by Forest Rohwer, head researcher on the Microbe Team
In 2005, we surveyed the reefs along the western and southwestern sides of Christmas Atoll. There were lots of microbes and algae, the coral was mostly dead, and big fish were almost non-existence. This was particularly striking when compared to nearby reefs such as Kingman. Since 2005, our working group has been back to Christmas five times extending our survey around the entire island. Coral cover is very high as soon as you get away from the population centers. The south side in particular has some amazing reefs. But the reefs around London (the main population center located on the western side of the island) and extending around the northwest corner are dead with massive algal blooms, lots of microbes, and only small fish.
So what is causing these dead reefs? The debate takes many forms. There are the naysayers that argue what we are observing is only the natural variation. There is no real evidence supporting this stance. The places in the Line Islands where we observe trashed reefs are always either next to human population centers or are where humans have had a strong effect (e.g., the black reefs associated with shipwrecks on Kingman, Fanning, and Millennium). Some people argue that the upwelling of nutrients around Christmas causes the reef death there. However, upwelling does not have such an effect at the other Line Islands; similar zones on Jarvis, Starbuck, and Kingman support some of the most spectacular coral reefs anywhere. Furthermore, paleontological studies show that the now dead reefs on Christmas died only recently.
The algal overgrowth of the corals on these dead reefs could be caused by either bottom-up or top-down effects. The black reefs associated with the shipwrecks, for example, result from disruption of a normal bottom-up control. Iron is normally a limiting nutrient in this part of the ocean, and the extra iron dispersing from the wreckage appears to stimulate growth of cyanobacteria and a “black” microbial community (the black may be sulfur). The reef dies, even though grazing on these reefs—a top-down control—is extremely high. We routinely see schools of hundreds of convict tangs , as well as other fish, chomping on the black reef algae. In this case, it is clear that bottom-up effects can cause the coral ecosystem to switch to one dominated by algae and microbes. More typically the top-down control by a robust population of fish and other algal grazers (such as sea urchins) can keep the algae in check. Heavy grazing means less algae, and less algae release less food for the microbes. We hope the data from this cruise will shed more light on this part of the puzzle.
Whatever the cause/mechanism of coral reef decline, the real message from the Line Islands as a whole is that these reefs are incredibly sensitive to human disturbances.