Eastern Waters

Gorgonian coral, found in the Ninepin Group

The area covered by the Eastern Waters includes three separate Water Control Zones: Tolo Harbour & Channel, Mirs Bay and Port Shelter. The Tolo Harbour & Channel WCZ is a nearly land-locked body of water, with just a narrow exit out into Mirs Bay to the east. Port Shelter, as its name implies, is also sheltered by islands and inlets, but its waters are much more open than those of Tolo Harbour, and its flushing ability better too. Mirs Bay is a body of water shared with the Shenzhen Special Economic Zone. The Mirs Bay WCZ includes both an inner bay to the east of Tolo Harbour and Port Shelter, and an outer bay to the south open to oceanic flow where the water is deeper and clearer. Generally, water quality in the eastern waters is good, and this area supports a wide variety of marine life including sponges, corals, and over 300 reef-associated species.

Yan Chau Tong Marine Park in Mirs Bay

Within the Eastern Waters, three areas have been gazetted by the Government as ‘secondary recreational waters’, meaning they are designated areas for sailing, surfing and similar activities. They are Port Shelter, Tolo Harbour, and three subzones in Crooked Harbour of Mirs Bay. In addition, the Government has established three Marine Parks in the Eastern Waters, which protect marine life from human activities. Throughout the area, 21 separate places have also been designated as ‘fish culture zones’. A major container port, Yantian port, is located at Shatoujiao on the Shenzhen side of Mirs Bay.

Tolo Harbour

Profile of dissolved oxygen, temperature and salinity in a typical stratified water column in the Tolo Channel

The harbour’s landlocked situation and narrow exit to the open sea mean that it suffers from weak water circulation. This is exacerbated by a north-easterly wind which blows against the direction of the outgoing tide. These two factors together prevent pollutants that enter the harbour from being easily flushed away by tidal flow. In addition, in summer the harbour sometimes experiences temperature and salinity stratification, where layers of water build up with quite different levels of temperature and salinity which prevent mixing of water masses, leading to hypoxia (low oxygen levels) at the sea bottom.

Between 1986 and 2001 the population in the Tolo Harbour catchment area nearly doubled, from 0.5 million to 0.9 million, as the new towns of Sha Tin and Tai Po were developed. The sudden rise in population was matched by a rise in wastewater generated in the area, which was discharged into the harbour from the Sha Tin and Tai Po Sewage Treatment Works. Though much of the increased discharge into Tolo Harbour received secondary treatment at the sewage works, the increased levels of nutrients proved too much for the harbour's limited abilities to assimilate them. Red tides (or algal blooms) began to occur with increasing frequency in the 1980s and into the 1990s. In 1988, a record total of 40 outbreaks of red tide took place in the harbour.

The Tolo Harbour Action Plan (extracted from "Environment Hong Kong 2000")

To halt the deterioration of Tolo Harbour, the Government drew up a Tolo Harbour Action Plan in 1986 consisting of a number of initiatives to control and reduce pollution in the harbour. The plan went into effect in 1987, when the keeping of livestock was prohibited or restricted in much of the harbour catchment area, and strict livestock waste controls were put in place. At the same time, sewage treatment works were improved, pollution from an old landfill site was reduced, and plans began to be made to extend the public sewer network to the rural areas. A further initiative came with the proposal to export the treated effluent from the Sha Tin and Tai Po Sewage Treatment Works for discharge in the better flushed Victoria Harbour, thus lightening the pressure on Tolo Harbour. This plan was put into action between 1995 and 1998.

The Tolo Harbour Action Plan has achieved very positive results over the last two decades since it was set up. Water quality in the harbour has improved substantially, and significant decreases in 5-day biochemical oxygen demand (BOD5) (organic pollutants), E. coli, nitrogen and phosphate have been recorded by the monitoring programme. Levels of chlorophyll-a in the harbour, meanwhile, have remained fairly stable but red tide incidents have reduced markedly, from over 40 in 1988 to around 10 each year in the last decade. Fish kills, which were frequently seen in the 1980s and 1990s, are rare nowadays.

Water Quality Improvements in Tolo Harbour (1986-2005)

Measured in terms of WQO compliance, Tolo Harbour has seen a major turnaround. For a number of years now, the Tolo Harbour and Channel WCZ has achieved 100% compliance with the WQO for E. coli. All stations apart from three in the Tolo Channel also achieved full compliance with the dissolved oxygen WQO.

Thanks to the success of the nutrient pollution reduction measures, Tolo Harbour has maintained total inorganic nitrogen and unionised ammonia at low levels over recent years. In 2005, compliance with the chlorophyll-a WQO was high, with all samples collected from the inner harbour and the outermost station meeting the WQO, and over 90% of samples collected from the other stations also meeting the objective.

Sediment sampling in the harbour tells a little more about the scope of pollutants that it has accumulated in the past. The monitoring programme has found highly anoxic sediment in some parts of the harbour, the long-term effect of organic pollution from local populations and from the fish culture industry.

The harbour's innermost monitoring stations have recorded levels of lead higher than the Lower Chemical Exceedance Level (LCEL). This suggests historical contamination, most likely from the use of leaded petrol before it was banned in 1992.

The monitoring of phytoplankton has shown that Tolo Harbour contains more phytoplankton species that can cause red tides than anywhere else in Hong Kong waters - a total of 55 species, as compared with just six such species in Deep Bay. Between 1980 and 2005, 309 of the 750 or 41% red tides recorded as occurring in Hong Kong waters took place in Tolo Harbour.

Generally, more species of phytoplankton occur in the eastern part of Hong Kong than the western because many species require oceanic levels of salinity to survive. In addition, the clearer waters of the east allow more light to penetrate, an important factor in encouraging phytoplankton growth. When this is combined with high levels of nutrients entering the water, as in Tolo Harbour, phytoplankton populations have the potential to develop into algal blooms.

Mirs Bay WCZ

Soft coral found in Hong Kong waters - one of the most beautiful marine creatures in the world

The largest Water Control Zone, Mirs Bay lies to the east of Hong Kong and includes water that is usually little influenced by the Pearl River to the west. Because the land adjoining Mirs Bay has relatively low population levels and little major development, the water quality of the Bay has always been good, and the area boasts most of the coral beds found in Hong Kong. Within the northern part of this Water Control Zone nine fish culture zones are located in sheltered embayments, and there are also two Marine Parks in Yan Chau Tong and Tung Ping Chau populated by a large number of species of fish and coral.

When the Mirs Bay WCZ was first monitored in 1986, only one monitoring station was established (near Port Island, in the outlet of the Tolo Channel). By 1991, the EPD had increased the number of monitoring stations to eight, and that number continued to rise until in 1998 it reached the current total of 13.

In 1993, Mirs Bay experienced massive hypoxia (oxygen deficiency) during the summer. It was a very wet year, and this incident may be related to the heavy rain and the particularly large outflow of fresh water from the Pearl River that year. Calm weather and water stratification followed (i.e. little mixing of the fresh water on top and the salt water underneath), which prevented oxygenation of the water column. Oxygen levels fell, and many organisms living on the sea floor died as a result.

Given the high ecological value of Mirs Bay and the clear need to protect its sensitive ecology and diverse marine flora and fauna in the face of port and other developments, the governments of the HKSAR and Shenzhen jointly drew up the Mirs Bay Action Plan in 1998. This was a cross-border initiative designed to control pollution from both sides. Environmental protection authorities from Hong Kong and Shenzhen exchanged monitoring data and agreed on pollution reduction plans. Both sides also agreed to restrict the development of heavily polluting industries in the Mirs Bay catchment area, especially in the inner Mirs Bay region around Shatoujiao.

Since the implementation of the Mirs Bay Action Plan, there has been a significant decrease in nutrients such as total phosphorus and total nitrogen. However, higher levels of chlorophyll-a (which indicates increased algal productivity) are still found at many northern stations around Starling Inlet, signifying the possibility of phytoplankton bloom. Water quality around Sha Tau Kok (Hong Kong) and Yan Tian/Inner Mirs Bay (Shenzhen) is being kept under close watch because of potential pollution sources in the area.

Despite this, the Mirs Bay WCZ has an excellent record of compliance with the WQOs, achieving 100% compliance in 2005 as it has since 2000. In addition, the three secondary contact recreation subzones in Crooked Harbour, Long Harbour and Double Haven, which are used for activities such as windsurfing and sailing, had very low E. coli counts and fully complied with the WQO in terms of bacteriological water quality.

Although the water in Mirs Bay is generally good, sediment monitoring has shown a few areas with highly anoxic sediments (below -300mV). These were in parts of Inner Mirs Bay, specifically Double Haven and Crooked Harbour, which have many fish culture zones. The high organic matter in the sediment was deposited over time, building up from fish food and excreta. Elsewhere, sediment was cleaner and had low levels of heavy metals and other toxic chemicals.

Port Shelter WCZ

This WCZ is made up of Rocky Harbour, outer and inner Port Shelter, and Hebe Haven. The whole area is partially enclosed by indented coastline and islands, and so is protected from the worst effects of adverse weather except when the wind blows from the south-east. The water in Port Shelter is generally good and well-mixed.

Boat sailing in Port Shelter

Its beautiful natural landscape and sheltered aspect makes this area ideal for recreational activities, and the whole WCZ has been designated a 'secondary contact recreation zone'. It has become one of Hong Kong's most important water sports and recreational resources. In addition, six fish culture zones are situated within the WCZ.

The catchment has no large urban developments within it, but there is a sizeable population scattered in small towns and villages, many of which are on the coast. The major settlements in the WCZ include Sai Kung Town, Pak Kong, Pak Sha Wan, Ho Chung, Tai Po Tsai, Silverstrand and Clear Water Bay. An advanced secondary sewage treatment plant is located at Sai Kung and provides a high degree of treatment to sewage, including UV disinfection and nutrient removal, before discharge. This facility has helped maintain good water quality within Port Shelter even as the population has increased over recent years.

The inner part of Port Shelter did, however, show the effects of population increases in the 1990s, when notable increases in E. coli were found at the monitoring station near Sai Kung town. Improvements were made in the sewage infrastructure, leading to a reduction in the amount of untreated sewage flowing into the sea. At the same time, pollution control measures were implemented that reduced or eliminated run-off waste from livestock farming and other activities. Most monitoring stations in Port Shelter recorded decreasing trends in Kjeldahl nitrogen, total nitrogen and total phosphorus over this period.

By 2005, water quality within Port Shelter was excellent and fully complied with the WQOs. It had high levels of dissolved oxygen and low levels of nutrients and E. coli bacteria, indicating its continued suitability for secondary contact recreation.