ENCORE: The effect of nutrient enrichment on coral reefs. Synthesis of results and conclusions

Coral reef degradation resulting from nutrient enrichment of coastal waters is of increasing global concern. Although effects of nutrients on coral re ef organisms have been demonstrated in the laboratory, there is little dire ct evidence of nutrient effects on coral reef biota in situ. The ENCORE exp eriment investigated responses of coral reef organisms and processes to con trolled additions of dissolved inorganic nitrogen (N) and/or phosphorus (P) on an offshore reef(One Tree Island) at the southern end of the Great Barr ier Reef, Australia. A multi-disciplinary team assessed a variety of factor s focusing on nutrient dynamics and biotic responses. A controlled and repl icated experiment was conducted over two years using twelve small patch ree fs ponded at low tide by a coral rim. Treatments included three control ree fs (no nutrient addition) and three + N reefs (NH4Cl added), three + P reef s (KH2PO4 added), and three + N + P reefs. Nutrients were added as pulses a t each low tide (ca twice per day) by remotely operated units. There were t wo phases of nutrient additions. During the initial, low-loading phase of t he experiment nutrient pulses (mean dose = 11.5 muM NH4+; 2.3 muM PO4-3) ra pidly declined, reaching near-background levels (mean = 0.9 muM NH4+; 0.5 m uM PO4-3) within 2-3 h. A variety of biotic processes, assessed over a year during this initial nutrient loading phase, were not significantly affecte d, with the exception of coral reproduction, which was affected in all nutr ient treatments. In Acropora longicyathus and A. aspera, fewer successfully developed embryos were formed, and in A. longicyathus fertilization rates and lipid levels decreased. In the second, high-loading, phase of ENCORE an increased nutrient dosage (mean dose = 36.2 muM NH4+; 5.1 muM PO4-3 declin ing to means of 11.3 muM NH4+ and 2.4 muM PO4-3 at the end of low tide) was used for a further year, and a variety of significant biotic responses occ urred. Encrusting algae incorporated virtually none of the added nutrients. Organisms containing endosymbiotic zooxanthellae (corals and giant clams) assimilated dissolved nutrients rapidly and were responsive to added nutrie nts. Coral mortality, not detected during the initial low-loading phase, be came evident with increased nutrient dosage, particularly in Pocillopora da micornis. Nitrogen additions stunted coral growth, and phosphorus additions had a variable effect. Coral calcification rate and linear extension incre ased in the presence of added phosphorus but skeletal density was reduced, making corals more susceptible to breakage. Settlement of all coral larvae was reduced in nitrogen treatments, yet settlement of larvae from brooded s pecies was enhanced in phosphorus treatments. Recruitment of stomatopods, b enthic crustaceans living in coral rubble, was reduced in nitrogen and nitr ogen plus phosphorus treatments. Grazing rates and reproductive effort of v arious fish species were not affected by the nutrient treatments. Microbial nitrogen transformations in sediments,were responsive to nutrient loading with nitrogen fixation significantly increased in phosphorus treatments and denitrification increased in all treatments to which nitrogen had been add ed. Rates of bioerosion and grazing showed no significant effects of added nutrients, ENCORE has shown that reef organisms and processes investigated ill situ we re impacted by elevated nutrients. Impacts mere dependent on dose level, wh ether nitrogen and/or phosphorus mere elevated and were often species-speci fic. The impacts were generally sub-lethal and subtle and the treated reefs at the end of the experiment mere visually similar to control reefs. Rapid nutrient uptake indicates that nutrient concentrations alone are not adequ ate to assess nutrient condition of reefs. Sensitive and quantifiable biolo gical indicators need to be developed for coral reef ecosystems. The potent ial bioindicators identified in ENCORE should be tested in future research on coral reef/nutrient interactions. Synergistic and cumulative effects of elevated nutrients and other environmental parameters, comparative studies of intact vs. disturbed reefs, offshore vs, inshore reefs, or the ability o f a nutrient-stressed reef to respond to natural disturbances require eluci dation. An expanded understanding of coral reef responses to anthropogenic impacts is necessary, particularly regarding the subtle, sub-lethal effects detected in the ENCORE studies. (C) 2001 Published by Elsevier Science Ltd .