The limpet Patella vulgata L. is an important microphagous grazer on intertidal rocky
shores of north-west Europe, occurring across the wave exposure gradient. Groups of
P. vulgata were selected at mid-tide level of two exposed shores and two sheltered,
fucoid dominated shores on the Isle of Man, British Isles, and manipulated to equivalent
densities and population structure. The level of grazing activity and growth rate
were determined over a 1-year period. At the same time, the abundance of epilithic
microalgae, measured as the concentration of chlorophyll a, was determined as an estimate
of food supply. Microagal abundance showed a seasonal pattern in both exposed and
sheltered conditions, with higher levels in winter compared to summer. In both seasons,
the microalgal resource was more abundant on the sheltered shore studied. The level
of grazing activity in P. vulgata showed a seasonal pattern on the exposed but not
the sheltered shores. Averaged over the year, grazing activity on the exposed shores
was over double that on sheltered shores. Thus, in sheltered conditions, food supply
for limpets was high and grazing activity low; in exposed conditions, food supply
was low and grazing activity high. The growth rate of P. vulgata, measured as increase
in shell length, showed no significant difference between exposed and sheltered shores.
Growth rate was also determined in P. vulgata at natural densities. Although the overall
density declined with decreasing exposure to wave action, the density per unit area
of grazeable substance was higher in shelter. In these populations, the mean growth
rate was over twice as high on exposed compared to sheltered shores.