Sylvia Chang
It’s easy enough for Hong Kong people to be complacent — even a bit smug — about the fantastic abundance of marine life in our coastal waters. What most people do not see is the frantic juggling act underway to save many of our marine species from extinction. It’s a work in progress, focused on scientific, regionspecific criteria for every area of Hong Kong’s marine environment.
Over 6,500 marine species live in the city’s coastal waters, which comprise only 1,650 square kilometers, or about 0.03 percent of the whole country’s coastal waters. Yet fully one quarter of all China’s marine species live here. The diversity of marine life makes Hong Kong special and of particular importance among coastal cities anywhere in the world.
Hong Kong is also one of the biggest seaports in the world. That’s where the juggling act starts. Our adjacent waters are polluted by chemicals from nearby industries and even farms. The chemical paint on ships’ hulls, mishaps involving ships’ cargo and other marine misadventures pose direct threats to our marine life.
Behind the evident worries about human activity are other vital concerns, painted against the broader backdrop of global warming.
Hong Kong’s marine environment is strikingly diverse. Take, for example, Hong Kong’s western waters, fed by the Pearl River. There’s an estuary effect. When there’s heavy rainfall, more freshwater pours into the ecosystem, and the salinity of the seawater drops correspondingly as rainfall increases.
Here we find the flathead grey mullet, an important food fish. It thrives in estuaries and rivers. Its optimal environment is found within this delicate balance of freshwater and seawater. What happens when there’s heavy rain? The water salinity falls — and the flathead mullet becomes more susceptible to disease and water pollution. Compound that with the effects from the unceasing surge of global warming and you get some sense of what scientists are up against trying to preserve marine species like the flathead grey mullet.
By contrast to the western waters, the waters on the eastern side of Hong Kong are fed primarily by the South China Sea. Water salinity is high and different species there are dependent on a different set of environmental values.
All these factors affecting the well-being of our marine life are in constant flux. Intensifying climate change with its emerging weather extremes, and day-to-day variables, present scientists with the challenge of defining site-specific water quality criteria to help preserve marine species from the sixth mass extinction — the ongoing extinction event of species resulting mainly from human activity.
“Each marine species has a tolerant limit for how much toxicant it can accept. Once the concentration of the toxicant in their 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,” warned Kenneth Leung Mei-yee, professor of aquatic ecology and toxicology in the School of Biological Sciences at the University of Hong Kong.
The case of 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 closely dependent on fluctuations of salinity and water temperature, Leung said. In the heat of summer, the mortality rate of these species climbs, especially of those living 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 (WQC), at or below which at least 95 percent of local species would be protected from chemical contaminants. Often, these WQC are based on toxicity data generated from standardized laboratory conditions, including fixed water salinity, temperature and pH value (balance of acidity versus alkalinity).
In tropical and subtropical regions, similar to Hong Kong, the laboratory temperature is often fixed at 25 C. Such a lab-based condition, however, doesn’t match real marine conditions which are subject to daily and seasonal changes. Hence, the lab-driven WQC may not provide adequate protection for marine organisms. What’s needed, argues Leung, is a gauge that takes into account aggregate sea conditions, including salinity, temperature and other potentially influential environmental factors.
This fluctuating condition of all or any of these factors plays a particular role in determining the metal toxicity of the immediate marine environment. Daily production of metal pollution has been increasing annually as a result of farming, industrial activity and just ordinary, day-to-day activities. Naturally, metal toxicity has an immense impact on the marine environment.
Water salinity is a major factor here. When heavy rain hits southern China during summer monsoons, more freshwater flows from the Pearl River. The water salinity along Hong Kong’s west coast falls. Lower salinity speeds the dissolution of metals (ionization), increasing the amount of iron being released into the surrounding environment which is chemically active and more toxic to marine organisms. Elevated concentrations of such metal ions can inhibit the growth and reproduction of marine organisms nearby.
By contrast, when water salinity is high, metal ions readily react with other ions present in the seawater such as chloride and sulphate to form complexes which are less toxic than metal ions, reducing the metal toxicity.
Water temperature is another factor. “Increasing 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 in 2017, Leung wrote: “At 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.” There is a great need to study how geographical and climatic conditions will affect what is currently known about the interactive, toxic potencies of metals, he added.
Metal detectors
Each organism has its own tolerance for metal toxicity. Leung and his team are mapping those tolerances on different marine species, as revealed within the naturally variable environment. The aim is to set up site-specific WQC to protect various 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, including two chordates, five arthropods, and a species of algae, under different combinations of water temperature and salinity. The study also made reference to natural regimes of temperature and salinity based on a global oceanography database.
The results found that predicted toxic potencies of the six metals on the eight species correlated directly to a temperature gradient between 10 and 30 C. When the temperature increased at a given site, mortality rates increased proportionately among the various species. These results imply that tropical marine species, particularly those inhabiting in estuary with a low salinity, are more susceptible to metal toxicity. The study was published in the peerreviewed journal Environmental Science and Technology.
The project is helpful to measure the biological values of marine sites in Hong Kong — providing the knowledge of how many marine organisms live there, which are rare and need protection, or which region has the most populated organisms.
With this information, the city’s Agriculture, Fisheries and Conservation Department will have a better understanding of which sectors of the marine sites should be demarcated for conservation, which are for fish farming and which for recreation facilities.
“This is a win-win situation,” Leung said. By knowing the acceptable risks of chemical contaminants (like sitespecific WQC), the optimum protection of marine organisms is ensured. Meanwhile, it supports some industry activities and economic development by allowing the discharge of certain pollutants without compromising environmental protection.
Toxic stew
In Hong Kong, existing water quality standards for marine waters do not set out limits for chemical pollutants. Other parameters, such as dissolved oxygen and nutrients, have specific guidelines limiting their volumes in marine waters, but the levels of toxic pollutants are not addressed. The statute states that toxicants are “not to be present at levels producing significant toxic effect” for all 10 water control zones. The guidelines, however, are not more explicit.
In comparison, the United States Environmental Protection Agency lists detailed criteria for aquatic life in ambient water, taking into account approximately 150 pollutants.
Leung also noted that Hong Kong lacks governance on “plasticizers”, chemical additives used to make manufactured plastic products more flexible or durable. Plasticizers are commonly used in consumer products such as footwear, food packaging, children’s toys, wire and cable. Many of these chemicals, such as phthalates and bisphenol A, are environmental estrogens that can lead to intersex in male fishes and interruption of their reproduction at low concentrations.
“The toxic effects of single plasticizers are known and there are WQC concerning those, but when two types of plasticizer combine in a marine environment, their combined toxicity may be enhanced,” Leung said. Further scientific research efforts are urgently needed to address the mixture toxicity of coexisting chemical contaminants in the environment, he emphasized.
Leung and his team also analyzed environmental statistics for temperature and salinity at different marine sites around the globe. Their findings may be applied to provide estimations of site-specific WQC, evaluating the toxicity of more than 30 metals against the backdrop of evolving temperatures and salinity.
Leung hopes that, if resources allow, each chemical contaminant may be tested according to the interplay based on water temperature and salinity. In that way, he anticipates that a better understanding of the toxin effects on marine organisms can be achieved.
While Hong Kong people feel proud of the biodiversity of the city’s marine life, that status quo cannot be taken for granted. Preserving the environment will demand that the existing record be given further study, providing a safer, more sustainable marine environment.