Lasting Effects of a Shipwreck

text and photographs by Cristi Richards

Typical site at Rose Atoll dominated by coralline algae
(light pink).

Coralline algae or CCA typically dominate the reefs surrounding Rose Atoll, making up 50% - 75% of the benthic cover according to preliminary results of our 2010 surveys. Scleractinian or hard corals comprise another 5% - 30% of the substrate. At the start of each dive, we are greeted by a light pink landscape accentuated by the greens and reds of fleshy macroalgal genera and the browns, purples and yellows of various genera of coral. The coralline algae are a combination of several genera including Mastophora, Porolithon and Peysonnelia.

In general, coralline algae play many important roles in the ecology of a reef ecosystem. They are a food source for many reef inhabitants including parrotfish, sea urchins and mollusks. Coralline algae also act as a stabilizing component or ‘cement’ for the reef and several genera of coral larvae will selectively colonize patches of this algae.

Site of ship wreck at Rose Atoll.
Brown and darker areas are dead coral
and CCA covered by turf algae and
cyanobacteria.

In October 1993, a 135-foot Taiwanese longline fishing vessel ran aground at Rose Atoll, releasing 100,000 gallons of diesel and 500 gallons of oil into the surrounding waters, across the reef flats and into the lagoon. The initial spill killed large numbers of giant clams, urchins, sea cucumbers and the dominant benthic organism, coralline algae, as well as endangering the health of 12 species of migratory seabirds and the threatened green sea turtle.

However there is still damage occurring to the reef ecosystem, 17 years later. Although there are on-going efforts to remove metal wreckage, with 37.5 tons having been removed to date, there are still iron contaminants being released by the remaining portions of the vessel. When we surveyed this site yesterday, it was immediately obvious that the habitat was vastly different than those seen at other areas of the Atoll. The percent cover of coralline algae was up to 50% less than at other areas, with corals comprising less than 1% of the benthic cover. We also noted an increase in the occurrence of cyanobacteria and turf algae.

Cyanobacteria covering a
colony of Favia stelligera

The iron contamination continues to cause a cyanobacteria or blue-green algal, bloom that is detrimental to the health of both the coralline algae and the corals. The cyanobacteria compete for space with coral larvae by settling on the CCA in thick mats, thus blocking the larvae’s access to the CCA. The thick mat also blocks sunlight from reaching the CCA. In addition, strands of cyanobacteria that detach from the bottom can settle and develop directly on corals. The end result is that corals and CCA are smothered and eventually die, thus changing the overall benthic composition of the reef. The presence of the increased levels of cyanobacteria causes a physical and chemical environment that is not suitablefor the growth of the original reef inhabitants.

This scar at the site of the shipwreck can still be seen today as a reminder of how fragile these ecosystems really are. With continued surveys, this will be a valuable documentation of how long it takes areef to recover from a wreck such as this.