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BIOERODERS

CORALS

F9

ON

What

What Is It?

BENCHMARK

Bioeroders are organisms that weaken or erode the

To be developed.

To

calcareous skeletons of reef-building corals. Their

presence on the reef is typically quantified in terms

TA R G E T

of their visible numbers (per unit area) on live coral or

coral rubble.

To be developed.

This group includes a diverse variety of boring algae,

sponges, worms and other species. These small

RED FL AG

"termites of the reef" are found everywhere -- on top

of the reef, within crevices and inside coral skeletons.

To be developed.

The biomass of bioeroders inside the reef is even

greater than on the reef surface. The immense

diversity of bioeroding species has not yet been

Usefulness

examined fully.

Why

Why Do We Measure It?

High densities of bioeroders can be a proxy indicator

of high nutrient concentrations and can indicate a reef

Bioeroders play a critical role in the competing

in trouble. If coral growth and reef-wide calcification

processes of reef growth versus erosion. The net

are not outpacing biological and physical erosion,

result of this competition is reef accretion if growth

then there will be no reef accretion and likely no

outpaces erosion, or loss if erosion rates are greater

persistence of the reef in the long term (with global

than growth rates. These "termites of the reef" convert

sea levels rising).

skeletal calcium carbonate to loose sediment, thereby

Bioerosion levels are expected to vary with the nutrient

weakening the overall reef structure.

content of the water, with higher-nutrient waters

Bioerosion intensity can be higher on nearshore reefs

generally supporting a higher density of bioeroders.

than offshore reefs, especially in nearshore, nutrient-

These variations in nutrient concentrations can be

rich environments.

natural or anthropogenic in origin. Bioeroder density

can also vary naturally depending on such factors as

For reef framework to grow, reef accretion rates must

reef type and types of bioeroders present.

exceed erosion rates. Bioeroders therefore play a key

role in whether a reef can sustain itself over the long

A high density of bioeroders greatly reduces a reef's

term.

ability to withstand the forceful waves generated

by storms and hurricanes. This effect will become

How

How Do We Measure It?

increasingly important given the projections for

stronger hurricanes associated with anticipated global

Bioeroders can be measured in two ways: the density

warming.

of bioeroders on coral rubble, or visible bioeroder

density on live coral. The "rubble method" and the

An optimal, "healthy reef" community of bioeroders

"live coral method" are described in more detail on

would allow for a positive balance between the rates

the Healthy Reefs website (www.healthyreefs.org).

of coral and algal calcification (growth) and biological

and physical erosion. However, no absolute numbers

Some earlier work documenting a relationship

can yet be established for "optimal," given the lack of

between bioerosion and high nutrient concentrations

relevant data.

relied on methods destructive to corals: slabs were

cut from live corals for x-rays of internal eroders.

Both procedures for characterizing bioeroders require

some technical experience to identify bioeroding

This indicator has not yet been incorporated into most

species (e.g., clionid sponges) accurately.

standard monitoring programs. A standardized method

for application in the MAR needs to be developed.

This indicator needs to be further developed.

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