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Millennium Lagoon Mysteries

Marine ecologist David Obura is the coordinator for CORDIO (Coastal Oceans Research and Development in the Indian Ocean) East Africa, which supports reef research and monitoring of coral reefs and coastal fisheries. CORDIO’s primary goal is to “conserve biodiversity in the context of improved livelihoods and sustainable development.”

Today’s mission: Survey Millennium Atoll’s lagoon, an exciting prospect for all of us. For me, lagoons give a picture of extreme environments for coral reefs. Each one is different in subtle ways, and this one has given hints of being unusual.

Millennium Atoll has only one major channel into the lagoon, and even that is not a real channel as it is almost a dead-end (it was widened artificially sometime in the last 150 years), forcing us to scrape and drag the Zodiacs through the final few meters.

Every time we’ve been in the channel, the water is flowing out, meaning that most of the time water flows into the lagoon from waves breaking all around the rim. The wash from the waves fills the lagoon, then has to flow out through the channel.

We’ve also seen that the lagoon is divided at various places by “line reefs” that come to the surface, blocking water flow and access. These are visible in satellite images and on Google Earth! They cut the lagoon into smaller sub-lagoons.

Unusually for a lagoon (and perhaps because of the waves lapping over the edge), here at Millennium there’s only a small difference in water depth between low and high tides. This is both good and bad news for us. The good: It means we have many hours during the day to navigate the reefs and channels within the lagoon. The bad: The line reefs all grow as close to the low tide limit as possible, leaving only a few centimeters of clearance over the tops of the reefs even at high tide, forcing us to find the gaps and mini-channels between them.

Getting through the channel was easy today as the sea was calm. We all started in the southern end of the lagoon at 10 a.m. waiting for the tide to rise the few centimeters that would let us cross the first of the line reefs. First mystery of the day: The corals in this southern part of the lagoon near the channel were mostly dead! We could see the old skeletons of the finger Acropora corals broken and scattered over the slopes, with live colonies in between. Also, we found no clams in this part of the lagoon. A report from a joint US/USSR expedition in 1988 found them here by the hundreds. We could see dead clamshells scattered all over the place, but no live clams. What happened to cause both the corals and the clams in this part of the lagoon to decline?

By noon, the tide crept up enough for us to move northward. We went as far as we could, all the way to the tip of the island, making it through two scrape-through channels and around many line reefs. The sun was bright and high, so seeing the obstacles was easy. This far up, surrounded by the small islets at the top of the lagoon, circulation was very restricted, and the corals showed how severe the environment could get here.

As in the south, we saw a lot of dead coral skeletons under the living corals, and some weird growth forms for common corals—or were they a different species? Corals are very variable animals, partly because of the symbiotic algae living inside them, and their growth forms can vary incredibly from place to place. Coral ecologist and taxonomist Jim Maragos found some samples of what he thinks may be a coral he has only seen in a couple places in Hawaii and the northern Line Islands. Will this mission extend the known range of that coral by thousands of kilometers, or is it just a weird growth form of a more common coral?

Time is short, so soon we picked our way south again, to stop in the middle of the lagoon. Rain showers were chasing us down the lagoon, making it hard to see the shallow reefs and channels. Dressed for sun and warm diving, we were chilled to the bone, and couldn’t wait to get in the water to escape the chilly rain!

Two days earlier, on a quick recon of the lagoon, some of us found highly developed coral communities while others found gardens of giant clams. Jackpot: This time, we found both! Two dives on different parts of the line reef structures turned up the most beautiful coral and clam communities—the small finger-Acropora corals lined all surfaces possible, giving a light tan tinge to the reef, and nestled between them were clams of all hues and colors. In some places, we found a single clam surrounded by a forest of coral. In other places, a cluster of 10 or 15 clams were all jumbled up with one another, their bright lips open to catch the sunlight. (Giant clams in the genus Tridacna have symbiotic algae living inside them, just as corals do.)

The other surprise finding of the day: One group made soundings of the lagoon as they went along. The southern lagoon is relatively shallow, with depths of six to seven meters (20 to 23 feet) quoted in reports. Our team found a maximum depth of more than 110 feet, or 33 meters. Two divers got down to 100 feet, about 30 meters, and found deep live corals and microbial mats.

Exploring the bottom of the lagoon, out from the base of the line reefs and into the basins between them, pillars of old dead corals poked up as much as six to seven meters. In the milky white lagoon water, they were like pinnacles in a misty landscape. Swimming up to them you could see how they were built—old coral skeletons, one layered on top of another, and the current set of live corals capping them all. But when were the pillars formed? In the last few thousand years of reef growth? Or maybe 11 to 12,000 years ago, when sea level was much lower, near the tops of the pillars? Or maybe over multiple low sea level stands going back hundreds of thousands of years? Other studies will be needed to answer these questions, but at least in this instance Millennium’s lagoon is similar to other lagoons in the Phoenix Islands, 1,000 kilometers to the west, which also contain line reefs and old submerged pillars, capped by living coral.

The question also lingers of what killed off the corals and clams at the lagoon’s southern end. Was it bleaching because of an El Niño/climate change event? Unlikely, because the outer reef corals don’t show evidence of this happening, and it would affect them too.

Was it caused by iron poisoning, as is visible on the outer reef south of the channel due to a shipwreck there? Perhaps the wreck had some effect on the southern lagoon, but the reefs don’t have the appearance of iron poisoning, a characteristic black algal community.

Perhaps it had something to do with water flow? In our discussions, we’ve speculated that perhaps the small channels through the line reefs separating the southern lagoon from the middle have slowly been blocked by corals growing up to the surface. Corals and clams in the southern lagoon might thus have lost their source of food and oxygen, and perhaps (another perhaps) they were less able to deal with some other stress, maybe a bleaching event that was not strong enough to affect outer reefs, but was enough to kill off corals and clams in this part of the lagoon?

Our work is often like a detective story … whodunit? We came here to try to understand how reefs function away from humanity’s influence, but even these reefs are not totally isolated. Perhaps most valuable for us as scientists is to see something different, a twist on a familiar story, a sight that helps us explain another reef in another study in a few years’ time. For now, though, we are still immersed in Millennium’s quirky lagoon—the corals, the clams, the line reefs and deep pillars … a dream landscape!