Salinity reduction from freshwater canal discharge: Effects on mortality and feeding of an urchin (Lytechinus variegatus) and a gastropod (Lithopoma tectum)

We conducted laboratory experiments to study the effects of rapid salinity fluctuation associated with freshwater canal discharge on survivorship and grazing rates of two common herbivores in South Florida seagrass beds, Lytechinus variegatus (Echinodermata) and Lithopoma tectum (Gastropoda). Urchins suffered 100% mortality when subjected to a short-term change in salinity from 36 to 2 to 36‰, a salinity change characteristic of a major canal discharge event. Gastropod survivorship, however, was unaffected by short-term exposure to reduced salinity. Urchins and gastropods both survived exposure to a less severe oscillation in salinity (from 36 to 18 to 36‰). Grazing rates of urchins and snails were affected by the 50% drop in salinity, but the effects were different for each group. Urchins demonstrated decreased grazing while gastropods increased their consumption of algae after exposure to reduced-salinity water. Our results suggest that water management strategies can significantly influence field distributions and feeding of invertebrate grazers in South Florida seagrass beds and other areas where freshwater runoff is controlled by canal locks.