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FL E S H Y MAC ROA LG A L IND E X

ORITY ON

F1 3

RI

RE

TH

EF

S FOR HEAL

What

What Is It?

BENCHMARK

Macroalgae are one of three main functional groups of

A regional macroalgal index of between

reef algae. (The other two are turf algae and crustose

20 and 40.

coralline algae.) The fleshy macroalgal index is simply

TA R G E T

a measure of how much fleshy macroalgae is present

on the reef.

Working target: Macroalgal index <20.

Macroalgal abundance is usually expressed in terms

RED FL AG

of percent cover or biomass. A macroalgal biomass

A macroalgal index >40. Site should be

index can be calculated as the product of percent

evaluated to determine why levels are

macroalgal cover and average algal height. The higher

so high and what actions might lower

the index, the greater the amount of macroalgae on

the reef.

the index.

Macroalgae  are  usually  larger  canopy-formers

(sometimes  called  "seaweed").  Common  reef

inhabitants include brown algae (Dictyota, Sargassum),

aquaculture, coastal populations and tourism) also

red algae (Gracilaria, Laurencia) and green algae

fuel macroalgal growth.

(Caulerpa, Microdictyon). Macroalgae can be either

How

How Do We Measure It?

soft and fleshy (e.g., Lobophora) or stiff and calcareous

(that is, able to deposit calcium carbonate in their

Various methods have been used to measure

tissue; e.g., Halimeda).

macroalgal percent cover and biomass in the MAR

Macroalgal community structure on tropical reefs

region (e.g., MBRS, AGRRA, CARICOMP). The same

is related to the abundance of other algal species,

transect measurements of benthic cover that are

herbivory intensity, and primary productivity (which

described in Indicators S4 ­ Coral Cover and S5 ­ Coral:

is in turn controlled by the availability of light and

Algae Ratio, along with an additional measurement of

nutrients).

average algal height (measured in five quadrats along

each transect line in the AGRRA protocol), are used to

Why Do We Measure It?

Why

calculate this index.

Healthier reefs tend to have less fleshy macroalgae.

The fleshy macroalgal biomass index is calculated as

Fleshy macroalgae are tough competitors for corals. In

the product of the percent macroalgal cover and the

high enough abundances, macroalgae can overgrow

average algal height3.

stony corals, interfere with coral recruitment and

reduce coral survival. A reef "takeover" by macroalgae

Usefulness

Usefulness

can potentially lead to the reduction of coral cover,

An increase in macroalgal abundance (easy to

reef structure and functional processes.

measure) may signal an increase in nutrients and/or

Macroalgae are also an important food source for a

a decline in herbivory (both of which are harder to

variety of reef herbivores.

measure). The exact cause of macroalgal changes is

not usually easily identifiable.

The loss of major reef herbivores through overfishing

and the 1983 Diadema die-off has led to dramatic

Analysts comparing different data sets need to keep in

increases of macroalgae on the Mesoamerican Reef

mind that macroalgal abundance may vary seasonally,

-- so much so that macroalgae have taken over

by reef type, water depth or wave energy. Apparent

many reefs formerly dominated by corals. Macroalgal

differences may also be introduced if different data-

dominance is further helped along by the chemical

collection methods were used.

deterrents or structural resistance most macroalgae

Macroalgal abundance is easy and inexpensive to

use to discourage consumption by herbivores.

measure.

Increasing

nutrient

levels

(from

agriculture,

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