Background
The authors write that "the residence time of pollutants such as fine sediment, nutrients, trace metals and pesticides is different from that of water," because these foreign substances "may also be affected by processes such as flocculation, sorption biological uptake and settling and/or resuspension," all of which are "processes of non-conservative transportation," as discussed by Wolanski (2007). This being the case, they state that "there is no justification to assume that river pollutant or solute residence times are equal to water residence times." And because "the circulation of coastal waters is sluggish," as described by Alongi and McKinnon (2005), Furnas et al. (2005, 2011) and Luick et al. (2007), they say there is "enough time for bio-available nutrients and organic matter to be taken up by biota and recycled several times before being exported to the surrounding seas by oceanic currents."

What was done
Brodie et al. collected and thoroughly analyzed the results of a host of published studies of various land-sourced contaminants of Australia's Great Barrier Reef (GBR), making their own best judgments of their several findings and ultimately developing a range of best estimates of residence times for materials discharged from GBR rivers into the GBR lagoon.

What was learned
For the following materials discharged from GBR rivers, the four Australian researchers derived their best estimates of associated residence times in the GBR lagoon: freshwater (15-365 days), coarse sediments (1000's of years), fine sediments (10's of years in bioactive form), reactive nitrogen (years-decades), reactive phosphorus (years-decades), PSII herbicides (1-3 years), and trace metals (10s of years in bioactive form, but variable with individual metals).

What it means
In response to the question "How long will it take to improve water and sediment quality to the extent required for reef recovery following catchment remediation measures?" - which Brodie et al. pose for themselves in the final sentence of their paper - they reply that in the case of the GBR lagoon, the answer is "probably several decades." Thus, it would appear that Australians had better get on with solving this local problem of theirs, for which only they are responsible. And if they would do so, they would probably find that with the resulting new vigor the GBR would likely experience, it may well be able to successfully deal with any further CO2-induced global warming and/or ocean acidification that might possibly occur in the future.

Furthermore, what holds for one place on earth holds for the rest of the planet as well. All nations must take individual actions to atone for their polluting of their own coastal waters. Grandiose schemes to alter the atmospheric composition and the climate of the entire world will never succeed; but if each nation with a seacoast took care of its own coastal waters, no such schemes would likely ever be needed.

References
Alongi, D.M. and McKinnon, A.D. 2005. The cycling and fate of terrestrially-derived sediment and nutrients in the coastal zone of the Great Barrier Reef. Marine Pollution Bulletin 51: 239-253.

Furnas, M., Alongi, D., McKinnon, D., Trott, L. and Skuza, M. 2011. Regional-scale nitrogen and phosphorus budgets for the northern (14°C) and central (170°S) Great Barrier Reef shelf system. Continental Shelf Research 31: 1967-1990.

Furnas, M.J., Mitchell, A.W., Skuza, M. and Brodie, J. 2005. In the other 90%: Phytoplankton responses to enhanced nutrient availability in the Great Barrier Reef lagoon. Marine Pollution Bulletin 51: 253-256.

Luick, J.L., Mason, L., Hardy, T. and Furnas, M.J. 2007. Circulation in the Great Barrier Reef lagoon using numerical tracers and in situ data. Continental Shelf Research 27: 757-778.

Wolanski, E. 2007. Estuarine Ecohydrology. Elsevier, Amsterdam, the Netherlands.