Effects of copper and reduced salinity on grazing activity and macroalgae production: a short-term study on a mollusc grazer, Trochus maculatus, and two species of macroalgae in the inner Gulf of Thailand

Grazing is an important structuring process in the marine environment, especially in the tropics where macroalgal standing crop is maintained at low levels by intense herbivory, often despite relatively high levels of primary production. There are several empirical examples of phase shifts following reductions in herbivory, where hard coral cover decreases while the abundance of large, fleshy macroalgae increases. In the present work the physiological effects of sublethal levels of copper (20 mu g Cu super(2+) l super(-1)) and lowered salinity (20 psu) were studied on the grazer Trochus maculatus and two species of macroalgae, Gracilaria tenuistipitataand Enteromorpha intestinalis.The study was performed in the inner Gulf of Thailand, a polluted area that frequently experiences salinity drops. The two stress factors were applied both in isolation and in combination to evaluate possible synergistic effects. Stress indices used were scope for growth for the grazer and ratios of gross production to respiration for the algae. This study indicates that moderate disturbance in terms of freshwater runoff and ecologically relevant amounts of the heavy metal copper can reduce grazing activity without a corresponding decrease in macroalgae production.