Cayos Perlas, Nicaragua

[Pdipal]

I am working on the Atlantic coast of Nicaragua. About 20 km offshore are
a group of islands with fringing reefs on the south east side (the Cayos
perlas). There is a tremendous amount of sediments on the reefs (viz. is
about 3 meters). It seems likely that the amount of sediment is having an
adverse effect on the reefs (coral cover is approx 5% and corals
frequently appear to be smothered by algae/sediment). Some people believe
that this amount of sediment is a recent phenomenon (perhaps due to land
use practices in the watershed). Can anyone advise me on how I could (1)
trace the source of the sediment, and (2) determine whether this amount of
sediment really has only started in the recent past. We are have a fair
amount of time, but not a vast amount of money, so very hugh-tech methods
are probably not going to be suitable.

For background...
The islands lie on the big continental plateau stretching from the border
with Costa Rica to Honduras with an average depth of 30 meteres. North of
the islands there is a big rivermouth carrying tons of sediments. The
prevailing currents tend to keep the main sediment plume close to shore,
but during the rainy season the sediment travels further offshore. During
calm days you can clearly see the border between the murky coastal water
and the blue seawater. However, visibility around the islands is very low,
3 to 5 meters and during the rainy season even worse. Judging from the
'graveyards' of Acropora palmata and the number of large boulders that
appear once to have been coral but are now overgrown by algae, it seems
that there had once been more healthy reefs. We don't know if white-band
disease has been a factor, but certainly the last major hurricane (1989)
has done a lot of damage to the reefs.

I would like to find out the origin of the sediments and whether or not it
got/is getting worse due to land activities upstreams. Is the plateau a
sort of 'sink', acumulating sediments and do they get stirred up and
transported to the islands and back again?

TO find a solution I was thinking of the possibility of (1) taking
coralcores to compare annual growth rings (what drills would one need?),
and (2) possibly coring sediment from the coastal shelf between the river
and the offshore islands to see if the type of sediment has changed over
the years.

Once again, equipment is hard to get but time is plenty (2 years). Do you
have any suggestions??

Thanks very much,
Mariska Weijerman
DIPAL (collaborative project between Holland and Nicaragua to help the
artisanal fisherman in the region)
RAAS, Nicaragua
tel/fax: +822 1777