THE EFFECT OF A HORN SNAIL ON ULVA EXPANSA (CHLOROPHYTA): CONSUMER OR FACILITATOR OF GROWTH?¹

Abstract

To determine whether California horn snails are more likely to be consumers or facilitators of Ulva expansa (Setch) S. & G. growth in estuaries, we conducted manip ulative expm.ments that evaluated algal growth and the movement of N between the water column, algal tissue, and, in the second experiment, sediments. Algae grew poor-4 in the absence of sediments, drawing on their own sequestered N supplies (3.5% of d? weight reduced to < 2 %) and N released by snails and by depleting inorganic N in the water column. There was no evidence of consumption when snail densities ranged from 0 to 900,m-2 (0, 3, 6, and 9 per aquarium), as algal growth was similar for all snail densities, and snail lengths did not increase during the 21-d expm'ment. when sediment was provided, N was depleted in the sediment and enhanced in the algal tissue. As in thejirst experiment, the water column was depleted of inorganic N and enriched with organic N, mostly in the dissolved form. Because both snails and macroalgae often dominate the shallow waters of southern California's lagoons and estuaries, our evidence that the snails are primarily facilitators of algal growth (via transfer of N from sediments to the water column) suggests that snails may play an important role in both food web and N dynamics.

Snail grazing facilitates growth of a bloom-forming alga

“…Herbivores can also positively affect macroalgal growth through the addition of nutrients (Fong et al 1997) and, in a situation similar to the removal of neighboring competitors, grazers can preferentially remove epiphytes that grow on algal blades (Dudley 1992, Littler et al 1995, Kamermans et al 2002, Jormalainen et al 2003, Råberg & Kautsky 2008. In our study, Ulva lactuca growth was greatest when snails grazed directly on the blades; this was true for experiments that simulated subtidal and low tide conditions.…”

“…Similarly, Kamermans et al (2002) observed an increase in U. lactuca growth in the presence of amphipods that consumed diatom films. Increased growth of U. expansa was also observed in the presence of horn snail grazing by Fong et al (1997), who attributed this to horn snail movement within the sediment layer releasing nitrogen to the water column. However, the researchers also observed that the snails were not consuming the U. expansa, but were grazing over the surface of the blade and the tank, assumingly consuming the microalgal and bacterial films growing on these surfaces.…”

“…In addition, grazer preferences can differentially influence which algal species are present in blooms, with less palatable species becoming dominant (Lotze & Worm 2000, Lotze et al 2001, Nelson et al 2008. Further complicating our understanding of herbivore-bloom relationships, herbivores can have a positive effect on bloom species by increasing the amount of nitrogen available (Fong et al 1997) or consuming epiphytes (Kamermans et al 2002).…”

Macroalgal Distribution Patterns and Ecological Performances in a Tidal Coastal Lagoon, with Emphasis on the Non-indigenous Codium Fragile ssp. Tomentosoides

“…These specific facilitation mechanisms were not described for polychaete worms in the review by Bruno & Bertness (Bruno & Bertness 2001). It is likely that summer growth and productivity also is enhanced by D. cuprea, by excretion of nitrogenous rich waste products (Fong et al 1997, Giannotti & McGlathery 2001, although this hypothesis were not tested in our study.…”

“…This was investigated by testing how common D. cuprea is within a shallow US east coast lagoon, how abundant and stable the algal-tube cap assemblage is, how fast the association recovers after different levels of disturbance, and if certain algal species are incorporated into tube caps more often than others. In addition to the investigated physically based links between D. cuprea and macroalgal abundance, nitrogen excretion by D. cuprea may also facilitate the incorporated algae, by stimulating growth (Fong et al 1997, Giannotti & McGlathery 2001, although this physiologically based aspect was not considered in the present study.…”