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

To determine whether California horn snails are more likely to be cons umers or facilitators of Ulva expansa (Setch) S. & G. growth in estuar ies, we conducted manipulative experiments that evaluated algal growth and the movement of N between the water column, algal tissue, and, in the second experiment, sediments. Algae grew poorly in the absence of sediments, drawing on their own sequestered N supplies (3.5% of dry w eight reduced to <2%) and N released by snails and by depleting inorga nic N in the water column. There was no evidence of consumption when s nail 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 leng ths did not increase during the 21-d experiment. When sediment was pro vided, N was depleted in the sediment and enhanced in the algal tissue . As in, the first experiment, the water column was depleted of inorga nic N and enriched with organic N, mostly in the dissolved form. Becau se both snails and macroalgae often dominate the shallow waters of sou thern California's lagoons and estuaries, our evidence that the snails are primarily facilitators of algal growth (via transfer of N from se diments to the water column) suggests that snails may play an importan t role in both food web and N dynamics.