Oliver et al (2004) Monitoring bleaching

• film purchase and development
costs eventually exceed cost of a
digital camera
initially much more expensive than film
cameras (but cheaper in the long term since
there is no film purchase and processing
cost)
Measurement of environmental variables
Temperature
Since elevated water temperatures are believed to be the primary cause of almost all mass bleaching
events, water temperature measurements are a key variable in confirming this relationship. Global
water temperature anomaly data, at 50km 2 resolution, are calculated by NOAA and made available on
their website as Hotspots and degree heating weeks (http://orbitnet.nesdis.noaa.gov/orad/coral_bleaching_index..html
). These data, which can also be obtained from
ReefBase together with bleaching records for the same period, can provide a quick indication if the
current bleaching event is associated with a regional or global temp erature event. Higher resolution
data (10km 2 ) are available to institution with a satellite receiving and processing station, and can
provide much finer scale comparisons, especially near continental margins.
Wherever possible, direct measurements of water temperature should also be taken since this provide a
direct indication of the physiological conditions that the coral are experiencing. At a minimum take a
single measurement of temperature using a dive computer or handheld thermometer at the depth of
maximum coral bleaching, every time you visit a site with bleaching. Maximum-minimum
thermometers can be used, if available and left on the reef between trips.
The best way of determining the temperature regime that is being experienced by corals is to install an
electronic temperature logger set to record temperatures every hour. These can be left in place for 6-12
months and provide accurate reading which can be downloaded and immediately analysed on a
computer.
Inexpensive data loggers can be purchased from the USA (Hobo Stowaway Tidbit or Water Temp Pro
http://www.onsetcomp.com/Products/3654_temp.html#Anchor-35882 ) for about US$120 each and a
further US$100-300 for software and downloading equipment. Loggers, whilst reliable, have a
tendency to fail or get lost, so it is recommended that 2 loggers be installed at each location so that a
backup is always available. Loggers should be installed at more than one location if the distance is
more than about 50km, or if the oceanographic features of the site are know to be very different (e.g.
shallow bays compared to steep shelf edge slopes with upwelling). Loggers should be wired firmly to
a metal stake hammered at least 1m into the reef. It is a good idea to put the logger in an inconspicuous
place where other divers are not likely to come across it and collect it as a souvenir. By wrapping the
logger in a dark plastic bag you can keep it free from fouling organisms and also disguise it somewhat
from curious divers.
Because corals are sensitive to small deviation above normal summer maximum temperatures, it is
important to use a thermometer or logger with a precision of at least 0.5ºC.
Salinity
Freshwater runoff, and (if intense) direct rainfall can lower the salinity of water in shallow reef
environments enough to cause bleaching, or to greatly increase thermally induced bleaching. While
rainfall records will often provide a good indicator of the likelihood of lower salinity (if there has not
been any substantial rainfall in the weeks preceding or during bleaching, you can rule salinity out as a
major factor). If salinity is a possible factor for consideration a simple refractive salinometer can be
used to take readings on each visit. If available, a portable conductive salinometer is very useful, and
can be used to take a profile for both temperature and salinity at different depths and location. This is
probably not necessary unless you are interested in determining the exact relationship between salinity
and bleaching threshold. In this case repeated measurements will be needed leading up to and during
30