagrasses and macroalgae) throughout the northern hemisphere (Williams and Ruckelhaus 1993;Jaschinski and Sommer 2008). The study species I. baltica is abundant on macroalgal and seagrass beds in the Baltic Sea, especially on brown algae of the order Fucales and on the seagrass Zostera marina (Franke et al 1999;Hemmi and Jormalainen 2002).…”

“…The study species I. baltica is abundant on macroalgal and seagrass beds in the Baltic Sea, especially on brown algae of the order Fucales and on the seagrass Zostera marina (Franke et al 1999;Hemmi and Jormalainen 2002). Idotea prefers feeding on epiphytes, however, if absent, they feed directly on Fucus and Zostera (Jaschinski and Sommer 2008). I. baltica relies on permanent access to food, in its natural habitat it only rarely experiences phases of food deprivation, and is thus severely aVected when food is temporarily limited ).…”