Hot weather, heavy rainfall, pollution from the Pearl River and other unknown causes have been blamed for one of the worst declines in the quality of marine water last year.
A report by the Environmental Protection Department says quality deteriorated in several districts, including central Victoria Harbour, Tolo Harbour, Southern and Deep Bay, which all fell short of the overall water quality objectives.
Eastern waters - including the planned route of the first cross-harbour swim in 30 years - all met the standards. The event, once held between Tsim Sha Tsui and Central, will be staged between North Point and Lei Yue Mun in October.
The environmental watchdog declined to comment on whether water quality had improved so far this year, saying only that it was comparable to last year's.
The annual marine water quality report issued by the department last week says the city's overall water quality objectives compliance rate hit the lowest level - 80 per cent - since 2001. This means that up to 20 per cent of the time the waters failed at least one of the water quality standards such as E coli bacteria, dissolved oxygen - which is vital to marine organisms - and total inorganic nitrogen or nutrients in the water.
Water samples are collected at a network of 76 water monitoring stations once a month.
The Deep Bay waters continued to have the lowest compliance rate at 40 per cent, while eastern waters in Tseung Kwan O met the objectives all year round. Tolo Harbour registered the worst deterioration, falling from 71 per cent in 2009 to 50 per cent, due to 'zero' dissolved oxygen compliance at all monitoring stations there.
Environment officials described it as a 'rare phenomenon' that had only happened once in the past 15 years. They believed it was caused by heavy rainfall rather than a rise in pollution discharged into the harbour.
The compliance in central Victoria Harbour also dropped significantly from 93 per cent in 2009 to 77 per cent last year, again due to poor dissolved oxygen readings. Most of the shortfalls took place during hot and wet months.
The department said high temperatures could slow the rate at which oxygen dissolved in water, and heavy rain could prevent mixing of the top and bottom layers of water columns to improve oxygen levels.
While water quality sometimes hinges on the weather, the report blamed excessive nutrient levels for poorer water quality in southern waters and Deep Bay. It says this might have been caused by pollution from the Pearl River after heavy rain.
Dr Man Chi-sum, chief executive of Green Power and a government environment adviser, said the department should not blame weather and external factors for the deterioration of water quality. 'Some of our sewage is still pumped untreated into the waters, while the wastewater collected at the sewage treatment works is not being treated to the most desirable standard,' he said.
The Environmental Protection Department plans to collect the remaining raw sewage, mostly from Hong Kong Island, for the city's largest sewage treatment works in Stonecutters Island by 2014.
Tonnes of sewage generated daily in Hong Kong
- About a quarter is pumped into the harbour after simple screening