Red Tides in Hong Kong
Where is the impact of red tides on our marine life and people commonly found in Hong Kong?
Introduction
Red tides are frequently sighted in Hong Kong's territorial waters. They are caused by algal blooms that refer to the rapid multiplication of tiny, single-celled algae in coastal waters ( AFCD ). The adverse effects of red tides on marine life and even people are sometimes realized in Hong Kong, though they are not large-scale. This project will explore the spatial extent of the impact of red tides in Hong Kong.
Characteristics of red tides
Are algal species red in colour?
Despite being called ‘red tide’, some algal species discolour the sea water and turn the water murky brown, red, green, bioluminescent or other colours, depending on the pigment of cells (see the photos on the right).
What are the causes of red tides?
Red tides occur when conditions are right for certain marine algae to grow rapidly. There are many causes for algal blooms in coastal waters and it is often difficult to pinpoint specific reasons for the formation of algal blooms. The favourable factors for the formation of bloom are increased nutrient content, warmer sea surface temperature and low salinity, strong light intensity, and calm seas ( ﾠBruckner, 2023ﾠ ). Human activities, such as farming and industry also contribute to their growth through the discharge of organic matter and nutrient-rich matter into the sea (see the figure on the right).
Are red tides harmful?
Most algae are beneficial because these tiny plants are food for animals in the ocean. They are the major source of energy that supports living organisms in the higher trophic level in the ocean food web. However, a small percentage of algae produces powerful toxins that can kill fish, shellfish, mammals, and birds, and may directly or indirectly cause adverse effects on people ( ﾠNOAAﾠ ). Scientists call them harmful algal blooms (HABs).
The situation of red tides in Hong Kong
In Hong Kong, the coastal waters have been profoundly influenced by red tides for many decades. According to the data provided by the ﾠCSDI portal on 28 April 2023, it was found that a total of 1817 red tide incidents had been recorded in Hong Kong since 1975. They occur about 14 times per year ( ﾠKnott, 2022ﾠ ). Among all these incidents, 70% of them were related to dinoflagellates, while about 12% were diatoms, as shown in the dashboard below. Most red tides are formed by many of the algal species that belong to these two common groups. Of course, other groups of phytoplankton species are being recorded to cause red tides in Hong Kong ( ﾠAFCDﾠ ).
ArcGIS Dashboards
Location of the occurrences of red tides
Incidents of red tide have been observed in various locations throughout Hong Kong. By clicking each orange circle on the map on the right, the number of incidents is shown in each cluster. Most of the red tides are spotted in Tolo Harbour & Channel, the southeastern part of Hong Kong and on the southern side of Hong Kong Island. On the other hand, the number of incidents of red tide was the least in the western shores of Hong Kong.
Refer to the map on the right. Most of the red tide events occur in the less exposed estuaries, bays, inlets and harbours such as Tolo Harbour where the flushing effects are small, especially in spring when the river discharge is low. Also, red tides are commonly found in waters which are near more populated rural villages (village clusters shown in dark blue) without public sewerage systems, like those near Tolo Harbour.
If untreated sewage containing nutrients from the unsewered rural villages in the streams and uneaten feed and waste from inshore fish farms ( Time Out Hong Kong, 2016 ) enter the coastal waters of Hong Kong, the limit of the coastal currents to remove the pollutants will be exceeded easily. This leads to eutrophication and then red tides. Nevertheless, not all red tides are caused by the discharge of pollutants locally.
In the recent two decades, pollutants from the densely-populated Zhujiang Delta Region have contributed to the local red tides, following the direction of water flow (see Figure (a) on the right). During the rainy summer, the Pearl River discharge is large (see Figure (b) on the right). It carries a large amount of river water containing nutrients to the western shores of Hong Kong. Some may flow to the southern waters following the SW wind. They can trigger the algal bloom easily in the western and southern parts of Hong Kong.
Spatial extent of the impact of red tides in Hong Kong
In Hong Kong, at least 105 algal species have been recorded to form red tides since 1975. A majority of them, including Noctiluca scintillans, the most popular marine species of dinoflagellate found in the eastern and southern parts of Hong Kong waters (see the map below), are not harmful (AFCD), even in high concentrations. Instead, they provide food for marine life up the food chain in the sea. Therefore, red tides rarely lead to serious effects.
Dominant type of algal species in each cluster in Hong Kong
However, some toxic algal species may still bring negative impacts on our marine life and people in different parts of Hong Kong waters. Let's explore where these impacts are commonly found.
Impact found in fish culture zones
The overlay analysis counts the number of red tide incidents that have occurred in the fish culture zones. It is found that fish culture zones mainly located in the eastern and southern parts of Hong Kong waters have been affected by red tides. Among them, the ones in Yim Tin Tsai and Yung Shue Au have been threatened most frequently since 1975, as shown on the map on the right.
Example of major recent damage caused by toxic algal species in Hong Kong:
Even though most of the algal species may not be toxic in the fish culture zones, red tides may also kill fish indirectly by depleting the oxygen in the water. The rapid growth of algae may consume a large amount of oxygen in the water during the night. Once the massive algae die, bacteria may decompose them and further deplete the oxygen in the water. Fish cultured in fish cages will die from a lack of oxygen (see figure on the right). This will result in great economic losses.
Impact found in marine parks and marine reserves
Some of the marine parks and marine reserves may be affected by red tides. As shown on the map on the right, the number of occurrences of red tides within a 1-km buffer zone of the marine parks or marine reserves is summarized. It is found that Hoi Ha Wan Marine Park, Yan Chau Tong Marine Park and Tung Ping Chau Marine Park in the northeastern part of Hong Kong waters have been threatened by red tides most frequently since 1975.
The red algal bloom can cause the reduction of coral ( Kao, 2016 ) in places like Hoi Ha Wan and Tung Ping Chau because the algae may attack the tiny organisms that the coral relies on for survival ( Todorova, 2008 ). When the coral dies, other living organisms will lose their habitat and become extinct. This will reduce the biodiversity and disruption of the marine ecosystem.
Also, the depletion of oxygen caused by the red tides may cause the death of fish and other marine life in marine parks or reserves. The whole ecosystem will collapse.
Impact found in the recreational facilities
Red tides contribute to serious impairment of recreational facilities. As shown on the map on the right, the number of occurrences of red tides within a 1-km buffer zone of the beaches or water sports centre is summarized. It is found that all beaches have been threatened by red tides since 1975. Among them, those in the southeastern part of New Territories and southern part of Hong Kong Island, such as Silverstrand Beach, Clear Water Bay Second Beach and Deep Water Bay Beach have had more frequent occurrences of red tides.
Also, four out of five water sports centres have been affected by the occurrence of red tides, but most red tides have been reported in Stanley Main Beach Water Sports Centre.
The occurrence of red tides in bathing beaches or water sports venues may lower their recreational value because it renders them unsafe for swimming.
In addition, the formation of red tides degrades the aesthetic value of an environment by discolouring the seawater. Some red tides may produce large and visible colonies, extensive foams or a fouling smell when the algae are washed on beaches. If a large number of marine organisms die in the coastal area, the dead bodies may be washed to the shore and decomposed, producing a bad smell and hygienic problems ( ﾠAFCDﾠ ).
Suggestions for improvement
Some mitigation and adaptation strategies should be adopted to tackle red tides in order to protect ecosystems, maintain fish harvest and improve people's quality of life.
Mitigation strategies help reduce the occurrences of red tides. First, the Drainage Services Department should carry out regular inspection and maintenance of the public sewage system, and connect the sewers into unsewered village houses. Therefore, any sewage containing nutrients can be properly treated before being discharged into the rivers and then the coastal waters, such as Tolo Harbour. Second, the fishermen should be advised not to dump uneaten feed and waste from their fish farms into the sea. The penalty should be given for illegal dumping.
Adaptation strategies can help reduce the negative impacts of red tides. First, the government should continue to monitor and study the algal blooms, so as to determine how to detect and forecast the location of red tides. This aims to provide fishing communities with advance warnings, so fishermen can adequately prepare for the adverse environmental, economic and health effects of red tides. Second, an app should be developed to tell the mariculturists where they should relocate their fish rafts in case of red tides so that they can reduce their economic loss. Third, education and promotion help raise public awareness and prevent the public from swimming and having close contact with red tides.
