The best place to see vibrant coral reefs in Mäalaea may be at the Maui Ocean Center. The popular aquarium displays an abundant range of marine life that no longer exists in the adjacent bay, which has experienced a total collapse of its underwater ecosystem in recent years, due largely to human impacts.

Coral coverage in the bay was estimated at 50 percent to 75 percent in 1993 by the U.S. Fish and Wildlife Service, similar to that found in the nearby Molokini Shoal Marine Life Conservation District. In 2006, a survey found only 8 percent coral coverage at Mäalaea.

The alarming speed at which the marine landscape has deteriorated offers an important lesson in coral reef conservation, according to Russell Sparks, an education specialist with the state Division of Aquatic Resources.

"When these reefs degrade, when they go off the edge, it happens very quickly," Sparks said. "When we see signs of it, we need to be willing to make changes, sometimes major changes, and do it very quickly."

The causes of coral reef decline at Mäalaea are complex, but severe overgrowth of invasive algae is one indication the ecosystem's natural balance is seriously out of whack, he said.

"We're not sure if it's the cause of the reef degradation or just the symptom. When you have a healthy reef, there's no place for that algae to grow," he said.

Shallow reefs in Mäalaea and neighboring Kíhei are almost totally overgrown with Acanthophora spicifera (spiny seaweed), Ulva spp. (sea lettuce) and reddish or brownish Hypnea musciformis (hookweed), which can double its biomass in just two days. Growth rates of the limu in South Maui are among the highest anywhere in the world.

The economic impact of the Kíhei seaweed invasion has been estimated at $20 million annually from lost hotel revenue, reduced property values and removal costs.

The ecological impact is incalculable.

"Algae is a natural part of the reef, but what happens when it's growing out of control is that it smothers some of the corals and acts as an irritant," Sparks said. "Healthy corals are not going to be affected by algae but when you have everything else stressing out the ecosystem, it can become a big problem."

University of Hawaii studies have found a correlation between severe algal growths and coastal areas with high human population densities. There also is evidence that concentrations of nutrients such as nitrogen and phosphorus are highly elevated in nearshore areas where algal blooms are found. These nutrients, from fertilizers and wastewater, are believed to accelerate seaweed growth.

Mäalaea Bay is in close proximity to vast agricultural fields and golf courses, which are sources of fertilizer runoff, and the area's extensive coastal development is served by injection wells that discharge treated sewage into the ocean.

"Mäalaea is in a wetland coastal area where you have groundwater and surface water funneling down there from the entire Central Maui isthmus," Sparks said.

Unmonitored grading of a major commercial development in Mäalaea in the late 1990s resulted in tons of sediment being deposited into the bay, further stressing the reefs, and piles of rotting seaweed encourage bacteria hostile to corals.

As a result of these multiple impacts, Sparks said, Mäalaea Bay has changed from "a dynamic, actively growing coral reef ecosystem" into eroding and relatively flat areas that don't provide structurally complex habitat for the herbivorous fish that can keep algae growth in check. Instead, the fish stocks are in poor condition, dominated by small wrasses, triggerfish and puffers.

It may be too late for Mäalaea's reefs to recover, but the state Department of Land and Natural Resources took action this year to prevent the same kind of swift decline off North Käanapali, which also suffers from algal blooms.

The newly created Kahekili Herbivore Fisheries Management Area bans removal of rudderfish (nenue), parrotfish (uhu), surgeonfish and sea urchins, all important limu-grazers.