Other physical and chemical insults to once-pristine reef environments are more subtle, such as the rising levels of nutrients and toxins in coastal waters caused by runoff from agricultural activities on land and associated increases in sediment delivery to reefs that are further augmented by deforestation, coastal development, construction, mining, drilling and dredging.  It has been difficult to determine just how much these phenomena have varied over the last few centuries.  Recently, however, our knowledge in this area has taken a quantum leap forward with the publication of a study that provides a 250-year record of sediment transfer to Havannah Reef -- a site on the inner Great Barrier Reef of northern Queensland, Australia -- by flood plumes from the Burdekin River (McCulloch et al., 2003).

How was this knowledge obtained?  Sediments suspended in river water contain barium (Ba), which is desorbed from the particles that carry it as they enter the ocean, where growing corals incorporate it into their skeletons along with calcium (Ca).  Hence, when more sediments are carried to the sea by periodic flooding and more gradual longer-term changes in land use that lead to enhanced soil erosion, the resultant increases in sediment load are recorded in the Ba/Ca ratio of coral skeleton material.  Inspired by these facts, McCulloch et al. measured Ba/Ca ratios in a 5.3-meter-long coral core from Havannah Reef that covered the period from about 1750 to 1985, as well as in some shorter cores from Havannah Reef and nearby Pandora Reef that extended the proxy sediment record to 1998.

What did the scientists find?  Prior to the time of European settlement, which began in the Burdekin catchment in 1862, they say there was "surprisingly little evidence for flood-plume related activity from the coral Ba/Ca ratios."  Soon after, however, land clearance and domestic grazing intensified and the soil became more vulnerable to monsoon-rain-induced erosion.  By 1870, baseline Ba/Ca ratios had risen by 30% and "within one to two decades after the arrival of European settlers in northern Queensland, there were already massive impacts on the river catchments that were being transmitted to the waters of the inner Great Barrier Reef."  During subsequent periods of flooding, in fact, the transport of suspended sediment to the reef increased by fully five to ten-fold over what had been characteristic of pre-European settlement times.

In a companion article, Cole (2003) reports that corals from East Africa "tell a similar tale of erosion exacerbated by the imposition of colonial agricultural practices in the early decades of the twentieth century."  There, similar coral data from Malindi Reef, Kenya, indicate "a low and stable level of barium before about 1910 which rises dramatically by 1920, with a simultaneous increase in variance," a phenomenon that was also evident in the Australian data.

What are the implications of these observations?  Cole concludes that "human activity, in the form of changing land use, has added sedimentation to the list of stresses experienced by reefs."  Furthermore, as land-use intensification is a widespread phenomenon, she notes that "many reefs close to continents or large islands are likely to have experienced increased delivery of sediment over the past century," which suggests that the stress levels produced by this phenomenon are likely to have increased over the past century as well.  In addition, Cole logically concludes that as coastal populations continue to rise, "this phenomenon is likely to expand."

We believe the historical and ongoing worldwide increase in sediment-induced stress -- which includes the debilitating effects of various nutrients and toxins of anthropogenic origin that are intimately associated with, and even carried by, the sediments -- is what is predisposing today's corals to bleach more readily than they did in the past in response to periodic increases in water temperature.  Perhaps it is also responsible for the historical increase in emerging diseases in coral reef ecosystems that has been studied by Hayes and Goreau (1998), which they attribute (falsely, we believe) to global warming.  These latter scientists note, for example, that "some of the emerging coral diseases may be explained by a decline in the capacity of coral colonies to mount effective protection against the increasing prevalence and varied invasive strategies of marine pathogens."  This reduced defense ability would seem to be equally well, if not better, explained as a consequence of the heightened and ever-increasing level of stress produced by the ever-increasing coastal concentrations of sediments and their associated nutrients and toxins that have become a fact of modern life throughout the entire world.

References
Cole, J.  2003.  Dishing the dirt on coral reefs.  Nature 421: 705-706.

Hayes, R.L. and Goreau, N.I.  1998.  The significance of emerging diseases in the tropical coral reef ecosystem.  Revista de Biologia Tropical 46 Supl. 5: 173-185.

Idso, S.B., Idso, C.D. and Idso, K.E.  2000.  CO2, global warming and coral reefs: Prospects for the future.  Technology 75S: 71-93.

McCulloch, M., Fallon, S., Wyndham, T., Hendy, E., Lough, J. and Barnes, D.  2003.  Coral record of increased sediment flux to the inner Great Barrier Reef since European settlement.  Nature 421: 727-730.