Top-down impact through a bottom-up mechanism. In situ effects of limpet grazing on growth, light requirements and survival of the eelgrass Zostera marina

Temporal changes in abundance, size, productivity, resource allocation and light requirements of a subtidal eelgrass (Zostera marina L.) population we re followed for 2 yr after the September 1993 appearance of a previously ra re oval form of the commensal limpet Tectura depicta (Berry) in Monterey Ba y, California, USA, By exclusively targeting the epidermis, limpet grazing impaired photosynthetic performance but left respiratory demand, meristemat ic growth and more than 90 % of the leaf biomass intact, The resulting low P:R ratios of grazed plants raised the light requirements for the maintenan ce of positive carbon balance almost 2-fold relative to healthy ungrazed pl ants and prevented the summertime accumulation of internal carbon reserves. Shoot density in this once-continuously vegetated 30 ha meadow declined fr om more than 50 shoots m(-2) (2230 g fresh wt [FW] m(-2)) to sparse patches supporting an average of 16 shoots m(-2) (380 g FW m(-2)). More than 50 % of the continuously vegetated meadow was converted to bare sand despite amb ient light availability and water temperatures that were favorable for grow th of healthy, ungrazed plants. Plant size declined by 50 % and internal su gar reserves declined more than 4-fold within 6 mo after the appearance of T. depicta, Plant losses were most extensive during winter, when internal c arbon reserves were minimal, The dramatic decline in eelgrass vigor and abu ndance reported here, despite a physical environment that was favorable for healthy eelgrass survival, illustrates the amplification of top-down contr ol by this relatively inconspicuous limpet through a feeding mechanism that specifically impairs photosynthesis, a bottom-up process.