Top-down impact through a bottom-up mechanism: In situeffects of limpet grazing on growth, light requirements and survival of the eelgrassZostera marina

Temporal changes in abundance, size, productivity, resource allocation and light requirements of a subtidal eelgrass (Zostera marinaL.) population were followed for 2 yr after the September 1993 appearance of a previously rare oval form of the commensal limpetTectura depicta(Berry) in Monterey Bay, California, USA. By exclusively targeting the epidermis, limpet grazing impaired photosynthetic performance but left respiratory demand, meristematic growth and more than 90% of the leaf biomass intact. The resulting lowP:Rratios of grazed plants raised the light requirements for the maintenance of positive carbon balance almost 2-fold relative to healthy ungrazed plants and prevented the summertime accumulation of internal carbon reserves. Shoot density in this once-continuously vegetated 30 ha meadow declined from 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 ambient light availability and water temperatures that were favorable for growth of healthy, ungrazed plants. Plant size declined by 50% and internal sugar reserves declined more than 4-fold within 6 mo after the appearance ofT. depicta. Plant losses were most extensive during winter, when internal carbon reserves were minimal. The dramatic decline in eelgrass vigor and abundance reported here, despite a physical environment that was favorable for healthy eelgrass survival, illustrates the amplification of top-down control by this relatively inconspicuous limpet through a feeding mechanism that specifically impairs photosynthesis, a bottom-up process.