Proceedings 9 th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000 Growth response of the reef coral Acropora longicyathus to elevated inorganic nutrients: do responses to nutrients vary among coral taxa? D. J. Bucher 1 and P. L. Harrison 2 ABSTRACT Rates of linear extension and changes in buoyant weight were measured for transplanted colonies of Acropora longicyathus exposed to elevated concentrations of ammonium and/or phosphate in the ENCORE experiment (One Tree Island, Great Barrier Reef). Linear extension was significantly faster in elevated phosphate treatments compared with controls. Rates of buoyant weight change were significantly increased by elevated ammonium on an annual basis, but significant reductions occurred in winter. Phosphate significantly increased the rate of buoyant weight change in some seasons but had no significant effect on an annual basis. Caution is therefore advised when extrapolating short-term growth data to represent annual trends. Chlorophyll a content of phosphate-treated corals was significantly higher than controls. Unlike other coral taxa in previous studies, elevated chlorophyll a content in Acropora longicyathus in this study did not correlate with reduced calcification. Growth of Acropora corals may be less sensitive to ‘clean’ (i.e. unaccompanied by reduced salinity, elevated sediments, or other pollutants) elevated inorganic nutrient concentrations than growth of other coral taxa used in nutrient enrichment studies to date, and wider use of this cosmopolitan genus in manipulative experiments is recommended. 1 School of Environmental Science and Management, Southern Cross University, P.O. Box 157, Lismore NSW, Australia. 2480. email: dbucher@scu.edu.au 2 School of Environmental Science and Management, Southern Cross University, P.O. Box 157, Lismore NSW, Australia. 2480 Keywords Acropora, Growth, Nutrients, Linear extension, Buoyant weight, Chlorophyll a Introduction Growth rate has been a widely used indicator of coral health (Buddemeier and Kinzie 1976). Two measures of growth, linear extension and changes in buoyant weight, have been used extensively in comparative studies, although rarely at the same time (Brown et al. 1985). Linear extension and increases in buoyant weight of corals have both been shown to be responsive to elevated nutrient concentrations. In aquarium experiments, elevated phosphate concentrations elicited an increase in extension of Acropora formosa branches (Rasmussen and Cuff 1990). Maté (1997) reported reduced linear extension when several coral species were experimentally exposed to a combination of elevated nitrate and phosphate. Stambler et al. (1991) found no effect of phosphate on Stylophora pistillata, but measured decreased linear extension in elevated ammonium treatments. Most field studies have shown a decrease in annual extension rates of corals associated with higher nutrient concentrations (e.g. Tomascik and Sander 1985, Tomascik 1990; Hudson et al. 1994). Most studies of buoyant weight changes have also measured decreased coral growth in more polluted environments (Davies 1989, 1990) or in elevated nutrient treatments in aquaria (Marubini and Davies 1996, McGuire and Szmant 1997). Field studies of the effects of eutrophication have rarely controlled for other environmental influences. Controlled aquarium manipulations of nutrient concentrations indicate that the effect of elevated nutrients may depend upon the type and ratio of nutrients that are added, and on the coral taxa used. The ENCORE manipulative field experiment (Steven and Larkum 1993, Larkum and Steven 1994) provided a unique opportunity to examine the growth responses of corals to elevated nutrients in situ, whilst controlling for other variables that might affect growth. In the present study, linear extension and buoyant weight changes of transplanted portions of A. longicyathus under conditions of elevated ammonium, phosphate, or both nutrients, were compared with controls. The chlorophyll a content of tissues immediately below branch apices was also measured so that coral growth rates could be related to photosynthetic activity of adjacent zooxanthellate tissues. Methods The experimental design of ENCORE has been described in detail elsewhere (e.g. Larkum and Steven 1994) and will only be briefly outlined here. Automated nutrient dispensing units were used to add concentrated nutrient solution to micro-lagoons within patch reefs in the main lagoon of One Tree Reef, Great Barrier Reef (Lat. 23°30’ Long. 152°06’). Three patch reefs remained unaltered as controls, three received ammonium (N-only), three received phosphate (P-only) and both nutrients were added to a further three patch reefs (N+P). Data presented here were collected during the ‘high-dose’ treatment period (see Koop et al.2001) when nutrient concentrations were elevated to approximately 20 times background levels (20μM NH 4 + , 4μM PO 3 2+ ) every low tide for 406 days. Sixty colonies of the branching reef coral Acropora longicyathus from around the main lagoon were used to provide sub-colonies for transplant into the micro- lagoons. Five colonies were randomly assigned to each patch reef. Transplanted sub-colonies were supported on racks made from PVC floor tiles on short (~10 cm) lengths of PVC pipe. Initially one rack was used percolony, but the racks were extended to accommodate the colonies as they grew during the study. All data sets