Biological Control of Reservoir Water Quality Revealing the Importance of Ecosystems from the Standpoint of Drinking Water Safety

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Recently, the Water Supplies Department released the Total Water Management Strategy, proposing measures for sustainable water use, in anticipation of potential acute climate change and the accompanying unstable supply and distribution of freshwater resources. At present, about 30% of Hong Kong's freshwater supply is from the reservoirs and 70% is from Dongjiang River, and is also stored in the local reservoirs. Sustainable use of water resources is certainly important. Water quality of the reservoirs must be well controlled too. You might think that as most reservoirs are located inside the country parks and the catchment areas are protected by law, reservoir water should be free from pollution. In reality, water quality will worsen even in a natural environment. To preserve water quality for safe drinking, we must rely on the natural power of ecosystems instead of technology and chemicals.

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In the early days, Hong Kong inhabitants relied on streams and wells as their water sources. As the population boomed, water supply from these sources could not fulfill the increasing demand for drinking water. The first local reservoir, the Pokfulam Reservoir, was completed in 1863. In 1968, the world’s first lake to be built in the sea, the Plover Cove Reservoir, was completed in Hong Kong. The High Island Reservoir, completed in 1979, was the last of a total of 17 reservoirs storing drinking water in Hong Kong, distributed across Hong Kong Island, Kowloon, and the New Territories including Lantau Island.

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In appearance, reservoirs resemble natural lakes. The difference lies in reservoirs being artificial, while natural lakes form over long periods of time. Most reservoirs in Hong Kong were built by artificially damming valleys. From a natural history perspective, a reservoir is just a newly formed freshwater habitat. However, a reservoir is generally much deeper than a natural lake, and more like a barrier lake in terms of geomorphic structure.

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A barrier lake is often formed by natural damming of water bodies by landslides, debris flows or lava. As its structure is usually unstable, it will eventually burst, restoring the original state. A natural barrier lake, existing for only a short period, cannot evolve into an independent ecosystem. This is similar to the case of a reservoir, though a reservoir will not “vanish” like a barrier lake. A reservoir is an artificially and newly created water pool in the upper or middle course of a river. The water level varies significantly and quickly. The water exchange rate is high, so it is difficult for aquatic and terrestrial flora and fauna to adapt. It is unlikely that a stable and mature ecosystem can develop.

As reservoir is not a mature ecosystem; the material cycle is inhibited and decomposing matters are easily accumulated in the water, causing algae blooms and worsening water quality.

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Algae threat to water quality

Algae are plankton that can carry out photosynthesis in water. They thrive in aquatic environments including natural streams and lakes, though they do not accumulate to a high concentration in flowing streams. In a lake ecosystem, algae are important producers which absorb sunlight and nutrients from the water and in turn become food for aquatic creatures.

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However, as water in the reservoir, which itself is an immature ecosystem, is stagnant instead of flowing, when stream water in the catchment carrying nutrients such as nitrogen and phosphorous enters the reservoir, problems arise. The accumulated nutrients lead to the excessive growth of certain algae species, which results in algae blooms. The bloom depletes oxygen levels in water and a low dissolved oxygen level favours the concentration of iron, manganese and sulphide in water, causing colouring, odour and taste problems with the drinking water.

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In addition, as dissolved oxygen is depleting, blue-green algae thrive. Some of the algae release toxins and impact water quality. In the low oxygen conditions, many fish, shrimps and amphibians die off. The large amounts of organic matter (corpses of the dead animals) further contaminate the water, as they undergo anaerobic decomposition and release toxic substances such as ammonia and hydrogen sulphides.

An oxygen-deficient environment also encourages the growth of parasitic protozoa, including pathogens such as Cryptosporidium and Giardi, which are spread through dirty water and often cause diarrhoea, acute gastroenteritis and malabsorption problems. Apart from thriving in reservoirs, Cryptosporidium and Giardi can grow in subsurface water and their ooxysts and cysts can survive in water for long periods. Therefore, water supply authorities all around the world take great precautions in controlling these two species of parasitic protozoa.

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Hence, without proper management, water quality in a reservoir with no outside pollution may still worsen to substandard level. Nature is needed.

Fish fry to recreate ecological balance

In Hong Kong, the Water Supplies Department utilises fish fry to build up reservoir ecosystems. Feeding habits of different species of fish help regulate algae growth and maintain water quality. The major fish fry used are Silver Carp (Hypophthalmichthys molitrix) and Big Head Carp (Aristichthys nobilis); the former feed on phytoplankton while the latter feed on zooplankton. Both can effectively control algae populations.However, the two fish are river species and are used to spawning in rapid flows. In the stagnant water environment, they fail to reproduce, and artificial replenishment of fish fry has to be routinely carried out.

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Tilapia (Tilapia spp.), Mud Carp (Cirrhinus molitorella), Goldfish (Carassius auratus), and Wild Carp (Hemiculter leucisculus) are also released into the reservoirs by the authority to control algae, insects, crustaceans and other small animals. Some of them are able to reproduce in the reservoir environment.

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On average, these fish have a life span of three to seven years. To regulate the fish populations, the Water Supplies Department opens all reservoirs to the public for fishing during non-spawning season. A fishing licence is needed, which requires that fish caught below a certain size have to be released alive back to the reservoir, and regulates the maximum number of fish kept by each angler in one day. The measure aims to prevent over-catching while keeping fish populations under control.

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To a certain extent, biological regulation of water quality interferes with the original river ecology. There is also the risk of introducing foreign species. Yet compared with addition of chemical substances, biological control is more comprehensive and effective. As reservoirs are the source of our drinking water, the use of chemicals poses a threat to public health. Besides, the cost of chemical treatment is also high, given the large capacity of reservoirs.

From the example of water quality control in reservoirs, we can see that ecosystems “serve” people at all times. They are vital to a healthy environment - unlike the conventional belief that nature is distant from human way of life, and conservation is a high-sounding ideal unrelated to our livelihood and is even an obstacle to urban development. The reality is, ecosystems are essential to every aspect of our daily life. And to protect nature is to protect the basic survival environment for humans - safeguarding our life support system.