Millenium Atoll: Discovering Some of Earth's Most-Pristine Reefs

Fish assemblage, including a mix of butterflyfish species, above a patch reef.

Among the most-remote coral atolls on earth, Millenium (formerly Caroline) Atoll is offering marine biologists the chance to survey coral reefs largely free of human impact.

In a new paper in the journal PLoS ONE, a team of researchers led by Katie Barott, right, of San Diego State University report on their explorations of the atoll and its reefs.

"It's one of the most spectacular places I've ever been," says Barott. "I hope that by showing people how special this place is — by doing scientific surveys on the sea life around the lagoon — that they will realize the beauty and diversity and protect them."

A series of surveys were carried out to characterize the physical and biological parameters of the Millennium Atoll lagoon during a research expedition in April of 2009. Millennium is a remote coral atoll in the Central Pacific belonging to the Republic of Kiribati, and a member of the Southern Line Islands chain. The atoll is among the few remaining coral reef ecosystems that are relatively pristine.

Blacktip reef shark swimming in the Millennium Atoll. Credit: Katie Barott

The lagoon is highly enclosed, and was characterized by reticulate patch and line reefs throughout the center of the lagoon as well as perimeter reefs around the rim of the atoll. The depth reached a maximum of 33.3 m in the central region of the lagoon, and averaged between 8.8 and 13.7 m in most of the pools.

The deepest areas were found to harbor large platforms of Favia matthaii, which presumably provided a base upon which the dominant corals (Acropora spp.) grew to form the reticulate reef structure. The benthic algal communities consisted mainly of crustose coralline algae (CCA), microfilamentous turf algae and isolated patches of Halimeda spp. and Caulerpa spp.

Fish species richness in the lagoon was half of that observed on the adjacent fore reef. The lagoon is likely an important nursery habitat for a number of important fisheries species including the blacktip reef shark and Napoleon wrasse, which are heavily exploited elsewhere around the world but were common in the lagoon at Millennium.

Cluster of Tridacna maxima giant clams growing amidst Acropora colonies.

The lagoon also supports an abundance of giant clams (Tridacna maxima). Millennium lagoon provides an excellent reference of a relatively undisturbed coral atoll. As with most coral reefs around the world, the lagoon communities of Millennium may be threatened by climate change and associated warming, acidification and sea level rise, as well as sporadic local resource exploitation which is difficult to monitor and enforce because of the atoll's remote location.

While the remote nature of Millennium has allowed it to remain one of the few nearly pristine coral reef ecosystems in the world, it is imperative that this ecosystem receives protection so that it may survive for future generations.

Acropora spp. dominates reefs that have almost 100% coral cover in large areas.

Original Paper in PLoS ONE: The Lagoon at Caroline/Millennium Atoll, Republic of Kiribati: Natural History of a Nearly Pristine Ecosystem

Funding: Funding for this work was provided by a National Science Foundation Grant.
 

Conclusions (Full text available at no charge from PLoS One)

This is the first comprehensive survey of the lagoon at Millennium Atoll, which contains some of the few remaining coral reefs that are relatively unaltered by human activity. The lagoon of the atoll is home to a variety of unique organisms that are threatened in many areas of the world. Reef-building corals were the dominant organisms in the lagoon covering greater than 60% of the benthos, a value that is much higher than the regional Pacific-wide average of 22.1% but similar to other uninhabited reefs in the Central Pacific (~44–56%,).

Other reef builders such as CCA were the second most abundant organism with fleshy turf and macroalgae representing a minor component on the benthos, again similar to other uninhabited reefs in the region. The benthic invertebrate community included a variety of filter feeders, including gardens of Tridacna maxima in the south to central regions of the lagoon, as well as holothurians along the reef flats and ascidians along the tops of the patch reefs. The clarity of the lagoon water observed may be due in part because of the filtering of these organisms [28]. Fish richness and abundance in the lagoon were generally low, and included occasional visitors from the oceanic reefs. It is likely that Millennium lagoon, like other coral lagoons, serves as an important nursery habitat for many fish species, including important and heavily exploited species such as the blacktip reef shark (Carcharhinus melanopterus) and the endangered Napoleon Wrasse (Cheilinus undulatus) which were observed here.

Protection of Millennium's coral reefs should be a priority for the Republic of Kiritbati as these habitats are not only unique but are some of the world's least impacted reef systems. The relatively undisturbed nature of the lagoon also serves a baseline with which to compare other lagoons in the Pacific Ocean, providing a reference for restoration, management or mitigation of disturbed lagoons. The lagoon of Palmyra Atoll, for example, has been heavily altered by dredging. Restoration efforts can be guided with the goal of achieving similar hydrological dynamics, water quality and coral cover currently found on Millennium.

There are several immediate areas for concern regarding conservation of Millennium lagoon. First, if a channel were created to allow for passage of larger vessels into the lagoon, more water would likely drain from the lagoon and the water level of the lagoon would decrease, exposing the corals within the lagoon at low tide. Circulation dynamics within the lagoon would also be altered, and there would potentially be a much higher oceanic influence (e.g. nutrients, temperature, plankton, etc.) and more tidal exchange at depth in the lagoon. Although the effects are unpredictable and complex, these changes would almost certainly alter the biological dynamics within the lagoon. For example, the abundant clam beds found here are typically found in enclosed lagoons around the Central Pacific, and opening the lagoon would likely have unexpected effects on the clam population in particular.

A second conservation concern is that there is little to no monitoring or enforcement in the area around Millennium, even though foreign fishing vessels have previously been reported to fish the atoll and temporary residents such as the one encountered during this study are exploiting the island's resources. Valuable resource species such as clams, sharks, Napoleon wrasse, sea turtles, and lobster are fairly abundant at Millennium but have been seriously overexploited elsewhere around the world. These organisms are sensitive to human activities and need to be protected. For example, the clams have been harvested from Millennium in the past, and their life histories and easy collection make them vulnerable to overexploitation. Monitoring and enforcement of fishing in the area is crucial, since even a modest amount of extraction of these resources could dramatically alter the lagoon ecosystem, and would likely require a long time to recover due to likely limited repopulation from elsewhere (i.e. the Allee effect).

Lastly, the atoll is likely to experience changes due to the effects of global climate change, such as sea level rise, rising sea surface temperatures (SST) and ocean acidification. Widespread bleaching has affected other remote reefs in the Central Pacific, indicating that Millennium may also be subject to this threat. For example, the most sensitive regions of the lagoon of Kanton in the Phoenix Islands experienced near 100% coral mortality due to bleaching caused by abnormally high SST in 2002–3. At present other reefs in the Phoenix (Baker and Howland Islands) and Line Islands (Palmyra Atoll) are experiencing mass coral bleaching at this time (February 2010, pers. obs. K. Barott, J. Maragos, and G. Williams), and there may be additional bleaching at other Line Islands and atolls to be visited in April 2010 (Jarvis, Kingman). This phenomenon indicates that the remote location of Millennium does not exempt it from the pressure of global climate changes.