TOP-DOWN IMPACT THROUGH A BOTTOM-UP MECHANISM - THE EFFECT OF LIMPET GRAZING ON GROWTH, PRODUCTIVITY AND CARBON ALLOCATION OF ZOSTERA-MARINA L (EELGRASS)

The unusual appearance of a commensal eelgrass Limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot(-1) ) has coincided with the catastrophic decline of a subtidal Zostera ma rina L. meadow in Monterey Bay, California. Some commensal limpets gra ze the chloroplast-rich epidermis of eelgrass leaves, but were not kno wn to affect seagrass growth or productivity, We evaluated the effect on eelgrass productivity of grazing by limpets maintained at natural d ensities (8+/-2 shoot(-1)) in a natural light mesocosm for 45 days. Gr owth rates, carbon reserves, root proliferation and net photosynthesis of grazed plants were 50-80% below those of unrated plants, but bioma ss-specific respiration was unaffected. The daily period of irradiance -saturated photosynthesis (H-sat) seeded to maintain positive carbon b alance in grazed plants approached 13.5 h. compared with 5-6 h for ung razed plants, The amount of carbon allocated to roots of ungrazed plan ts was 800% higher than for grazed plants. By grazing the chlorophyll- rich epidermis, T. depicta induced carbon limitation in eelgrass growi ng in an otherwise light-replete environment. Continued northward move ment of T. depicta, may have significant impacts on eelgrass productio n and population dynamics in the northeast Pacific, even thought this limpet consumes Fiery little plant biomass, This interaction is a dram atic example of top-down control (grazing/predation) of eelgrass produ ctivity and survival operating via a bottom-up mechanism (photosynthes is limitation).