What was done
Annual density, linear extension and calcification rates were determined from several coral cores retrieved from 35 sites at the Great Barrier Reef from 9° to 23°S, providing information about long-term variability in coral growth there since the late 15th century.

What was learned
All three of the parameters studied showed a general decrease in values from the north to the south.  Temporally, the authors' results indicated that growth characteristics for Porites are "highly variable," with extension and calcification rates varying "as much as ±20-30% about the mean, both from year to year and over the 10-30-year periods."  The temporal variability of density was much less at ±10% about the mean, both inter-annually and over 10-30 year periods.  The authors also note that their data showed frequent and extended periods of coral growth characteristics that were above or below the long-term mean, cautioning that "it would be unwise to rely on short-term values (say averages over less than 30 years) to assess mean conditions," adding it would be "rash to compare one year's value with another and it would be reckless to compare individual years in different decades without analyzing the long-term trends."

Analysis of average annual calcification rates revealed a statistically significant correlation with available sea surface temperature records, suggesting (though not confirming) that long-term variations in Porites calcification rates on the Great Barrier Reef are driven by variations in sea surface temperatures.  Using this relationship, the authors calculated that a rise in sea surface temperature from 20° to 21°C would increase the calcification rate by about 3.5%.

What it means
Because calcification, extension and density were shown to decrease with increasing latitude (or with decreasing water temperature), the authors suggested that calcification rate in Porites may not be an acceptable proxy for assessing the health of coral reef communities.  "Unfortunately," they note, "declines in calcification in Porites on the Great Barrier Reef over recent decades have been interpreted to be as a result of degradation of the marine environment."  And true it is, their record indicates a recent decline in calcification rates for this region.  However, their data show that "a decline in calcification equivalent to the recent decline occurred earlier this century and much greater declines occurred in the 18th and 19th centuries."  Furthermore, analyses of annual density banding from the coral samples "indicate that the 20th century has witnessed the second highest period of above average calcification in the past 237 years."  As a result, the authors conclude that "the observed decline in coral growth in recent decades may be, simply, a return to more 'normal' conditions."