Sediment is bad news for coral reefs. In Australia, large increases in sedimentation were a by-product of introducing intensive agricultural practices, and that may also apply to other parts of the world.
Modern agriculture has transformed Earth's surface, and one consequence has been the increase in continental material that is eroded and transported to the oceans. There, it poses particular hazards to reef-building corals, by decreasing the availability of light and interfering with feeding. If sediment delivery to coral reefs can be evaluated and attributed to specific processes, then the effects of anthropogenic erosion on reefs can be quantified and managed more effectively. In a study described on page 727 of this issue, McCulloch and others1 have cleverly developed a history of sedimentation on Australia's Great Barrier Reef by quizzing the corals themselves. Their results show that sedimentation on the reef increased dramatically following European settlement and agricultural expansion in northeast Australia.
Corals preserve a history of terrestrial sediment delivery because, as they build their skeletons from calcium and carbonate, trace amounts of other elements are incorporated. Suspended sediments in river water contain barium, which desorbs from these particles as the river water enters the ocean; during periods of high discharge, levels of the element rise in coastal waters2. As a coral grows, it inadvertently incorporates barium into its skeleton, in rough proportion to the ambient concentration. The skeletons grow at 1–2 centimetres per year, and annual bands allow ages to be assigned to the skeletal material. By measuring the amount of barium (normalized to calcium) in the skeleton, McCulloch et al.1 have developed an effective proxy record of terrestrial sedimentation on the reef that extends from ad 1750 to ad 1985. Their record shows that the influx of sediment onto the reef increased after 1870, shortly following European settlement. Both average and maximum values of barium are significantly higher after this time.
Could these changes in coral barium content be a consequence of other factors? Although such changes can reflect the upwelling of barium-rich deep waters3, this process is not significant at the inner-reef site analysed by McCulloch and colleagues. A climate-driven increase in the frequency and intensity of flooding, independent of land-use change, might also cause variations in barium. But the authors use additional evidence from the coral skeletons (fluorescent banding, strontium content and oxygen isotopic content4,5) to show instead that, before 1870, flood waters still influenced the reef but contained much less barium. As domestic grazing and land clearance intensified after 1870, soil became more vulnerable to erosion during monsoon rains, and the barium content of river waters increased. From the change in the relationship between river discharge and barium calculated from coral data, the authors infer a five- to tenfold increase in the transport of suspended sediment to the reef following European settlement. This study is based on a single core, and reproducibility would add confidence to these quantitative estimates. But the basic conclusion that sedimentation has increased and become more variable is almost certainly robust.
Such results are not unique to the Great Barrier Reef. In East Africa, corals tell a similar tale of erosion exacerbated by the imposition of colonial agricultural practices in the early decades of the twentieth century. Unpublished data, collected by myself and colleagues on coral 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. This interval coincides with documented increases in soil erosion in Kenya during the early twentieth century6.
In both the Kenyan and Australian records, the increase of variance following land-use change is one of the most striking features. Recent maxima of barium reach two to three times the levels of eighteenth- and nineteenth-century maxima. The increased variance probably occurs because the soil is more easily eroded by intense rains under modern agricultural regimes, so a given amount of rainfall can mobilize more sediment (Fig. 1). These changes in variance are important for corals, which are likely to be sensitive to maximum sediment levels. In East Africa, modern sedimentation patterns appear to influence the distribution of reef species7. But we lack ecological data from before the transition in sedimentation regimes that would allow us to understand the full impact of these changes.
In both locations, increased river discharge is associated with extreme states of the El Niño–Southern Oscillation (ENSO) system. In Kenya, El Niño events bring the heaviest rains and greatest likelihood of flood, along with warmer seawater temperatures8. In northeastern Australia, major floods and increased likelihood of tropical storms are associated with La Niña events9. Human activity, in the form of changing land use, has added sedimentation to the list of stresses experienced by reefs during these ENSO extremes, making this interannual variability more problematic for the corals. Moreover, it is possible that the ENSO system itself may be intensifying as a consequence of global warming10.
Another common feature of these records is that the increase in sedimentation occurs as a baseline shift around the time of agricultural intensification and not as a long-term, continuing trend. Is this because erosion has levelled off at the new, higher levels? Soil conservation measures and river damming could stabilize the amount of sediment transported to a reef. Alternatively, does the system that allows preservation of the barium signal in the coral saturate at some level, so that higher values cannot occur? Maybe the coral dramatically slows skeletal growth during times of highest sedimentation, so leaving little or no record. To understand these records better, more research is needed on how this tracer behaves as it moves from terrestrial sediments, to rivers and the oceans, and then into the coral skeleton.
Land-use intensification is widespread, so that many reefs close to continents or large islands are likely to have experienced increased delivery of sediment over the past century. As coastal populations increase, this phenomenon is likely to expand. Sedimentation is just one of many large-scale stresses threatening coral reefs11. Rising temperatures lead to bleaching, a response that can result in coral death, and increasing concentrations of atmospheric CO2 make the oceans' carbonate chemistry less favourable for calcification12.
Reef preservation efforts often focus on stemming the impact of locally significant threats, such as those from destructive fishing, mining and tourism. But mitigating large-scale reef stress from changing climate, ocean chemistry and land use will require regulation of the driving forces of global environmental change — which, thus far, nations have been reluctant to undertake.
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