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SECTION
5 ASSIMILATIVE CAPACITY CAPITAL STOCK
| 5.2 |
Assimilative
Capacity of Marine Waters |
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| 5.2.1.12 |
In
order to show a broad picture of marine
water quality in Hong Kong, trends in the
average concentrations of four key marine
pollutants (DO, SS, TIN and E. coli)
across each of the ten WCZs are shown in
Figures 5.2a,b
to Figures 5.2c,d.
The figures, calculated as averages of the
summary statistics presented for each pollutant
in EPD's marine water quality reports for
1994 to 1998 inclusive, show that levels
of E. coli are highest in the Deep Bay and
Victoria Harbour WCZs and that Deep Bay
WCZ also has the highest levels of suspended
solids and TIN.
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| 5.2.1.13 |
A
more comprehensive analysis of the current
state of Hong Kong's marine waters is included
in tabular form in Annex
F2 which reviews marine water and sediment
quality issues by WCZ as published in the
EPD (1998) Marine Water Quality in Hong
Kong in 1997. From this information a number
of overall points can be made about marine
water quality:
- Hong
Kong waters are under increasing pressure
from local wastewater discharges and as
a result levels of E. coli have increased
considerably in most Hong Kong waters
in the past decades. Some WCZs have also
shown increasing trends in nitrogen and
orthophosphate levels
- Discharges
of poorly or un-treated sewage are resulting
in increasing levels of bacteria in parts
of Junk Bay, Southern (near Lamma Island),
Mirs Bay (Starling Inlet only), Western
Buffer, Deep Bay and Victoria Harbour
Water Control Zones in particular.
- In
a number of locations, the natural capacity
of the marine environment to assimilate
waste inputs appears to be taxed and has
perhaps been exceeded. Problems are particularly
acute in Deep Bay which is subject to
discharges of untreated sewage and inputs
of poor quality water from rivers draining
into the Bay, combined with a naturally
low level of flushing.
- Where
pollution control measures have been implemented
(for example in Tolo Harbour), improvements
in water quality are being registered.
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| 5.2.1.14 |
Marine
water quality is also monitored in a number
of Hong Kong's typhoon shelters. These waters
have high levels of pollution as a result
of discharges from vessels and adjacent
built up areas which are exacerbated by
the limited flushing capacity of the area.
In 1997, Kwun Tong, Yau Ma Tei and Causeway
Bay Typhoon Shelters had the poorest water
quality and Yim Tin Tsai and Cheung Chau
had amongst the best water quality (EPD
1998a).
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| 5.2.1.15 |
Marine
waters in Hong Kong have been subject to occurrences
of Harmful Algal Blooms (or 'red tides' as
they are commonly known) which are blooms
of phytoplankton (micro-algae) turning the
seawater into different shades of red brown
colour. For algal blooms to occur three conditions
must be met: the right proportion of nutrients
(nitrogen: phosphorus: silicate ratio), suitable
temperature and light conditions and presence
of the algal organism. Although naturally
occurring, red tides are commonly found in
waters with high nutrient input and are thus
often linked to pollution (although some red
tides are not directly linked with pollution).
Red tides can have various effects on water
quality, in particular through depleting oxygen
at night and reducing light required for photosynthesis
by other organisms and in exerting a further
oxygen demand when the blooms collapse and
decompose. Several phytoplankton species are
toxic to fish, shellfish or humans (the latter
usually through consumption of the contaminated
shellfish or occasionally through dermal exposure
to the contaminated water in beaches). |
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| 5.2.1.16 |
From
1980 to 1997, 481 red tides have been recorded
with most occurring in the eastern WCZs of
Tolo Harbour and Channel (48% of occurrences),
Mirs Bay (16%), Port Shelter (13%) and the
Southern WCZ (13%). A more detailed breakdown
of red tide occurrences since 1990 is included
in Annex F3, for
Hong Kong waters and for beaches. The data
from this table show that the greatest incidence
of red tides is from the months of December
to May with no discernible pattern in the
number or percentage of these occurrences
at bathing beaches. The annual numbers of
red tide occurrences have declined since the
start of the decade, although a significant
proportion of these (between 20% and 50%)
have affected bathing beaches since 1993.
In 1998, harmful algal blooms resulted in
significant losses to the mariculture industry
through several fish kill events. Mariculture
feeding practices are thought to have lead
to an accumulation of nutrients on the seabed
underneath the mariculture rafts. The presence
of such nutrient rich sediments may at times
encourage algal blooms in the area which,
when they collapse, may result in depressed
levels of dissolved oxygen affecting the cultured
fish in the cages. |
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