The Channel Deepening Proposal
Deepening of shipping Channels in Port Phillip Bay is proposed
to enable vessels with a draft of 14 metres to enter the Bay and
the Port of Melbourne under all tidal conditions.
The proposal involves continuous capital dredging of 32 million
m3 of material over 2 years and maintenance dredging
of a further 11 million m3 is required over the life
of the project. This is unprecedented in scale.
Figure 1. Map showing proposed channel deepening 'northern works'
The dredge spoil is to be disposed in 'designated' areas in the
bay. The existing spoil ground in the north central bay area (8
km west of Ricketts Point marine Sanctuary) will receive 15 million
m3 of spoil including at least 2 million m3
of contaminated spoil.
Figure 2. Map showing proposed spoil ground West of Ricketts Point
A proposed new spoil ground north-east of Hovell Pile is to receive
28 million m3.
The north and south sites are ecologically unique as they are transition
zones where neighbouring ecosystems overlap.
Freshwater flows from the Yarra merge with marine waters in Hobson's
Bay. The relatively enclosed bay system merges with Bass Strait
at Port Phillip Heads. Both areas have their unique set of issues
associated with the proposal.
Some major issues with southern dredging include turbidity, potential
dieback of seagrasses, impact on marine species of noise associated
with rock removal, and overburden smothering substrate below.
The deepening at the Heads will increase tidal regimes around the
bay which will change coastal plant communities and result in some
beaches eroding.
However, although there are significant issues with the southern
works, to avoid confusion, this web page deals only with issues
associated with the northern works.
Further Information
Current Status
- Planning Panel has listed 128 matters that need further work
to ensure environmental risks are adequately addressed
- Government has ordered a Supplementary Environment Effects Study
- The Panel recommended 'Trial Dredging' be conducted near Port
Phillip Heads to monitor impacts, but did not limit the extent
or timing of the 'trial'
Channel Deepening Issues Associated with the 'Northern Works'
The Channel Deepening works as proposed in northern Port Phillip
Bay (PPB) are considered to pose a serious risk for the Bay's food
chain. Icon species such as dolphins and penguins ultimately rely
on successful breeding of smaller species to survive.
For example, a crash in the Southern anchovy population would be
disastrous for the St Kilda penguin colony and have serious implications
for the significant proportion of the Phillip Island penguins that
overwinter in PPB.
In summary, the central issues associated with the 'northern works'
are:
- Primary production (the basis of the marine food chain) requires
sufficient light to penetrate the water column.
The combination of dredge impacts and natural background levels
of turbidity and nutrient influx in the Yarra River and Hobsons
Bay during spring and summer represents a high risk to production.
This ultimately translates to reduced breeding success of fishes
and seabirds.
- Toxic materials have accumulated in the Yarra sediments over
the past 150 years from the urban and agricultural catchment.
Dredging contaminated sediments from the Yarra and Hobsons
Bay will remobilise toxins to the water column and impact on
fish and other marine life.
Disposing these sediments in the bay will result in their longer-term
dispersal across the wider bay area.
- Dredging in the Yarra River and Hobsons Bay during spring and
summer over two consecutive years will significantly reduce spawning
success of Southern anchovies.
This species is of critical importance to the higher food
chain; and black bream, an important species for the recreational
fishery.
The dredge plume may also impact on snapper spawning along
the north-eastern coast of the bay.
Figure 2. Map of Port Phillip Bay illustrating flows
Rainfall Events and Increased Nutrient Input from the Yarra River
Heavy rainfall events in Melbourne occur predominantly
in spring-summer. They translate as increased nutrients
flushed to the Yarra from the wider catchment. Excessive nutrient
loads combine with seasonal greater average hours of sunlight
to cause algal blooms that de-oxygenate the water leading to dieback
of organisms.
Note: The EES Water quality and sediment transport
modelling did not include freshwater pulses associated with thunderstorms.
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Rainfall Events... - continued
The following chart shows that heavy rainfall events in Melbourne
predominantly occur in spring and summer. Increased rainfall
can be taken to roughly translate as increased nutrients,
as heavy rain events flush nutrients from the wider catchment into
the bay.
Figure 3. Australian Bureau of Meteorology chart of heavy rainfall
events in Victoria
Impacts of increased turbidity (murkiness
of the water): During Yarra flood events the freshwater plume
extends well into Hobsons Bay, occasionally as far south as Beaumauris.
Freshwater overlies seawater until currents and wave action cause
the two layers to mix. Fine 'mud' particles and nutrients remain
in the freshwater plume until mixing with the seawater layer and
eventually settling.
Increased turbidity due to dredging will reduce light reaching
the bottom and therefore limit microphytobenthos growth; with consequences
for organisms higher up the food chain. Microphytobenthos in this
area play an important role in processing nutrients in the bay.
Ability of predators to see their prey will also be reduced. Breeding
colonies of seabirds which feed in the bay include Australasian
Gannets, Pied Cormorants, Crested Terns, and Little Penguins.
Resuspension of nutrients: Dredging
releases 'pore water' held within the sediment. Pore water typically
contains higher levels of nutrients than in the water column above.
Pore water from Hobsons Bay has substantially higher nutrient concentrations
than all other areas studied in PPB. Released 'pore water' nutrients
that were previously unavailable to plants, add to the nutrient
loads already existing in the water column.
Increased nutrient loading: Nitrogen
in the water column is derived mainly from organic (ie plankton
and dead material) and particulate forms, but also from oxidised
N and ammonium. The former are higher in northern PPB where algal
blooms are more common. The latter are highest around the major
inputs of the Western Treatment Plant and the Yarra River.
Reduced nutrient assimilation: Microphytobenthos
(single-celled algae on the seabed) play a key role in nutrient
cycling. Loss of microphytobenthos and reduced photosynthesis may
therefore have a significant impact on denitrification efficiency
in the Bay.
Mobilisation of toxic algal cysts:
EPA Guidelines state, 'if possible, dredging in Hobsons Bay should
avoid December to mid April when Alexandrium catanella
blooms are most likely.' Remobilisation of cysts may result in blooms
of toxic algal species Alexandrium catanella and A.
tamarense, that cause paralytic shellfish poisoning, and are
potentially fatal in humans.
Seasonally vulnerable fish spawning in Northern
PPB during spring and summer include:
- Snapper breed on reefs between St Kilda and Ricketts Point between
Nov - March.
- Black bream spawn in the Yarra estuary from October to December.
- Southern anchovies spawn October to March. At least two Expert
Witnesses engaged by the Port of Melbourne Corporation have acknowledged
that the impact of dredging on the Southern anchovy spawning season
is uncertain.
The reliance on anchovies as prey for penguins, other seabirds,
larger fishes, and dolphins has become critical since the pilchard
population crashed in the 1990s.
Note: 'Irreversible Damage'
to a species is most likely to occur when all ages (adults, eggs,
larvae, and juveniles) are concentrated within a restricted geographic
area. Destruction of habitat at this time can cause the
death of a significant sample of the population resulting in overall
loss of genetic stock available to future generations.
Port Phillip Bay Food Chain
Werribee Sewerage Farm and the Yarra are the major sources
of nutrient to the bay. Plants (primary producers) use nutrients
and sunlight to produce organic material. Consumer organisms feed
on the plants or on other animals and in this way energy and nutrients
are passed along the wider bay food chain.
Plants (primary producer organisms) are at the base of any food
chain as they are eaten by animals (consumer organisms).
In the marine environment, the phytoplankton (free-floating microscopic
plants) and microphytobenthos (microscopic plants anchored to the
seabed) are grazed on by a range of zooplankton (animals) that are
ultimately eaten by larger animals.
In northern PPB there are also larger plants such as seagrasses
and seaweeds. The growth and productivity of all these plants is
predominantly influenced by water temperature, light availability
and nutrient levels.
Southern Anchovy; where do they fit in?
Southern Anchovy (Engraulis australis) spawning occurs
at the northern end of PPB from October to April and usually peaks
in January. Adult anchovies feed primarily on zooplankton.
Commercial Catch (humans): In Victoria,
PPB is the most important commercial fishery for anchovy. Commercial
catches have traditionally been greatest along eastern PPB (Blackburn
1950). During 1998- 2001, the largest commercial catch of southern
anchovy was taken from an area just west of the Port Melbourne Channel
between Altona and Williamstown (Fisheries Victoria 2001).
Colonies of seabirds breeding within Port Phillip
Bay:
- White-faced Storm-petrels: September to March
- Australasian Gannets: July to mid-April
- Crested Terns: October to January
- Little Penguins: July to March
- Pied Cormorants: February to September-October
Except for Pied Cormorants, seabird breeding in PPB occurs mostly
in spring-summer. Again, with the exception of Pied Cormorants,
these seabirds feed primarily on schooling fish species such as
pilchards and anchovies. In 2004 the St Kilda penguin colony has
been found to rely almost exclusively on southern anchovies.
Note: Penguins from the Phillip Island colony
are also known to over-winter in PPB where adequate fish stocks
enable them to attain breeding condition. A
serious decline of anchovy stocks would clearly impact on these
birds.
Refrences
Blackburn M (1950). A biological study of the anchovy Engraulis
australis (White) in Australian waters. Aust. Journal of Marine
Freshwater Research. 1: 3-84.
Glossary of Terms
- Biological Oxygen Demand - combined oxygen requirements
of all organisms within a waterway.
- Eutrophication - the enrichment of water as a result
of introduction of nutrients, often casing excessive growth of
aquatic plants
- Flocculation - The change which takes place when the
dispersed phase of a colloid forms a series of discrete particles
which are capable of settling out from the dispersion medium.This
is an almost inevitable result of its mixing with a solution containing
electrolytes e.g. Seawater
- Microphytobenthos - microscopic plants living on the
seabed
- Nutrient assimilation - uptake and dispersal of nutrients
to the wider food chain
- Photosynthesis - production by plants of organic compounds
from water and carbon-dioxide using energy absorbed from light
- Phytoplankton - microscopic floating plants
- Turbidity - a complex composite of several variables
that collectively influence the transparency
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