T. Elfwing ® M. Tedengren 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 Received: 10 August 2001 /Accepted: 29 November 2001 / Published online: 27 February 2002 Ó Springer-Verlag 2002 Abstract 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 empir- ical 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 lg Cu 2+ l –1 ) and lowered salinity (20 psu) were studied on the grazer Trochus maculatus and two species of macroalgae, Gracilaria tenuistipitata and Enteromorpha intestinalis. The study was per- formed 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 macro- algae production. Introduction Degrading reefs often undergo a ‘‘phase shift’’ where hard coral cover decreases while the abundance of large, fleshy macroalgae increases, and numerous such exam- ples have been reported (e.g. Done 1992; Hughes 1994). Two factors have received special focus in explaining increased abundance of coral reef macroalgae, namely, water quality, particularly increases in nutrients (i.e. eutrophication), and reduced herbivory (i.e. overfishing; for example, Littler and Littler 1984; Szmant 1997). In a review, McCook (1999) argues that despite the wide- spread assumption that oversupply of nutrients is a sufficient single cause for macroalgal overgrowth, this is poorly supported by empirical data. However, there are now numerous reports showing that under most cir- cumstances the macroalgal standing crop is maintained at low levels by intense herbivory, often despite rela- tively high levels of primary production (e.g. Steneck 1988; Carpenter 1997), with several empirical examples of phase shifts following reductions in herbivory (e.g. Caribbean Diadema die-offs, Carpenter 1990; Hughes 1994). Due to grazing, the standing crop of macroalgae in coral reef ecosystems is 1–4 orders of magnitude smaller than in temperate coastal macroalgae beds (Hatcher and Larkum 1983). This transfer between pri- mary producers and herbivores is accepted as one of the largest trophic fluxes on coral reefs (Hatcher 1981, 1988). Empirical data indicate that even in areas with low herbivore abundance, large differences in algal tissue production are closely followed by changes in herbivore consumption (Hatcher and Larkum 1983; Carpenter 1986). Factors that increase algal production are thus unlikely to change the standing crop in most reef habi- tats, whereas factors influencing herbivore consumption usually will (Hatcher and Larkum 1983; McCook 1999). The shallow inner Gulf of Thailand is exposed to reduced salinity (Moberg et al. 1997) and increasing pollution by, for example, heavy metals from domestic, industrial, and agricultural sources (Menasveta and Cheevaparanapiwat 1981; Phillips and Tanabe 1989). Large amounts of freshwater are delivered by four major rivers and cause frequent salinity drops from 30 practical salinity units (psu) (ambient) to well below 20 psu (Menasveta and Hongskul 1988) and occasionally to levels as low as 10 psu for short periods of time (Moberg Marine Biology (2002) 140: 913–919 DOI 10.1007/s00227-001-0763-8 Communicated by G.F. Humphrey, Sydney T. Elfwing (&) ® M. Tedengren Department of Systems Ecology, Stockholm University, 106 91 Stockholm, Sweden E-mail: tina@system.ecology.su.se Fax: +46-8-158417