China Daily Global Edition (USA)
Battle along coast
Efforts ongoing to salvage Hong Kong marine species
It’s easy for people in Hong Kong to be complacent — or even a little smug — about the fantastic abundance of marine life in the waters surrounding the city. However, what most people do not see is the frantic juggling act underway to save many of these marine species from extinction. It’s a work in progress, focused on scientific, region-specific criteria for every precinct of Hong Kong’s marine environment.
More than 6,500 marine species live in Hong Kong’s coastal waters. These waters comprise just 1,650 square kilometers, or about 0.03 percent of the nation’s coastal waters, yet they are home to 25 percent of all China’s marine species.
The diversity of marine life makes Hong Kong special and particularly important among coastal cities anywhere in the world.
Hong Kong also is one of the biggest seaports on the globe. That’s where the juggling act starts, because the waters adjacent are polluted by chemicals from nearby industries and even farms. The chemicalbased paint on the hulls of vessels, accidents involving ships’ cargoes and other misadventures pose direct threats to the city’s marine life.
However, behind the concerns about human activity are other vital issues, painted against the broader backdrop of global warming.
Falling salinity levels
Hong Kong’s marine environment is strikingly diverse. For example, the western waters, fed by the Pearl River, experience an estuary effect. During heavy rainfall, more freshwater pours into the ecosystem, and the salinity of the seawater drops correspondingly as the rainfall increases.
The area is home to the flathead grey mullet. The fish, an important source of food, thrives in estuaries and rivers, and its optimal environment occurs within a delicate balance of freshwater and seawater. What happens when there’s heavy rain? Water salinity levels fall, and the flathead mullet becomes more susceptible to disease and the effects of water pollution. Compound that with the effects of global warming and you get some sense of what scientists are up against as they try to preserve marine species such as the flathead grey mullet.
In contrast to the western waters, the waters on the eastern side of Hong Kong are fed primarily by the South China Sea. Salinity levels are high and different species in the area are dependent on a different set of environmental values.
All the factors that affect the well-being of marine life are in constant flux. Intensifying climate change and day-to-day variables present scientists with the challenge of defining site-specific criteria for water quality to help preserve marine species from the Sixth Great Extinction.
“Each marine species has a tolerance limit for how many toxicants it can accept. Once the concentration of the toxicant in the body goes beyond that limit, the health of the species is affected, leading to poor growth, reproductive decline, higher mortality and ultimately to population decline and even extinction,” said Kenneth Leung Mei-yee, professor of aquatic ecology and toxicology at the School of Biological Sciences at the University of Hong Kong.
The flathead grey mullet is by no means unique. Many species of cold-blooded marine life and algae are affected in the same way, as they are highly sensitive to fluctuating salinity and water temperature, Leung said. In the heat of summer, the mortality rate of these species climbs, especially in proximity to agricultural or industrial activities.
Fluctuating fortunes
Almost every region in the world has drawn up its own set of water quality criteria, which protect local species from chemical contaminants.
Often these criteria are based on toxicity data generated under standardized laboratory conditions, including fixed water salinity, temperature and pH levels (the balance between acidity and alkalinity).
In tropical and subtropical regions similar to Hong Kong, the laboratory temperature is usually fixed at 25 C. However, lab-based conditions don’t match real marine conditions, which change according to the day and the season.
Hence, the lab-driven water quality criteria may not provide marine organisms with adequate protection. According to Leung, what’s needed is a gauge that takes account of aggregate sea conditions, including salinity, temperature and other potentially influential environmental factors.
The fluctuating condition of these factors plays a specific role in determining the metal toxicity of the immediate marine environment. Production of metal pollution has been rising annually as a result of farming, industrial activity and ordinary day-to-day activities. However, metal toxicity has an immense impact on the marine environment.
Water salinity is also a major factor. When heavy rain hits southern China during the summer monsoon season, a greater volume of freshwater flows through the Pearl River, which leads to levels of water salinity falling along the west coast.
Lower salinity accelerates the dissolution of metals, raising the amount of ions, which are chemically active and more toxic to marine organisms, being released into the environment. Elevated concentrations of these ions can inhibit the growth and reproductive ability of marine organisms in the vicinity.
In contrast, when water salinity is high, metal ions readily react with other ions present in seawater such as chloride and sulfates to form complexes that are less toxic, reducing metal toxicity.
Water temperature is another factor. “Raising the temperature of seawater can interrupt the physiology and growth of cold-blooded marine organisms. It may cause death and ultimately lead to the disappearance of those species if the temperature goes beyond their thermal tolerance limit,” Leung said.
In a study published last year, he wrote, “By the end of the 21st century, coastal marine organisms, such as phytoplankton, will likely live in a warmer environment, with more frequent periods and longer duration of low salinity.”
He added that it will be essential to study how geographical and climatic conditions will affect what is known about the interactive, toxic potencies of metals.
Metal detectors
Each organism has its own tolerance level for metal toxicity. Leung and his team are mapping the tolerances of different marine species, as revealed within the naturally variable environment.
The aim is to establish sitespecific water quality criteria to protect a range of marine ecosystems around the world. The team examined the toxic elements of six metals (mercury, cadmium, copper, zinc, nickel and chromium) and their effects on eight marine species under different combinations of water temperature and salinity. The study also made reference to natural temperature and salinity regimes, based on a global oceanography database.
The results showed that the predicted toxic potencies of the six metals on the eight species correlated directly to a temperature gradient between 10 C and 30 C. When the temperature rose at a given site, mortality rates among various species rose proportionately.
The results suggested that tropical marine species, particularly those inhabiting estuaries with low levels of salinity, are more susceptible to metal toxicity. The study was published in the peer-reviewed journal Environmental Science and Technology.
The project has helped scientists to measure the biological values of marine sites in Hong Kong, providing knowledge of how many marine organisms live in them as well as identifying those that are rare and in need of protection, and which region has the largest populations of organisms.
With this information, the Hong Kong Agriculture, Fisheries and Conservation Department will gain a better understanding of which sectors should be designated as conservation sites, fish farms or recreation facilities.
Toxic stew
In Hong Kong, limits for chemical pollutants are not set in accordance with water quality standards for marine waters. Other parameters, such as levels of dissolved oxygen and nutrients, have specific guidelines that limit their volumes in marine waters, but levels of toxic pollutants are not addressed.
The regulations for all 10 water control zones state that toxicants are “not to be present at levels producing a significant toxic effect”. However, that’s as explicit as the guidelines get.
Leung noted that Hong Kong lacks governance on “plasticizers”, chemical additives used to make manufactured plastic products more flexible or durable. They are commonly used in consumer products such as footwear, food packaging, children’s toys, and wire and cable products. However, many are environmental estrogens that can lead to intersex status in male fish and can interrupt reproduction, even at low concentrations.
“The toxic effects of single plasticizers are known, and there are water quality criteria concerning those, but when two types of plasticizer combine in a marine environment, their combined toxicity may be enhanced,” Leung said, adding that further scientific research is needed to address the toxicity of coexisting chemical contaminants in the environment.
Leung and his team have also analyzed environmental statistics for temperature and salinity at different marine sites around the world.
Their findings may be applied to provide estimates of site-specific water quality criteria to evaluate the toxicity of more than 30 metals against the backdrop of evolving levels of temperature and salinity.
Leung hopes that if resources allow, each chemical contaminant could be tested according to the interplay of water temperature and salinity.
He believes that the method would provide a better understanding of the effects of toxins on marine organisms.
While the people of Hong Kong are proud of the biodiversity of the city’s marine life, that situation cannot be taken for granted. Preserving the environment will demand that the existing record must be studied further to provide a safer, more sustainable marine environment.