Spatial and temporal variation in grazing damage by the gastropod Lacuna vincta in Nova Scotian kelp beds

Population increases of the gastropod Lacuna vincta have been associated with significant damage to kelp blades and decreases in kelp biomass in subtidal kelp beds off Nova Scotia, Canada. We measured the total level and along-blade distribution of grazing damage by Lacuna vincta on the dominant kelp species at 5 sites in Nova Scotia, Canada, over a 15 mo period. Grazing was typically low or absent in the basal regions of blades, consistent with seasonal fluctuations in growth and physical properties of blade tissues. Grazing was largely concentrated in middle and distal sections, although this distribution varied with site exposure and over time. The cover of the invasive bryozoan Membranipora membranacea on the surface of kelp blades did not have a strong effect on grazing by L. vincta. The total level of grazing damage (max. 1% of blade area for Saccharina longicruris and 1.5% for Laminaria digitata) varied seasonally, with peaks in September at some sites. Spatial variation was driven in part by a negative relationship with site exposure. In a field experiment, simulated grazing damage that exceeded a threshold value of 0.5 to 1.0% of blade area caused a significant increase in blade loss during a period of heavy wave action due to a passing hurricane. Our results show that direct reductions in kelp biomass through grazing by L. vincta are relatively small, but can indirectly lead to significant losses of kelp biomass during large wave events.