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

Abstract

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. KEY WORDS: Lacuna vincta · Laminaria digitata · Saccharina longicruris · Grazing · Mesograzers · Kelp beds · Detritus · Membranipora membranacea Resale or republication not permitted without written consent of the publisherAquat Biol 13: 163-173, 2011 164 na vincta is the only mesograzer known to consume macroscopic kelp sporophytes (Brady-Campbell et al. 1984, Johnson & Mann 1986. On the Atlantic coast of Nova Scotia, L. vincta is seasonally abundant in kelp beds, with peak densities (up to 2000 individuals m −2 ) during recruitment in February, and seasonal lows between August and October (John son & Mann 1986). L. vincta preferentially grazes kelps (Chavanich & Harris 2002), creating perforations or superficial excavations on the surface of blades. Johnson & Mann (1986) estimated that the cumulative effect of grazing by L. vincta was a removal of only 0.05% of standing blade biomass of Saccharina (=Laminaria) longicruris in a kelp bed in St. Margarets Bay, Nova Scotia. However, population explosions of L. vincta in the Gulf of Maine have been associated with high levels of damage to kelp blades and canopy loss (Fralick et al. 1974). linked variation in the rate of kelp blade fragmentation and erosion to damage along the distal ends of blades, suggesting that grazing by L. vincta can indirectly decrease kelp biomass and increase detrital production.Spatial and temporal variation in grazing damage to kelps and other macroalgae in shallow marine habitats is caused by various abiotic and biotic factors. Wave exposure generally has a negative effect on gr...

Production and fate of kelp detritus

“…For example, grazing damage by snails and sea urchins on kelp stipes and holdfasts is linked to stipe breakage and holdfast dislodgement (Koehl & Wainwright 1977, Biedka et al 1987, Duggins et al 2001. Per forations generated by snails reduce the strength of kelp blade tissues (Black 1976, Krum hansl et al 2011, which accounts, in part, for seasonal variation in erosion rate (Krumhansl & Scheibling 2011a) and loss of kelp biomass during storms (Krumhansl & Scheibling 2011b). Encrustation of kelp blades and stipes by bivalves and bryozoans also increases drag and induces flaws in blade tissues that lead to dislodgement and breakage (Brady-Campbell et al 1984, Brown et al 1997, Krumhansl & Scheibling 2011a, Krumhansl et al 2011.…”

Positive feedback between large-scale disturbance and density-dependent grazing decreases resilience of a kelp bed ecosystem

“…However, we expect increased grazing intensity at low kelp abundance to indirectly enhance the loss of biomass from individual thalli. Grazing damage by L. vincta increases the erosion rate of tissue from the distal end of kelp blades (Krumhansl & Scheibling 2011a) and causes considerable blade fragmentation from S. latissima during large wave events if grazing intensity exceeds a threshold at 0.5−1% (Krumhansl & Scheibling 2011b). Our findings suggest that the removal of kelp by hurricane damage and recurrent outbreaks of Membranipora membranacea may intensify the direct impact (i.e.…”

“…2A,B). Measurements were taken at Duncan's Cove Protected, Duncan's Cove Exposed, Splitnose Point, Cranberry Cove and The Lodge in September in both 2008 and 2009 at the peak of grazing intensity (Krumhansl & Scheibling 2011b). Grazing damage was measured on 10 to 23 haphazardly collected thalli of each species (> 25 cm blade length), except for L. digitata at Cranberry Cove, which was not measured in either sampling period because it was rare at this site.…”

“…7), and this relationship was non-significant (Table 2). If grazing intensity on S. latissima by L. vincta exceeds a threshold at 0.5−1%, there is a stepwise increase in blade fragmentation with large wave events (Krumhansl & Scheibling 2011b). The increase in grazing intensity with decreasing standing kelp biomass in our study exceeded even the more conservative 1% threshold (Fig.…”

Seasonal variation in kelp phlorotannins in relation to grazer abundance and environmental variables in the Alaskan sublittoral zone

“…The perennial kelp species may persist through the winter as complete individuals or as stipes (O'Clair and Lindstrom 2000, Chenelot 2003). The gastropod grazer L. vincta is seasonally abundant and has been reported to have significant grazing impact on canopy (Duggins et al 2001, Chenelot 2003, Carney et al 2005) and understory kelp thalli (Fralick et al 1974, Johnson and Mann 1986, Krumhansl and Scheibling 2011. The same kelp individuals observed for gastropod densities were then removed from the substrate below the holdfast and kept in cold seawater in the dark for transport (n = 3 thalli per species).…”