While they remain rich with corals and abundant marine life, the reefs at Kwaj are not pristine. Shipwrecks, pollution, litter and construction have all taken their toll, and in addition there are the ever increasing effects of warming ocean water and rising sea level associated with climate change. (Links embedded in the text lead to pages with more information and photos.)
After World War I, Japan took possession of the Marshall Islands under a Leage of Nations mandate. As the second World War approached, they fortified the islands, constructing defenses and supporting a good-sized military population. The southern part of the Kwajalein lagoon was a center for shipping, and attacks and an invasion of the atoll by American forces in 1944 resulted in the sinking of a number of freighters and smaller craft.
While they can be fascinating dives and serve as artificial reefs for a large number of fish and invertebrates, the wrecks also can be environmental hazards. Sunk in war, they usually went down with considerable fuel in their tanks that continues to seep out and float to the surface even 75 years later. You can sometimes locate one of the wrecks by its surface oil slick. What the surface of the water and the downwind island beaches must have looked like immediately after the ships went down is painful to consider. After the war, tons of metal debris including live ordnance was dumped, at least some of it in the lagoon. Nowadays the deteriorating wrecks and other wartime metal debris stain the surroundings with unsightly and possibly unhealthy rust.
Other factors that affect the reefs include reef-modifying activities, runoff and litter. The reef has been modified in a number of ways. Kwajalein Island itself has been enlarged by dredging sand from the lagoon and depositing it on the shallow reef, extending the island in both directions. Much later, a causeway built of reef rock and dredged sand by the Marshall Islands' government connects half a dozen islands along the east reef. Some shallow lagoon pinnacles have been blasted to reduce the hazard to shipping, and a few that were not deepened ended up with wayward ships plowing into them. the most recent in May 2015 when a freighter flattened the top of a once lush living reef. In former days, quarrying of the intertidal reef flat on the seaward sides of some islands for aggregate for concrete and other building materials left large holes in the reef. Some rather extreme reef modification activity took place in the northern Marshalls atolls of Enewetak and Bikini between 1948 and 1958 with the test detonation of more than 60 nuclear weapons. Some very large and deep "quarries" (craters) were created on some reefs and parts of the atolls were contaminated with radioactive materials. Some of the tests annihilated entire islands.
Runoff of who-knows-what from some of the islands also takes place. From an airplane, it is easy to see multiple surface slicks continually blowing downwind from crowded Ebeye and other inhabited islands. A once coral-rich pinnacle just downwind from Ebeye is now mostly dead coral, while similar pinnacles just out of the typical flow are still lush and living. Runoff and trash can include a variety of items floating across the lagoon. In some areas directly downwind of crowded islands one can sometimes see plastic bags or disposable diapers caught in the coral. On a larger scale, ocean currents carry plastic and other trash from all around the Pacific and deposit it on the beaches.
Fortunately, such problems are not common and most of the reef is unaffected. Some reef modifications even seemed to have increased diversity. The quarries blasted and dug out of the reefs for building materials are now mostly full of healthy living coral colonies and lots of fish, stuff that just wasn't there when the reef was flat and featureless. Even areas affected by such traumas as blasting or grounded ships seem to spring back quickly into diverse living reefs.
So under healthy conditions, a reef can recover. The question is, will it remain healthy? That may depend on the extent of climate change. Increasing levels of carbon dioxide and other greenhouse gases in the atmosphere are causing the earth to warm up, and through multiple lines of evidence it is clear that this warming is really happening. Ice caps and glaciers are measurably diminishing. Sea level at Kwaj is rising, according to the weather station measurements of the tidal gauge on Kwaj's Echo Pier, and the rate or rise has apparently increased significantly in the 21st century. Further, warmer oceans are not good for a lot of present day coral reefs. It seems that many corals live near the upper limit of their temperature tolerance range.
But here a little background on corals is necessary. Reef building corals mostly live in warm tropical waters, which usually contain few nutrients, especially in the middle of the ocean where the Marshall Islands lie. The lack of nutrients keeps the density of plankton much lower than it could be, much lower than it is where there is higher nutrient content such as areas of upwelling or seasonal mixing of shallower and deeper ocean waters. There is little enough plankton in tropical water that by itself it cannot support a reef full of so many filter feeding plankton-eaters. So what makes these reefs possible? These reefs can exist because corals have formed a partnership with plants. Large numbers of single-celled plants called zooxanthellae (really photosymthetic protozoans rather than plants) live within the tissue of living corals. They use the carbon dioxide produced by the coral animal and sunlight to produce oxygen and carbon compounds--food--both of which can then be used by the coral. It is this partnership, called symbiosis, literally "living together," that allows a coral reef to survive in nutrient poor water.
So what's the problem? I'm not sure if anyone really knows why, but it is a fact that many corals when subjected to warmer temperatures lose their zooxanthellae. Since much or sometimes all of a coral's color comes from the symbiotic plants, their loss causes the corals to turn mostly bright snow white. If you look closely it is still alive; there are still tentacles of the coral polyps sticking out. But they need their symbiotic plants. If the plants are gone too long--if the water stays warm too long--the coral animal dies. The loss of the zooxanthellae from the coral tissue is called coral bleaching.
There have been some severe coral bleaching episodes in the Caribbean and western Pacific, and probably other areas, over the past few decades. Until recently, Kwaj seemed to have been spared. We had some bleached sea anemones in the fall of 1995 and 1996 (anemones are related to corals, and are similar in physiology and in their anemone-zooxanthellae symbiotic relationship). But at that time we did not notice any bleached corals. I'd been feeling rather smug thinking that the small coral atolls of the Marshalls might be unaffected by the bleaching blight affecting more continental areas and some of the larger islands.
However, in September of 2009, we noticed the water we dove in was comfortably warm; our dive computers were reporting a temperature of 86°F (30°C). That's only one degree F higher than our typically warmest water in the fall of every year. But that was apparently enough to trigger a bleaching episode. By early October, we noticed the large Heteractis magnifica anemones bleaching. Then I was startled to see a branching coral, Stylophora, easily noticed for its normally brighter yellowish color and rounded colony shape, turn pure bright white. Along one reef we dive frequently there were quite a few colonies, and all of them bleached. The brightness of their bleached white lack of color jumped out at us. Over the next few weeks, most colonies of one of several species of coral in the genus Goniopora also fully bleached, as did many colonies of shallow-water branching fire coral, Millepora. Curiously, the more encrusting and columnar forms of Millepora did not bleach this time around. Some colonies of a few other corals also completely or partially bleached: some Acropora, Pavona minuta and a few others. Most corals were apparently unaffected by the warmer water, making the scattered bleached colonies stand out all the more.
A broken ankle in early November kept me out of the water for the next few months, and by the time I got wet again in February, the temperature was down to 82°F (27.8°C) and no bleaching of corals or anemones could be found. However, most of the bleached colonies of at least the Stylophora, Goniopora and Millepora had died and were now covered with algae. In the Stylophora, a few branches out of each bleached colony had survived and continued to grow, isolated islands of life in an otherwise dead colony. Colonies of Stylophora on the seaward reef appeared to fare better; most of those were still mostly or all living.
So we watch the temperature more closely now. During 2011, it oddly stayed at a relatively low 82°F throughout most of the year (normal variation is 80-81°F in late winter to 84-85°F in late summer and fall). However, in the fall of 2013 it peaked again at 86°F in the fall, triggering another round of coral bleaching. Since many of the most susceptible corals were already dead from the previous episode, a number of other species were targeted as well as the survivors of 2009. Alarmingly, in 2014 there was yet another serious bleaching episode affecting even more groups of corals. In 2015 the water peaked at 85°F and no bleaching was observed. However, in 2016 a strong El Niño helped boost the temperature again to 86°F for a long lasting bleaching episode that caused more coral bleaching than we had previously seen. Effects were still present when we departed Kwajalein for the last time in January, 2017. I have been told that in the fall of 2017 there was a mild bleaching episode, mostly sensitive anemones and a few more corals, but a year later in 2018 came the most widespread bleaching event yet at Kwajalein, and divers estimated that about half the remaining corals bleached and many died.
This series of bleaching events, all since 2009, have taken a heavy toll.
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