[Coral-List] Regarding dams, sediment, and coral reefs

[Kaufman, Leslie S]

Hi everybody.  I am tapping into the recent discussion on the effects of dams on coastal turbidity, and hence, coral reef health.

Dams do of course trap sediments.  The effect of actually removing sediment from runoff to coastal waters is temporary.  It may be more accurate to say that dams alter the temporal distribution of coastal sediment loads.  For example, barring disaster, a dam will trap sediment and reduce coastal turbidity for a period of maybe 15 to 20 years.  At the end of its useful life, a dam will either burst- possibly releasing a sudden and dense plume of sediment- or else the dam might be taken down (similar outcome), or the dam might be serviced as newer dams now are, by flushing some of the accumulated sediments.  This, again, will create a downstream and potentially a coastal plume.  What actually happens is complex, because there is rarely only one dam on a river.

As some of you have been following, a few weeks ago in Minas Gerais (Brazil), disaster was unbarred as a mining dam came down, killing a bunch of people, decimating the aquatic biota of the river, and releasing sediment and tailings into the Rio Doce, which happens to mark the southern end of our study area on the Abrolhos Bank.  After a brief reprieve from favorable winds and currents, the plume is now trailing north into the National Park and across the most important concentration of coral reefs in the South Atlantic Basin.  Fortunately, this is a coral reef system that is well adapted to chronically high sediment loads; the endemic genus Mussismilia is a champion under such conditions, along with the Siderastrea, Montastrea cavernosa and other corals, hydrocorals, bryozoans, and crustose coralline algae that dominate framework construction in this system.  Unfortunately, there is a lot of sediment, plus we are having trouble believing that the toxin load is now insignificant as claimed by the mining company (these are, after all, mine tailings).  We have to wait for the data.

Dams can also influence coral reefs by starving coastal geomorphological processes.  For example, the Mississippi Delta is disappearing, as we all know…ditto the Mekong Delta, the latter likely much more so once the 100 to 200 dams planned for the Mekong Basin are completed.  Will periodic turbidity plumes still be deposited at the river mouth, or in the near future might they instead spread the wealth over areas occupied by coral reefs that may not be so ready for it?  Not being a geomorphologist, I can’t say.  In Bahia, however, we do have evidence that dams have starved the coastal beaches for sand. These beaches are now eroding, endangering the coastal tourism-based economy.  This causes people to do all sorts of things, including clamoring for beach nourishment and perhaps eventually giving up on the coastal tourist industry that also brings people to the coral reefs, jacking up their perceived value and generating revenues for coral reef conservation.

Look beyond the mud.

Les



Les Kaufman
Professor of Biology
Boston University Marine Program
and
Marine Conservation Fellow
Betty and Gordon Moore Center for Science and Oceans
Conservation International
lesk at bu.edu<mailto:lesk at bu.edu>