Grazing rates of organic matter and living fungal biomass of decaying Spartina alterniflora by three species of salt-marsh invertebrates

The pathway for the flow of salt-marsh grass production into marsh food-web s is still not well defined. We compared the abilities of three marsh macro invertebrates [salt marsh periwinkles, Littoraria irrorata (Say) (=Littorin a irrorata), salt-marsh coffee-bean snails, Melampus bidentatus (Say); and a talitrid amphipod, Uhlorchestia spartinophila Bounsfield and Heard] to ac cess standing-dead leaves of smooth cordgrass (Spartina alterniflora Loisel ). The invertebrates were incubated with naturally-decaying leaves, and the rates of removal of organic matter and living fungal biomass (ergosterol) were measured. The impact of invertebrate activity upon fungal growth rates was measured as rates of fungal-membrane synthesis (incorporation of radio acetate into ergosterol). The removal rates of organic leaf biomass per mg individual biomass were highest for amphipods (700 mu g mg(-1) d(-1)) and l owest for periwinkles (90 mu g mg(-1) d(-1)), but the relatively large biom ass of the snails made their removal rates per individual greater than thos e of amphipods. Net removal of ergosterol by all three invertebrates was >5 0% for yellow-brown (early-decay) leaf blades. For fully-brown (advanced-de cay) blades, >50% removal of ergosterol was found only for periwinkles; exp osure to coffee-bean snails and amphipods resulted in a net ergosterol redu ction of less than or equal to 20%. The lower net reduction of living funga l biomass by coffee-bean snails and amphipods may have been due to fungal-g rowth stimulation (2.3-fold stimulation in coffee-bean snails and 1.5-fold stimulation in amphipods). Grazing by periwinkles did not stimulate fungal growth, possibly because of its high intensity. Grazing by these three salt -marsh shredders may affect marsh-grass shoot-decay in different ways. Peri winkles may abbreviate the period of fungal production, and incorporate the decaying material relatively quickly into snail biomass and fecal-pellet r ain to the sediments. Coffee-bean snails and amphipods may enhance and prol ong fungal production, along with the formation of fecal-pellet rain. All t hree invertebrates fed preferentially on leaf blades rather than leaf sheat hs, and feeding rates of gastropods were higher during the night than durin g the day.