Functional diversity of mesograzers in an eelgrass–epiphyte system

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. Introduction Marine benthic macrophyte communities are regulated by abiotic conditions, resource availability and food web structure. Small invertebrate consumers, primarily crustacean and gastropod species, are supposed to play a crucial role in controlling ecosystem processes. Most of these ‘‘mesograzers’’ preferentially feed on epiphytic algae and thus, promote seagrass growth and survival by releasing the plants from competition for light and nutrients (Brush and Nixon 2002; Hauxwell et al. 2003; Hughes et al. 2004). Thus, the detrimental effect of eutrophication on macrophyte communities may partially be mitigated by high mesograzer abundance (Williams and Ruckelshaus 1993; Hillebrand et al. 2000; Worm et al. 2000). Furthermore, mesograzers are important in the transfer of primary production to higher trophic levels including commercially important fish species (Edgar and Shaw 1995; Taylor 1998). Historically, mesograzers have been considered as a homogeneous functional group in many studies (Steneck and Watling 1982; Edgar 1990a). They are thought to feed rather unselectively on epiphytic algae and detritus. This view is indirectly corroborated by field experiments demonstrating a rapid compensatory response of mesograzers to manipulation of single mesograzer species abundances (Edgar 1990b; Edgar and Aoki 1993). However, some experimental studies showed a significant species-specific impact of mesograzers on biomass and taxonomic composition of primary producers in macrophyte assemblages (Jernakoff and Nielsen 1997; Duffy and Harvilicz 2001; Duffy et al. 2001; Duffy et al. 2003). A meta-analysis approach (Hughes et al. 2004) found that the effects of grazers were generally strong in seagrass systems, but were highly dependent on grazer species and experimental conditions. In particular, invertebrate Communicated by U. Sommer. S. Jaschinski (&) U. Sommer IFM-GEOMAR Leibniz Institute of Marine Sciences, Düsternbrooker Weg 20, 24105 Kiel, Germany e-mail: [email protected] 123 Mar Biol (2008) 154:475–482 DOI 10.1007/s00227-008-0942-y