ARE NATIVE AND NONINDIGENOUS SEAWEEDS OVERGROWING FLORIDA’S EAST COAST REEFS?

by Charles Jacoby1, Brian Lapointe2, and LeRoy Creswell3

Florida Sea Grant College Program • www.flseagrant.org • SGEF–156

1 Department of Fisheries and Aquatic Sciences, University of

Florida, Box 110600, Gainesville, FL 32611-0600;

phone (352) 392-96117, ext. 272; cajacoby@ufl.edu

2 Division of Marine Science, Harbor Branch Oceanographic

Institution, 5600 US 1 North, Ft. Pierce, FL 34946;

phone (772) 465-2400, ext. 276; lapointe@hboi.edu

3 Florida Sea Grant Marine Extension Service, 8400 Picos Road,

Suite 101, Ft. Pierce, FL 34945-3045; phone (772) 462-1660;

Lcreswell@ifas.ufl.edu

Seaweed? What seaweed?

Brian Lapointe and his colleagues from the Harbor

Branch Oceanographic Institution (HBOI) first became

concerned about excessive growth of green macroalgae

or seaweeds around 1990. They spotted drifting mats of

the green seaweed Codium isthmnocladum on reefs in

24–40 meters of water (78–131 feet) off southern Palm

Beach and northern Broward counties. These mats were

up to 2 meters thick (6.5 feet), and they covered other

plants, corals, sponges and other animals. When the algae

washed ashore, it made beaches less attractive to

residents and tourists.

By 1995, C. isthmocladum had spread onto reefs in

northern Palm Beach County, and some of the plants had

attached to the bottom. Although extensive growth of

C. isthmocladum was of concern, this alga had been

seen in Florida’s coastal waters, so this was not a case of

an invasion by a nonindigenous or non-native seaweed.

The discovery of C. isthmocladum was followed by the

discovery of large beds of another green alga, Caulerpa

verticillata. In the fall of 1997, HBOI scientists

discovered C. verticillata growing on reefs off Singer

Island just north of the Princess Anne, an artificial reef

that was sunk by Palm Beach County in 1992.

C. verticillata is a small plant that typically grows in

nutrient-rich mangrove habitats and near seabird

rookeries where guano adds nutrients to the water.

Although found in such habitats in Florida, C. verticillata

had not been observed on Palm Beach County’s reefs.

Surveys during following years documented its spread

northward to Jupiter Inlet. Large areas of reef became

covered by this small plant, which can inhibit settlement

and growth of coral and other plants and animals.

Codium isthmocladum (Brian Lapointe)

Caulerpa verticillata (John Reed)

Another larger, native, green alga, Caulerpa racemosa,

also became increasingly conspicuous and abundant

during the late 1990s. Unlike Codium, Caulerpa does not

rely entirely on nutrients in the water column for growth.

It can draw nutrients from sediments through its root-like

stolons and rhizoids.

Caulerpa racemosa (Lazaro Ruda)

Codium isthmocladum (Brian Lapointe)
What problems arise from blooms of

macroalgae?

Macroalgal blooms can produce undesirable ecological

and economic consequences. They can be especially

damaging to coral reefs where macroalgae overgrow

and replace these animals. The resulting reduction in

biological diversity lowers habitat quality and aesthetics,

which can cause decreases in fisheries production and

tourism. Macroalgal blooms also overgrow seagrasses

resulting in similar effects.

In addition, when macroalgae respire or die and

decompose, they use oxygen. Large amounts of

macroalgae can use enough oxygen to cause hypoxia

(low oxygen levels in the water) or anoxia (essentially no

oxygen in the water). If oxygen levels fall low enough

for long enough, other plants and animals will die.

If blooms of macroalgae cause ecological and economic

damage, they can be classed as harmful algal blooms.

Macroalgal blooms can be as destructive as “red tides”

that produce toxic compounds capable of killing fish or

marine mammals.

What does this have to do with me?

We fit into this picture by being potential contributors to

the cause of macroalgal blooms. Blooms of macroalgae

can be symptomatic of increased nutrients in coastal

waters. Increasing nutrient loads occur near coastlines

around the world, especially in areas with rapid

population growth or extensive agricultural or industrial

activities. Sewage, fertilizers in runoff, and atmospheric

deposition of nitrogen from car exhaust or industrial

emissions all contribute nitrogen or phosphorus that can

fuel macroalgal blooms. Natural inputs of nutrients can

also fuel macroalgal blooms, but the frequency and

extent of blooms appears to be increasing as populations

increase along the coast of Florida. In contrast, natural

inputs of nutrients by upwelling or other events do not

appear to be increasing. In addition, surveys funded by

Sea Grant and conducted by LaPointe and his colleagues

indicate that many green algae seem to use nitrogen

from sewage rather than nitrogen from natural inputs.

We can all work to lower the amount of nutrients that

find their way into Florida’s coastal waters. Maintaining

septic systems and using fertilizer wisely are two key

ways to help lower nutrient loads. Working with

authorities to reduce nutrient loads from sewage

treatment plants represents another way to lower nutrient

inputs and avoid fueling macroalgal blooms.

In May 2001, HBOI scientists funded by Florida’s

Harmful Algal Bloom Task Force were surveying algae

throughout Palm Beach and northern Broward counties.

On the first dive off Singer Island, they discovered

extensive blooms of the green alga Caulerpa

brachypus at a depth of 43 meters (141 feet).

C. brachypus is not native to Florida’s east coast. It is a

native of the Pacific Ocean rather than the Atlantic.

Caulerpa brachypus (Geoff Oldfather)

Subsequent surveys showed that C. brachypus was

widespread in waters 20–43 meters deep (65–141 feet)

off northern Palm Beach County. The algae had

overgrown or displaced other macroalgae, sponges and

corals. C. brachypus has since been reported off

Pepper Park in Fort Pierce, and in early March 2003, it

was found in the Indian River Lagoon around the Jensen

Beach causeway and Jupiter Inlet.

C. brachypus will probably continue spreading north and

south from Palm Beach County. At this time, the total

area covered remains unknown. This nonindigenous

species may have become invasive because it may be

causing harm. Dive operators and fisherman have

reported that C. brachypus is so thick off Palm Beach

County that it is forcing fish and lobster away from

reefs.

There is good reason to be concerned about the spread

of C. brachypus in Florida’s coastal waters. A related

green alga, Caulerpa taxifolia, spread from a small

patch (1 square meter or 10.8 square feet) to cover

almost everything for 131 square kilometers (50.6 square

miles) along the Mediterranean coast. This same species

has been found at sites in California where authorities

are spending millions of dollars to eradicate it before it

spreads. (For more information, please see the related

fact sheet Can we stop “killer algae” from invading

Florida? [SGEF-155] by Linda Walters and Charles

Jacoby.)