Shenzhen has 169.7 hectares of mangrove forest, which make up one of China's most important wetland conservation zones. But because of the city's rapid development, the size of the wetland has shrunk by half in the past two decades.

Over 60 per cent of species have disappeared from the mangroves, according to a Shenzhen government study. In its heyday, more than 100 bird, plant and fish species, some of them endangered, lived in the Futian mangrove forest conservation zone, one of only two national mangrove reserves in China.

An official in charge of the mangrove forests in Shenzhen attributed the problem to reckless urbanisation and industrial pollution. Many factories and residential communities discharge untreated sewage into Deep Bay.

According to the government report, the level of heavy metal contamination in waters inside the conservation zone meant they failed to meet even the lowest measurement of water quality - grade five. Grade-five water is deemed heavily polluted.

The mangroves are surrounded by pig and fish farms, runoff from which makes the waters overly nutrient-rich, allowing non-native seaweed species to grow rapidly.

Xu Hualing, of the Shenzhen Mangrove Forests Administration, told Southern Metropolis News that the seaweed had 'seriously affected' mangroves in Hong Kong.

The director of the Shenzhen Bird Watching Society, an expert on mangroves, said: 'The problems affect the general environment in Deep Bay ... We could face an ecosystem collapse at any time.

'If the [Shenzhen] mangrove forests were gone, it could be a fatal blow to the Hong Kong mangrove forests. Fewer birds would fly to the Pearl River Delta.'

The Mai Po nature reserve is a major overwintering ground for scores of migratory bird species, including the endangered black-faced spoonbill.

Academic research in Hong Kong has shown that high levels of heavy metals in Deep Bay hamper birds' ability to raise their young.

Paul Lam Kwan-sing, of City University's department of biology and chemistry, said heavy metals were found in eggs and feathers of birds at Mai Po during research conducted from 2000 to 2002.

He urged the Hong Kong government to identify sources of pollutants immediately and ensure sewage treatment plants across the border operate properly.

Mai Po reserve manager Lew Young said that once heavy metals entered the food chain, birds there would be endangered.

The Environmental Protection Department does not monitor heavy metals in seawater, saying concentrations are usually very low.

To prevent further depletion, the Shenzhen government has said it will invest 300 million yuan in the coming years to restore the mangrove forests. The money would be spent on water treatment, rebuilding habitats for birds and moving traffic away from the conservation zone. But mainland government experts say it would take 10 to 20 years for these measures to take effect.