Anthropogenic disturbance of environmental signals retained in massive corals

Samples of Porites coral were removed from seven different reefs (No Name, Hastings, Upolu, Thetford, Batt, Green Island and Brook Islands Reefs), including two from the same reef (Green Island Reef), within Australia's Great Barrier Reef Marine Park. Chemical and morphological data retrieved from these samples were used to investigate historical change in water quality surrounding the growing corals, particularly as this related to the increased use of phosphatic fertiliser on the nearby mainland. The study focused on a narrow section of the continental shelf adjacent to the mouth of the Barron River. In this area coral reefs are close to a mainland significantly altered for anthropogenic purposes. No Name Reef (approximately 300 km north of the study area and reasonably distant from known anthropogenic input) was included as a control. The Brook Islands Reef (approximately 300 km south of the study area) was added for geographic variability. Support data was gained by a two year nutrient monitoring programme of the Barron and Mossman River catchments, and marine waters adjacent to these two river systems. Additional information was acquired by the experimental supplementation of Acropora formosa corals with 2.0 μM, 4.0 μM and 8.0 μM PO₄.\ud \ud Stream phosphate levels responded rapidly to rainfall. During the April 1989 flood period between 710 and 2,850 tonnes of Superphosphate were delivered into the ocean. Estimates vary according to the presumed ratio of soluble to particulate phosphate in the flood plume (e.g. see Cosser 1987 and Brodie & Mitchell 1992). The lower figure is nearly half the total industry estimated fertiliser usage (1810 tonnes) for the Far Northern Statistical Division during 1988. The greater figure is more than a third higher than Australian Bureau of Statistics figures for the same period (Pulsford 1990). The marine monitoring program suggested that nutrient monitoring was an insufficient method of estimating water quality.\ud \ud The phosphate supplementation experiment showed that elevated levels of phosphate hindered calcium carbonate production with considerable alteration to the internal and external morphology of the coral skeleton.\ud \ud Chemical analysis of the Porites samples suggested that intra-reefal variation in the coral skeleton was minor, but with sufficient inconsistencies in the annual record to suggest that localised, micro-environmental conditions exist and should be recognised when attempting to reconstruct environmental records from the coral skeleton. Statistically, the inner- and inner/mid-shelf sample sets were significantly different from each other and from all other sample sets. No significant statistical difference could be distinguished in the mid- to outer-shelf sample sets, but the relative contribution of each element differed for each of these sample sets.\ud \ud The difference between the samples from nearer to shore and those from the outershelf was also apparent in the reconstructed coral inferred environmental record. Strontium-based sea-surface temperature estimates provided an adequate record of historical sea surface temperatures from the mid- to outer-shelf samples (No Name, Hastings, Upolu and Thetford Reefs), but were unreliable for the closer to shore samples (Batt, Green Island and Brook Island Reefs).\ud \ud Similar inconsistencies were noted in associations with climatic variables. Data from the outer-shelf No Name Reef sample correlated with climatic data associated with the Southern Oscillation Index (SOI) while data from the Green Island Reef sample correlated with Mean Sea Level Pressure read at Darwin. These inconsistencies were inherent in the coral-inferred record and were not associated with anthropogenic influence.\ud \ud However, ambiguities in the chemical composition and morphological structure of the Green Island sample were noted post-1950. These inconsistencies were statistically correlated to the use of fertiliser on the nearby mainland. Changes to the internal porosity of the coral skeleton were similar to those noted in the experimental phosphate supplementation programme.\ud \ud The study concludes that chemical and morphological variations in the skeleton of the massive Porites coral are suitable for hindcasting paleoenvironmental conditions laid down in the skeleton at the time of precipitation. The study further concludes that there is significant evidence of anthropogenic influence in the Green Island sample, and that this influence is related to the use of fertilisers on the nearby mainland