Part 1 of 2





MAY 2007

Chapter 8



The food chain of Port Phillip Bay is a complex web of synergistic and trophodynamic interactions which conceptual models cannot effectively reproduce or mimic. In nature the consequences of anthropogenic influences can produce negative consequences beyond the predictions of modelling in orders of magnitude. Our concerns lie in the proponent’s predictions and modelling not meeting their best predictions. As with other chapters, the consequences of the proponent’s predictions not being fulfilled poses economic threats which outweigh the purported economic benefits potentially by orders of magnitude. As with other chapters, the economic consequences of a less than best outcome has not been predicted or modelled. We see this as reckless outmoded  management behaviour.

As turbidity is a major influence over primary production in the bay, this subject has been examined closely and the modelling for this particular aspect of the SEES found to be flawed. This has implications for social impacts and human health.
Some parts of this section may overlap as many of the ecological processes discussed are interrelated and connected. 


1    Introduction

As the turbidity modelling will inform other studies undertaken for the Channel Deepening Project, the importance of reliable and robust modelling cannot be understated.  The turbidity modelling used in the SEES will be used by consultancies engaged in the CDP. 

Related studies include:

  • Social Impact
  • Human Health 
  • Water Quality 
  • Denitrification
  • Primary production – food chain
  • Impact on industries, eg aquaculture, Ecogen

Given the environmental and economic significance of these matters a rigorous review of the turbidity modelling is warranted.

2    Flaws in the modelling

a)    Modelling is based on data collected during Trial Dredge

The Trial occurred during a period when background turbidity was relatively low due to drought conditions (low stream flows and consequently low nutrient levels).  We do not know what will happen during wetter years.

b)    The model does not include input of ‘background levels’ of turbidity

Background levels of turbidity is contributed to by a range of factors including stormwater run-off (rainfall events) and physical displacement of sediments by currents, ships propellers, and bow waves. 

Exclusion of background turbidity from the model also eliminates any consideration of the future behaviour of dredge generated particles once they’re assumed to have settled on the Hobson’s Bay seabed. In reality these fine particles will be highly likely to be resuspended by subsequent wave action and tidal currents and remain mobile until eventually deposited on beaches.  They will in effect contribute to ‘background’ turbidity level which is not included in the model.  This is a major concern.

c)    The Trial Dredge did not include dredging in the Yarra
The Turbidity Model Calibration and Validation Report1 – Executive Summary states:

“The models used to simulate the turbidity and sedimentation generated by the proposed Channel Deepening in Port Phillip Bay have been calibrated and verified using data collected during the Trial Dredge program (TDP). The hydrodynamic aspects of the models have been examined in a separate report and this study concentrated on the production and fate of suspended sediment as reproduced by the models.”

“The final measure of confidence in the performance of the turbidity modelling must come from the analysis and modelling arising from the TDP. These results are presented above and are considered to demonstrate that the models are able to provide robust descriptions of the levels of TSS averaged over time.”

However, the ‘Trial Dredge’ did not include Trailer Suction Hopper Dredging in the Yarra.  Consequently, use of Trial Dredge data for turbidity modelling relating to Yarra plumes has to be seriously questioned, particularly in light of generally smaller particle size of Yarra sediments.

Information on sediment particle sizes in the Yarra is provided by Golder & Associates2 who report:

“Grainsize percentages in the following discussion are based on laboratory data. The percentage of gravel (>2mm) in all samples varied from 0% to 18% (mean 0.5%; SD: 1.9%). The proportion of sand (0.63mm to 2mm) in all samples varied from 0% to 91%, although the mean sand content in all samples was17% (SD: 18.7%). The content fine fraction (<0.063mm, i.e. silt/clay) material in all samples varied from 5% to 100% (mean: 82.6%; SD: 19.4%).”

This information serves to highlight the variation in particle sizes in different sedimentary layers and locations. There is no information provided as to how this information was applied to enable the turbidity modelling to be calibrated and validated for the Yarra.

It is noted that Appendix 34 (Vol 3) Channel Deepening Project Turbidity Model Outputs Dredge Configurations Assessment June 2006 states that:

 “the objectives of the paper are to:
  • Determine and model the turbidity plumes resulting from an initial    dredging configuration to inform the initial risk assessment for the SEES; and
  • Undertake a comparison of the turbidity plumes and estimated project duration from varying dredge configurations to inform the selection of the feasible dredge schedule.”
Despite these objectives, the above document makes no reference to dredging in the Yarra River. In fact the model seems to rely primarily on data collected during the Trial Dredge in the south of the Bay and at the southern extremity of the Port Melbourne Channel.

In designing a turbidity model it can be expected that the model will simulate the impacts on a worst case scenario basis of the dredge technology to be used. Eg Trailing Suction Hopper Dredge in overflow mode will cause worst case turbidity. Any other dredge technology should also be included. Sediments in the Yarra have a greater percentage of fine silts and clays than the materials which were dredged during the Trial Dredge. These ‘fines’ can be expected to remain in the water column much longer than coarser grained materials and therefore are capable of dispersing greater distances. 

A further concern is that the modelling does not include the future behaviour of particles once they’re assumed to have settled on the Hobson’s Bay seabed. In reality these fine particles will be highly likely to be resuspended by subsequent wave action and tidal currents and remain mobile until eventually deposited on beaches.  They will in effect contribute to ‘background’ turbidity level which is not included in the model.

d)    Modelling uses flow volumes from the River Yarra, Maribyrnong River, and Moonee Ponds Creek between 2003 and 2005

Due to prevailing drought conditions this period cannot be considered as representative of flows that could reasonably be expected to occur during the dredging period.  For the modelling to have credibility, flood events should also be modelled.

Cardno Lawson Treloar3  (2006 p.15) state:

 “Average flows at 30 minute intervals from the Yarra, Maribyrnong and Moonee Ponds Creek were obtained from Melbourne Water for the years 2003, 2004, and 2005. These were input into a one dimensional model and the total flow, including the tidal flow in and out of the estuary, was then computed at the entrance of Yarra R into Port Phillip Bay. This total flow was introduced as a vertically-averaged discharge at the northern boundary of the turbidity model.”

Due to the relatively low rainfall experienced in the Melbourne region over the years from 2003 to 2005, use of figures from these years does not represent a worst case, or even an average scenario in terms of flow volumes.

Cardno Lawson Treloar4 also state “Average flows from the Yarra, Maribyrnong, and Moonee Ponds Creek were included in the model. These were introduced uniformly through the water column as a total discharge at the northern boundary of the model.”

The indication that the river flow volumes were introduced uniformly through the water column is not representative of the behaviour of freshwater flows which sit on top of the saltwater layer until mixing occurs due to wave action.

Due to the tidal nature of the Yarra below Dight’s Falls, streamflow stations below the Falls are unreliable. The most reliable station on the Yarra is at Chandler Highway, Fairfield.

Omission of streamflow data from Merri Creek and Gardiner’s Creek in the model, suggests that:
  • readings from the Yarra were taken from unreliable stations below Dights Falls; or
  • Merri Creek and Gardiners Creek were overlooked. 
Analysis of streamflow data obtained from Melbourne Water for Maribynong R, Yarra R, Moonee Ponds Creek, Gardiners Creek, and Merri Creek for the years 1988 to 2005 shows that the 2002-2005 period in no way represents typical flows.

Of the six consecutive ‘3 year’ periods that occur between 1988 and 2005, the 2002–2005 period ranks fourth in terms of flow volumes. The relatively low flow was despite the fact that a one-in-one hundred year storm event occurred on February 4 and 5, 2005, highlighting just how unrepresentative the 2003-2005 period is. The two highest ranking periods for steam flow (1991-1993 and 1994-1996) both had more than double the flow of 2003-2005.

e)    Modelling uses wind data from 2003 as this year was deemed the most representative of the 5 years before model design

The scientific basis for using wind data from just one year must be questioned. Data from one year cannot be considered as representative as data collected over a longer period. On comparing wind roses for January to March over the 20 years from 1987 to 2007 with the 2003-04 period, a significantly greater percentage of southerly winds are evident in the 2003-04 data.

As a result of the approach specified by PoMC5, northerly winds are under-represented and southerly winds are over-represented. Northerly winds would push the turbid plume further into the Bay. Interaction between the plume and clockwise tidal currents increases the likelihood of impacting on north-eastern PPB beaches during the peak beach use (January to March) period. 

Wind data from extreme years should be used to gain a comprehensive idea of possible outcomes. Under-representation of northerly winds and streamflow volumes presents modelling results that under-represent the probable area of plume dispersal. 

This clearly represents a flaw in the turbidity modelling.  Any conclusions drawn from the turbidity modelling by other consultants, such as those assessing water quality and human health issues, will also be flawed.

f)    Although a backhoe/grab dredge is proposed to operate continuously over a 12 month period in the Yarra River and Williamstown Channel, turbidity generated by this machine was “not required to be modelled for impact assessment purposes”.

g)    Modelling outcomes conflict with known circulatory patterns in Port Phillip.

The indicative plumes mapped by the turbidity model developed for the project suggest that plumes generated by dredging in the Yarra and North of the bay will not impact on any eastern bayside beaches south of Albert Park.  This is despite numerous references in the EES technical appendices acknowledging that turbid plumes from the River Yarra travel along the north eastern coast of the Bay sometimes as far south as Beaumaris. In particular, Cardno Lawson Treloar, 20077 states:

“The estuary opens out into Hobsons Bay and, after rainfall events, naturally turbid plumes flowing from the river tend to move down the eastern shoreline.”

h)    PoMC Specifications for Turbidity Modelling were not met

The PoMC Specifications contain a number of apparently unmet requirements which give cause to doubt the reliability of the modelling. For example, they state (p. 2):

“each of the models have been through extensive calibration, verification and peer review. Turbidity modelling will only be carried out with those models developed specifically for the CDP and agreed as appropriate for use in the SEES through the peer review process.” 8

The SEES document does not include a peer review report in relation to the turbidity modelling.

The Specifications state (p3):

“During maintenance dredging in the Yarra River area using a backhoe, a turbid plume was not measurable or noticeable more than 100m away from the dredge and did not persist for more than 2 hours after dredging (Hale, J. Dec 2006. Minor maintenance Dredging Campaign, Water Quality monitoring in the dredge and disposal plumes). This comparable to a plume from TSHD which can disperse several kilometres from the dredge.” 

“For this reason, turbidity plumes from planned backhoe/grab dredge operations is not required to be modelled for impact assessment purposes.”

This is a further flaw in the turbidity model.  The source strength for a backhoe /grab dredge is estimated at 7kg/s. This is not insignificant, particularly when combined with the source strength of the Trailing Suction Hopper Dredge and natural background levels of turbidity.  Backhoe Grab Dredge is scheduled to be operating for 12 months in the Yarra and Williamstown Channels.

This specification places a further constraint on modelling for worst case scenario.

i)    Modelling of turbidity plume during capping operations not required.

Proposed capping of contaminated materials “is likely to be discharged from the dredge through a pipeline to a spreader pontoon which will finely distribute the material over the contaminated material on the seabed.”  

Again, based on the estimation that the source term for this operation will be relatively low (16 kg/s) the Specifications state:10

“As for the backhoe operation discussed above, in comparison to the source term of 200 kg/s for normal dredging operations, turbidity modelling of the turbidity plume during the capping operations is not required”.

No mention is made to possible displacement of the contaminated material during capping.

Impacts of turbidity on seagrass.

The impact of dredging on seagrass is identified as one of the key ecological issues.

The SEES states that there will be no long-term impacts on the health of the Bay.

The SEES contains numerous references to the importance of seagrass habitat
for a large suite of species endemic to the Bay. It identifies the extent of
the Bay's seagrass beds as being 68 square kilometres. It states that 5% of
all seagrass in the bay will be affected and that 20% of seagrass in the
south of the Bay will be affected. It also advises that in places such as
Portsea its recovery from the effects of the dredge plume in water depths
greater than 3 metres is uncertain. We do not believe that this is
acceptable. Neither do we believe that the effect on seagrass and intertidal
communities in the north of the Bay have been adequately addressed,
particularly at Altona, Williamstown and St Kilda and are concerned that in
taking a "pragmatic" approach that these matters may have not been fully

Impacts occasioned by reduction in light caused by turbidity, sedimentation
and the release of nutrients fostering the growth of fine algae on the
seagrass appear to have been assessed singularly. It is not clear whether
the cumulative affects of these three factors have been assessed. In
circumstances where the species may be stressed by any one of the three
threats or all of them in combination we believe further work is required.

Additionally no mention has been made how these factors may impact on the
plants capacity to flower when stressed by reduction in light or other
factors for several months prior to its flowering or how it might impact on
the species when in flower should turbidity persist or there be issues
related to the re-suspension of sediments.

Technical Report 50 states that sediments do not attach to seagrass leaf. It
has been observed in Westernport that some sediments (derived from the Lang
Lang area) do adhere to seagrass (per coms Dr Tim Ealy, Westernport Seagrass
Partnership). The SEES cites a wide variety of particulate size and physical
property. There is no evidence of work being undertaken to validate or
otherwise that the assertion that sediments being handled at dredging and
dumping sites will not adhere to seagrass.

It is widely reported that nutrients released to the water collum foster
epiphytic growth on seagrass, which in turn prejudices the plant's
viability. In our opinion inadequate work has been undertaken to prove or
otherwise that this will not occur within the footprint of the plume created
particularly in the north of the Bay.

In circumstances where reasons for the loss of seagrass and its failure to
regenerate in Westernport is still not understood, we can identify no work
regarding impediments to successful regeneration or risks that affected
sites will not be colonised by foreign or pest species.

In the area where the highest loss of seagrass is predicted there appears to
have been no study of the effects of displaced species on the balance of the
habitat. In addition there has been no study of the effect on local sediment
transport following the death of seagrass.

The work undertaken regarding seagrass and the intertidal reefs is entirely
reliant on the reliability of the plume modelling and stated settlement
times. However, technical Report Appendix 42 suggests that fine particulates
comprise between 2 and 8% of the dredged material taken from the north will
take approximately 30 days to settle. We believe that with the normal
weather patterns this matter could be further exacerbated by the
re-suspension of fine particulates. 

The plume modelling suggests that the plume will not reach the shoreline between St. Kilda and Brighton. However, detritus and other materials washed up on local beaches is often collected whose origin is obviously the immediate Yarra catchment. Indeed City of
Melbourne and City of Yarra polypropylene parking tickets have been found washed up on beaches as far south as Seaford.  We believe that it is highly improbable that the plume will not reach seagrass beds at St.Kilda or impact on the seagrass beds and inshore reefs of the Northern end of the Bay.  We believe that further work is required inclusive of physical testing of the transportation of fine sediments.

We believe that turbidity thresholds proposed in the SEES for the dredging of the south channel should be lowered and that, should the project proceed; there should a reduction in the level of overflow during the dredging to the same level as that proposed in the Northern section; that it is critical that work be halted immediately should turbidity exceed levels quoted in the final EMP and that significant penalties apply for breaches; that a real time observations regime be put in place that is independently monitored by
EPA personnel and that this be backed up by community (Beach Watch, Water Watch and Reef Watch) groups.

In addition, we believe close monitoring for a period of at least two years is imperative in order that management actions can be immediately implemented should there be issues. The concept of an ‘Environmental Bond’ should seriously be considered to ensure any damage is rectified.


We submit that the Water Quality assessments in the SEES are flawed due to reliance on the flawed Turbidity modelling; and the SEES does not address the possible human health effects of contact with contaminated waters.

Australian and New Zealand Guidelines for Fresh and Marine Water Quality, 2000. includes section 5. Guidelines for Recreational Water Quality and Aesthetics, which states (p. 5-8):

“Waters containing chemicals that are either toxic or irritating to the skin or mucous membranes are unsuitable for recreation.”

“In general, there are two kinds of human exposure in swimming areas: contact with the waterbody and ingestion of the water…… Special care must be taken to check for substances that can enter the body through absorption through the skin.”

Golder, 200711 cites ‘Hale. J. 2006. Minor Maintenance Dredging Campaign. Water Quality Monitoring in the Dredge and Disposal Plumes. Draft 1. August 2006’  as the basis for conclusions on Water Quality. However, although this work is referenced in the bibliography of the Golder Report, but the Minor Maintenance Dredging Campaign report is included in the SEES Technical Appendices (neither hard copy or CD form). Nor are there any other reports in the Golder HHRA bibliography relating to “maintenance dredging” and water quality.
In the absence of this report it is impossible to determine what sampling methods were used, the time and place of the maintenance dredging. However, it is stated that a backhoe dredge was used. As backhoe dredges do not produce a plume of anywhere near the magnitude of a TSHD there can be no reasonable assumption that surface water data sets resulting from sampling associated a backhoe dredge operation would be equivalent to sampling associated with a TSHD.


In relation to ‘Surface Water Data Set screening and modelling’ Golder12 states:

“The surface water data set was derived from Jennifer Hale (2006) as these were measured water quality values resulting from maintenance dredging activities in the Yarra Port area (where sediments are known to be contaminated by a range of chemicals). Water samples were filtered after collection and analysed for metals and a range of other parameters. The reported concentrations are considered to be predictive of dissolved phase chemical concentrations in contaminated areas.”

“It was concluded that the empirical data set derived from maintenance dredging activities (Hale, 2006) was the most representative of the available data of the surface water concentrations representing dredging activities.” 

However, there appears to be no available data on the concentration of contaminants associated with particulate bound contamination.


Due to the benign conclusions reached by the flawed turbidity and water quality modelling these issues have not been considered serious enough to warrant inclusion in the SEES Risk Assessment report.

a)    Risk Assessment

The URS Risk Assessment includes Appendix C, Risk Register, in which a table lists issues specifically excluded from SEES Risk Assessment.

“Modelling inaccurate” is listed as an “excluded issue”. The “Reason for exclusion” being “Modelling uncertainty accounted for through conservative risk assessment approach and expert interpretation.”

In relation to Risks associated with contamination URS Australia 200713 states:

“This issue is about release of toxic material into the environment during removal of contaminated sediments from the Yarra River and during placement in the DMG in the North of the Bay, and release over time during operations. There will be some potential for release of available contaminants that could have a direct toxic effect on organisms or an indirect effect on human health through accumulation via the food chain.”

“Inspection of the event trees indicates that there is no pathway that leads to the predicted effect relating to contamination.”

“The event trees show that public safety is a potential risk issue with respect to toxicity of contaminated sediments.  In Yarra River and Hobsons Bay, when dredging with the Trailing Suction Hopper Dredge, backhoe or grab dredge, there is expected to be release of particulate bound contaminants from sediment. It is unlikely (1 in 1,000 chance) however, that toxicity will accumulate in fish and benthic invertebrates and lead to public health and safety effects.  If that does happen, the specialists advised that the effects would be Minor.”  

Risk mitigation – Project Delivery Standards

Risk Evaluation Section 714 discusses risks to Amenity – recreation. Although it cites issues such as fishing, diving, beach use, and boating, there is no reference to swimming. Similarly, risks to beach use are described on the following page, but this is confined to discussion  of th south of the Bay.  Again, swimming is not mentioned.

The Risk Evaluation Section makes no reference to the ‘Minor’ human health risk (reported by Golder 2007) associated with recreational swimmers in contact with contaminants associated with particulate bound contamination.

As the risk of swimmers in contact with contaminants is overlooked in the Risk Assessment there appears to be no Risk Mitigation strategy. For example, even a moderately heavy rainfall event would transport particulate bound contaminants along all beaches from Williamstown and Sandridge to Brighton and possibly beyond. This would reasonably warrant cessation of dredging and closing beaches.

The Risk Register15 also notes “heavy rains compounding the effects of turbidity” as an “excluded issue”, the reason being “Modelling uncertainty accounted for through conservative risk assessment approach and expert interpretation.

The flawed modelling of turbidity and water quality (as previously described) has resulted in the area to be affected by plumes in the north of the Bay to be understated. 

b)    Social Impact

Sinclair Knight Merz, January 200716, is a further example of a consultant relying on the turbidity modelling to draw conclusions in their field.

“The diagrams above (figure 5-3: Plumes in Yarra River and Hobsons Bay (5mg/L, surface layer)) indicate that the plume from the TSHD, while dredging in the Yarra and Williamstown Channels, will be present in varying densities along the banks of the Yarra River downstream from Appleton Dock to a point some distance to the south-east of Williamstown.  The greatest turbidity would be found between Williamstown Road and Breakwater Pier and could be most noticeable along the Williamstown foreshore.  The effects of turbidity could persist for a period of several months as described under the dredging schedule above”.

(p. 96)  “On occasion, users of Williamstown beach or beaches extending eastwards towards Albert Park would be able to notice the plume…. Much of the dredging in this project area is scheduled to occur between December 2008 (early summer) through to and beyond Easter 2009, where a plume of 5mg/L may occur with a frequency that might vary from one day in 8 to almost two or more days in a fortnight or more. The plume is expected to be noticeable against background though not strong and this could discourage activities involving contact with the water at particular times – for example parents taking their children to the beach on fine summer days when they might expect clearer water, especially if they felt that the plume was unsafe, or was contaminated in any way.”
“… over 400,000 Victorian adults who had visited the Bay in the last twelve months spent the bulk of their time in the St. Kilda, Elwood and Brighton area of the Bay on their last visit. Nearly half of these visitors were in the two age cohorts 25-34 and 35-44 years old, some 41% of the total visitors had children at home under 18 and a third visited with children, suggesting the total visitation in the area could be over 500,000. This area also attracted some of the most loyal visitors with some 53% who mostly visited just this area rather than spreading their visits around the Bay and only 18% who had only visited the area once.”

“This project area also includes Williamstown which attracted some 216,000 adult visitors or potentially nearly 300,000 visitors in the last 12 months who spent the bulk of their time in the area.”

c)    Human Health

Golder and Associates17 acknowledge that recreational swimmers using beaches in the north of the Bay over the two summers that dredging is proposed face a ‘minor’ risk. This is a concern in itself. However, even more of a concern is that critical water quality documentation has not been included in the SEES.

Golder and Associates state:

“….. the available water quality data with which to compare against water quality screening criteria were for dissolved phase contaminants only.   Suitable data were not available on the concentration of contaminants associated with particulate bound contamination in each project area.  Therefore, for the purposes of a conservative assessment, the consequence rank for direct exposure of swimmers to contaminants from sediments in the North of the Bay and Yarra/Hobsons was increased from negligible to minor. A higher consequence rank than minor was not considered to be warranted on the basis of the short exposure duration of swimmers and other people in the waters of the Yarra River and Port Phillip Bay.”

Despite this finding, the Channel Deepening Project Summary Brochure18 states: 

“Extensive research by PoMC has identified no health risk concerns for recreational swimmers or consumers of fish caught in the lower reaches of the Yarra River as a result of the CDP.”

As has been shown, the research was not been as extensive as claimed. This gives rise to serious Social Impact and Human Health implications for the hundreds of thousands of beachgoers who use northern bayside beaches in spring and summer.


Seriousness of the issue

The SEES grossly underestimates the potentially catastrophic affect on our bay if denitrification is compromised and the nutrient cycle altered during the Channel Deepening Project (CDP).  If denitrification is disrupted or disabled, toxic algal blooms may occur in the bay rendering the water unsafe and unusable for any human activity and reducing biodiversity significantly.

Longmore states “Port Phillip Bay appears to be unusually efficient at denitrification, and denitrification has been identified as a key indicator to be maintained.  The model predicts that a substantial drop in denitrification efficiency would lead to large increases in plankton production, even at current inputs, and in the extreme, cause a shift to a highly enriched state. This is clearly a situation we would wish to avoid”.19 

Understanding the nutrient cycle in our Bay


Nitrogen enters our bays ecosystem in several ways; from the atmosphere, from the Yarra and from other smaller creeks and rivers and from the Western Treatment Plant.20 Nitrogen influences plant growth and is a vital nutrient for much plant life in the bay, including phytoplankton.  Phytoplankton is the basis of the food chain in our bay and thus the health of this life form is paramount21


Denitrfication is the process by which microbes convert nitrate to N2 Gas.22 By turning excess nitrogen in the Bay into a gas that is then lost to the atmosphere it removes the source of nitrogen from our Bay.  If the Nitrogen was not removed it would become a source of accelerated growth for phytoplankton and could lead to blooms.  Toxic algal blooms have already been observed in Port Phillip Bay23.  The bay’s water quality and thus ecosystem is highly dependent on this process24

Microphytobenthos (MPB)

Microphytobenthos (MPB), a single-celled microscopic form of plant life  play an important role in denitrification, therefore, an assessment needs to be made on how proposed dredging would affect their ability to denitrify.  Longmore estimates that these MPB take up 45% of the remineralised Nitrogen in our bays sediments.  For the MPB to survive and thrive it must photosynthesize, as all plants do.  However, dredging would radically reduce this plants ability to photosynthesis due to increased turbidity.  The loss or reduction in photosynthesis ability of MPB’s in the ecosystem will have a significant impact on denitrification.

The Port Phillip Bay Environment Study illustrates the critical role of MPB’s in nutrient cycling in our bay.

The shallowness and clarity of the Bay allows some sunlight to reach the Bay floor. This enables particular microalgae mats (microphytobenthos) to flourish on and in the sediment. These algal mats intercept the inorganic forms of nutrients in the sediments before they enter the Bay waters, and use them for growth. The MPB are then eaten and excreted by the bottom invertebrates, so that most of the nutrients are “bypassed” back into the sediments.

Resilience and Biodiversity

Port Phillip Bay is among the most biodiverse marine environments in Australia, as evidenced by the presence of four marine national parks.  Biodiversiy is a key contributor of a resilient system.  For example, less biodiverse systems are more vulnerable to invasions by pest species.  By impacting on this biodiversity it will lose this resilience. 

It is true that over the past ten years our Bay has been impressively efficient at denitrification thanks to a reduction in nutrient inputs since 1975.  However, the bay is still under an array of environmental pressures from the 1.3 million people living on its edges, exotic organism invasions and all the industry and shipping that use our bay.  Resilience should not be taken as a given and for the purpose of gauging any ecosystem should always be measured alongside biodiversity.  Our bay is only as resilient as far as it is biodiverse. There is only a certain threshold of external pressures that an ecosystem such as our bay can withstand.  Something as extensive as the current dredging proposal, and any short and long term impacts may tip the bay over the threshold and cause a significant reduction in biodiversity and thus long term changes.  

The Port Phillip Bay Environmental Study states that balancing the Nitrogen budget in the bay is a crucial factor.  The natural processes, including denitrification and nitrification remove 80-90% of the Nitrogen input into our bay’s ecosystem25.  The study concludes that “The Nitrogen cycle in the Bay is therefore almost entirely balanced by this process, with 10 to 20% of input being exported to Bass Strait or buried in the sediments in refractory forms. Even though Phosphorous levels are higher than Nitrogen levels, the processes in play ensure that Nitrogen levels are the limiting factor in the growth of algal populations in the Bay.

Dredging effects

The Longmore report26 lists ten mechanisms by which dredging may plausibly affect nutrient cycling and denitrification. On their own, each mode of dredging impact is of low risk to our bay, however in partnership with each other they have the potential to form a greater risk to the bay. 

It should be noted that this list does not include a risk assessment of potential deoxygenation due to dredging effects.

Movement of Dredged sediments- risk for algal blooms

There is also potential for the dredged material from Hobson’s bay area to be infested with algal cysts.   Blooms can occur once these cysts are disturbed and mobilised into the water column.  In The Age Article titled ‘Toxic Plume may reach Docklands’, 23 March 2007, Clay Lucas reports “A technical report by consultants SKM completed for the report says the dredging will generate vast amounts of sediment and remobilise toxic algal cysts.”These can cause toxic algal bloom," Monash University water studies centre biologist Simon Roberts said. "Paralytic shellfish poisoning is caused by algal bloom. It can kill us, it can kill fish — it is deadly," Dr Roberts said.” 

Risk Assessment

The majority of risk assessments measure the likelihood of an event occurring and the consequences should that event occur.  Each measure is rated on a scale and then they are multiplied together to reach the overall risk of the event or practice.  In the SEES the risk to our bay of toxic algal blooms occurring from a reduction in denitrification has been assessed as low.27  This is a questionable assessment given that the consequences of this risk could be very high.

Blue Wedges believes that the risk of toxic algal blooms impacting recreational swimmers in the Hobsons Bay area has been understated due to flawed turbidity modelling wrongly indicating that plumes from the Yarra would not impact on beaches south of Albert Park.

The Executive Summary of the SEES also states that “all impacts identified were short term and restricted to parts of the bay rather than being baywide".28  This is a very presumptuous and misleading statement as many fish species move around the bay and seasonally congregate in the north of the Bay. In Longmore (2006 p. 35) in a summary for the Yarra River and Hobson’s Bay the “Yarra River sediments are not thought to have a significant capacity to denitrify: Hobson’s Bay sediments have a denitrification efficiency of about 50%, indicative of a system under stress.” 

Denitrification capacity increases moving further south out of the Hobson’s bay area and into the North of the Bay.  Longmore (2006) states that the North of the bay has a “denitrification efficiency much higher (85%) than in Hobson’s Bay”.  Therefore, it is not unreasonable to conclude that if the denitrification capacity of Hobson’s Bay was further reduced by the CDP, this would have a flow on affect to the North of the Bay area.  It is questionable that impacts are only restricted to parts of the bay and not baywide.

The SEES justifies these claims by focusing on the low amount of Nitrogen inputs dredged sediments would release into the bay.  However, it ignores the possibility of additional Nitrogen introduced in storm water run off, or already in the water column.  It is not only a question of how much extra Nitrogen will move into the bay but whether the natural processes that currently exist to remove the Nitrogen would still exist during and after the CDP.  Nitrogen will continue to enter the bay well after any type of dredging project. The question we need to be able to answer is whether the dredging will significantly alter the nutrient cycling process in the bay. 

The bay is an efficient ecosystem that is currently very good at cycling nutrients through the system.  One reason for this is the shallowness of the Bay and thus the light penetration to the floor of the Bay.  This light allows all the single and multi-cellular plants to photosynthesize and form the foundation for the food chain within the bay.  There are a lot of complex interactions at play that ensure nitrification and denitrification remove Nitrogen from the bay.

The CDP will cause significant turbidity in the bay. Turbidity reduces the amount of phytoplankton and other plant life which then sends a sizeable ripple down the food chain that would eventually cause the loss of species of conservation concern such as dolphins and weedy sea dragons.  Other iconic species that are important to ecotourism such as penguins, seals and sea lions would also decline.


  [1] Technical Appendix 41 Cardno Lawson Treloar RM2133 – FINAL VER 1.3 
  [2] FINAL REPORT Northern Channels Sediment Investigation, Port Phillip Bay SEES. URS prepared for Golder & Associates. 17 January 2007
  [3] Technical Appendix 41 Page 15
  [4] Ibid Page 26
  [5] Specifications for Turbidity Modelling of Dredge Strategy. PoMC,2006 (p. 2) Appendix No. 40
  [6] Specifications for Turbidity Modelling of Dredge Strategy. PoMC,2006 (p. 3) Appendix No. 40
  [7] Hydrodynamics and Coastal Processes. Head technical Report RM2124. Apendix 45 (p.45)
  [8] Specifications for Turbidity Modelling of Dredge Strategy. PoMC,2006
  [9] Specifications for Turbidity Modelling of Dredge Strategy. PoMC,2006
  [10] Specifications for Turbidity Modelling of Dredge Strategy. PoMC,2006 (p. 4)
  [11] Human Health Risk Assessment Head Technical Report, January 2007. Appendix 60. (p. 5)
  [12] Human Health Risk Assessment Head Technical Report, January 2007. Appendix 60. (p. 19)
  [13] Channel Deepening Project Risk Assessment for SEES, URS Australia, 2007. Appendix 5 (p.26)
  [14] Channel Deepening Risk Assessment for SEES, URS, 2007 Appendix 5 (p.28 &29)
  [15] Channel Deepening Risk Assessment for SEES, URS, 2007 Appendix C, p. C14).
  [16] Social Impact Assessment – Final Report. SKM, 2007. Appendix 59 (p.95 & 96)
  [17] Human Health Risk Assessment Head Technical Report.  Golder & Assoc, 2007. Appendix 60 (p.67)
  [18] SEES Channel Deepening Project Summary Brochure. PoMC, 2007 (p. 32)
  [19] Technical Appendix 43
  [20] Ibid Page 8
  [21] Ibid Page 9
  [22] Ibid Page 9
  [23] Ibid Page 61
  [24] CSIRO Port Phillip Bay Environmental Study, 1996
  [25] CSIRO PPBay Environmental Study (Harris et al 1996)1996
  [26] Technical Appendix 43 Page 58
  [27] Technical Appendix 43 Page 86-88
  [28] SEES Executive Summary Page 30

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