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W  AT E R Q  U A L I T Y :

ORITY ON

S8

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RI

TE M P E R A T U R E , S  A L I N I T Y, TR A N S P A R E N C Y

RE

TH

EF

S FOR HEAL

What Is It?

BENCHMARK

Water quality, as an indicator, refers to the

Temperatures between 25 and 29°C.

Temperatures

physical, chemical and biological properties of the

seawater bathing the reefs. Three key parameters

Salinity between 34 and 37.

important to reefs and associated ecosystems are:

Water clarity ­ norms to be developed

water temperature, salinity, and water clarity (or

for different classes of reefs.

for

transparency).

Following a general guide: 35% of

Following

Why Do We Measure It?

incident light reaches 5-m depth for

inshore reefs, and 60% of incident light

Reefs depend on highly specific environmental

reaches 5-m depth at offshore reefs.

conditions, and corals and many of their inhabitants

have adapted to tolerate relatively narrow ranges

TA R G E T

of these conditions. Water quality has declined in

Monthly average temperature values

several areas in the MAR, affecting reef habitat,

no greater than 0.5°C above historical

critical nearshore nursery habitats and feeding

areas. Chronic (long-term) declining water quality can

monthly average for that month.

eventually exclude reef growth altogether.

Salinity

Salinity values not in excess of 34-37.

Optimal coral growth occurs when water temperatures

For water clarity: No net decrease

For

are 25-29 °C. Monthly averages that exceed 0.5 °C

in "baseline" water transparency (to

above the historical average for that month are likely

be determined after more data are

to result in coral bleaching.

available).

In some nearshore areas, salinity and water clarity

Following a general guide: ~35% of

can serve as indicators of fluvial (river) run off, which

incident light at 10 m depth for inshore

may also contain contaminants (nutrients, metals,

reefs; 60% of incident light at 10 m

chemicals) that are more difficult to measure. Inorganic

depth for offshore reefs.

nutrient inputs are rapidly taken up by phytoplankton,

thus water clarity can serve as a proxy for nutrient

RED FL AG

concentration. Optimal coral growth generally occurs

To

To be developed.

at salinities of 34-37, but more site-specific `norms'

could be developed for specific areas within the MAR,

based on actual long-term data collection.

temperature data are freely available on the Internet.

Measures of water transparency can be used

to  characterize  light  penetration  through  the

Salinity measurements are usually made with a

water column. Light is essential for the symbiotic

handheld refractometer (US$150) or a conductivity

zooxanthellae that provide the corals with much

sensor.

of their food. The intensity of light reaching the sea

bottom affects the coral's growth and nutrition, and

Water clarity can be assessed using a Secchi disk (a

ultimately the depth to which coral reefs can exist.

circular black-and-white disk attached to a calibrated

rope: (cost US$100) or a turbidity sensor and logger.

How

How Do We Measure It?

Usefulne s s

Usefulness

Temperature measurements are relatively easy and

inexpensive to obtain with small recorders that can

These basic water quality indicators are very important

be deployed for months of automatic and frequent

for inclusion in long-term monitoring programs at

recording. Temperature data loggers are inexpensive

well-established monitoring sites, where routine

(US$200) and can be used in most reef monitoring

measurements can be taken (or using automated

efforts. Remotely sensed (satellite) sea surface

stations).

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