La faute au ballast

MARITIME TRANSPORT HAS MORE TO DO WITH CORAL DISEASE than you might think. Just like most animals, corals can get diseases. Researchers first recognized coral disease in the early 1970s. It has increased over time and become a significant threat in many areas of the world.

Changes in tissue color indicate coral disease; when tissue loss accompanies those changes, the coral is no longer alive. There are more than 20 recognized coral diseases, but many have unknown origins. While some known pathogens naturally occur in the water, research has shown that humans also play a role in this threat. 

Some human influences are indirect, such as warming waters, increased nutrient loads, and sedimentation, but humans have had a direct impact in causing coral disease. For example, a bacterium found in human sewage was identified in 2010 as the source of whitepox disease, which targeted elkhorn coral and served as a primary contributor to the drastic loss of this species throughout the Florida Keys in the early 2000s.

Stony coral tissue loss disease (SCTLD) has made headlines more recently throughout Florida and the Caribbean, which are hot spots for coral disease. First appearing in waters off Miami, Florida, in 2014, SCTLD has now spread throughout most of the Caribbean. As is typical of coral disease, SCTLD has had an impact on the afflicted corals as well as the entire ecosystem. 

Coral bleaching, which drastically affected coral reefs this past summer, is also connected to coral disease. Corals bleach when stressed, and while a bleached coral is not dead, it is weakened and more susceptible to disease. Some studies have shown that corals can become diseased by ingesting bacteria or zooplankton carrying the pathogen. While corals feed on zooplankton as a secondary form of energy, bleached corals have expelled the symbiotic algae that provide their primary energy source and rely solely on zooplankton. Bleached corals, therefore, potentially have an increased chance of ingesting food that carries pathogens.

cruise ship at port

Ships use ballast water to manage their draft, trim, and stability. Scientists are still studying the connection between coral disease and ballast water, but they suggest it is reasonable to assume ballast water serves as a vector for some diseases. Photo by Stephen Frink

Ballast Water Connection

The maritime industry is connected to coral disease because of ballast water, which ships use to manage their draft, trim, and stability. Ballast water can be fresh or salt water brought into and released from tanks in the ship’s hull, depending on the cargo load. The maritime industry has used this practice for hundreds of years. 

Ballast water is often loaded at one port and unloaded at another, potentially on the other side of the world. A diverse community of microorganisms such as bacteria, parasites, microbes, small invertebrates, eggs, and larvae fills this water. Researchers have documented that hitchhikers in ballast water are a primary source of marine exotic species (a plant or animal taken from its native region and introduced to a new area). 

Scientists are still studying the connection between coral disease and ballast water, but they suggest it is reasonable to assume ballast water serves as a vector for some diseases. An unidentified pathogen caused the die-off of Diadema antillarum, long-spined sea urchins, throughout the Florida Keys and Caribbean in the early 1980s. When the same thing happened in 2022, science was better prepared to identify the cause. 

The Smithsonian Tropical Research Institute (STRI) determined that the culprit is microscopic organisms called ciliates. “Ciliates seem to thrive in nutrient-rich shoreline habitats,” STRI reported. “The disease seems to originate in calm water ports and harbors. Then it appears to spread through the water across a wide space, perhaps in currents, with floating vegetation or carried by migratory fish or sea birds.” Or it could possibly spread with ships taking on water for ballast in one port and discharging it in another.

The spread of white band disease, which decimated staghorn and elkhorn populations throughout the Florida Keys and the Caribbean in the late 1970s, is thought to be due to pathogen introduction via the Panama Canal or ballast water transfers. An analysis of the spread of SCTLD, which evaluated outbreaks in distant or isolated locations, suggests that ballast water likely played a role in the spread of that disease as well.

Managing the Problem

Advancements in ballast water management occurred after recognizing some large ecological impacts from the spread of exotic species in the 1980s. The International Maritime Organization issued guidelines for ballast water in 1991, and in 2004 they adopted the International Convention for the Control and Management for Ships’ Ballast Water and Sediment, which took effect in 2017. 

Regulation can be complicated because it entails international and national management. The Coast Guard set out rules effective in 2012 prohibiting ships from releasing untreated ballast water in U.S. waters. Various agencies, such as the Environmental Protection Agency, have since enacted new rules and regulations.

One strategy to reduce the transport of hitchhikers is ballast water exchange (BWE), which entails flushing out a ship’s coastal ballast when the ship is at a minimum distance from land and replacing it with open ocean water. While BWE reduces the concentration of coastal organisms in the ballast water, one strategy alone won’t completely eliminate them. 

Ballast water management systems include techniques such as mechanical filtration, gravity separation, ultraviolet (UV) radiation, chlorination, and ozonation. The effectiveness of each technique is species dependent. UV radiation is commonly used, for example, but it does not mitigate SCTLD spread through ballast water. Filtration, chlorination, and ozonation — all of which have logistical and financial challenges — are likely more effective for this disease. Regardless of what mitigation protocols are most efficient, it presumes that ships, whether freighters or cruise ships, will invest the time and expense to do it properly.  

What Divers Can Do

As a diver, you can play a key role in preventing the spread of coral disease. A coral’s primary defense mechanism against disease is a layer of mucus that covers the coral. If you touch, stand on, kick, or otherwise disturb the coral, you remove the mucous membrane and increase the coral’s susceptibility to disease. In a time of thermal stress, exacerbated by water contamination, corals need all the help they can get.  

Always control your buoyancy, especially when taking photos — research suggests that is when most damage from divers occurs. Some pathogens might survive on dive gear, so decontaminating from one dive destination to the next or renting gear locally could be beneficial. (See Risk Mitigation on Page 60 for best practices for decontaminating dive gear.)

Coral reefs worldwide are up against a lot of threats; ballast water is just one of many. As divers, let’s do our part! AD

© Alert Diver - Q4 2023