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.