Narrabeen Lagoon, Warringah NSW, Australia

There are plans in the works to dredge a nearby lagoon where seagrasses are meant to be protected.  Today I worked on a letter to the Warringah Council to enter my comments about the proposed plan.  It is such a rare and beautiful feature.  I hope they can agree to a no-dredge option.


Warringah Council, NSW Australia

To Whom it May Concern,

Thank you for the opportunity to provide comments on your forthcoming plan to dredge the Narrabeen Lagoon.  I heard of this plan through my friend Dr. Frank Gleason.  I am a seagrass biologist from the United States.  After visiting the lagoon, reviewing the proposed plan and reading supplemental literature your council has prepared, I had some thoughts on the matter I thought I would share with you.  

In reading the council’s management plan, I was happy to see that the council was planning to prioritize preventing nutrient and sediment loads from entering the lagoon through strategic infrastructure investments in catchment areas.  This type of investment makes sense, as the problems with water quality and sediment control have been exacerbated by increases in the area of development within the watershed.  This type of solution successfully attacks the problem at its origin and limits the need for costly and invasive corrective actions, such as dredging, in critical habitats.

I understand that the management of an urban lagoon has many inherent challenges as you attempt to sustainably manage it for human health and safety, ecological conservation and restoration, and the recreation needs of the public.  I was able to find and read through a variety of preparatory the management studies and plans prepared for the lagoon about 10 years ago which detail the critical objectives you have developed for managing the lagoon for a variety of uses.  In these studies it is abundantly clear that seagrass conservation and restoration is of paramount importance in your management planning and to the public to whom you serve. 

In the list of priority management prescriptions for the lagoon, seagrass protection is ranked number 1, while dredging, for any reason, is not listed at all. In successfully managing this lagoon for seagrass stabilization and improvement, some concessions must be made that limit other uses, such as certain types of recreation and fishing practices that degrade precious seagrass and other habitats. Dredging must really be abandoned as a management strategy. Seagrass habitat is stressed by pollution, turbidity, hypersalination, disease, mechanical injury/removal and eutrophication, among other things.  To prevent losses, certain anthropogenic activities that are known to adversely impact seagrass health in the lagoon (such as dredging, boating and sailing) should be limited as well, not expanded, if seagrasses are to be successfully conserved as proposed in the management plan. 

Seagrasses are very sensitive species, and as such, are known to be key indicators of ecological health.  Seagrasses also serve as critical nursery habitat for juvenile fishes, especially when they are located next to mangrove forests, such as you have in the lagoon.   For this reason alone, abandoning the plan to dredge the lagoon at all can be argued.  

In Narrabeen Lagoon it is reported that seagrasses have been declining steadily since the 1960’s. This was determined to be caused by direct removal from dredging and indirectly through changes in the water level due to entrance management strategies.  Losses have continued even since dredging ended in the 1980’s.  Mapping efforts following known dredging events in seagrass beds have shown that losses from dredging may function as areas of permanent loss overtime.

Additional specific concerns related to dredging in Narrabeen Lagoon exist.  I was concerned about the findings of one sediment study that found disturbing the sediments may generate acid sulphate runoff upon oxidation.   This is not an area of expertise of mine, but it may be of interest to research this risk further.  Additionally, the water quality in dredged areas is known to be worse than other areas due to collection of nutrients and low levels of dissolved oxygen.  These assertions should also be studies further before dredging begins as the effects of the dredging may be much worse than what has been considered thus far in the preparatory studies.   

Indirect effects and losses must also be accounted for. Great losses of seagrasses occur in coastal areas around the world, the causes of which often remain unclear. Thus, seagrass conservation and protection are more effective strategies in maintaining seagrass populations than restoration because seagrass species recolonization is often very slow, direct planting efforts often fail, and the projects become financially challenging or ineffective all together.  Thus, all disturbances to seagrass beds must be considered carefully because there is always a risk that the seagrass will not recolonize a site following planned physical disturbances to seagrass habitat and areas surrounding planned removals or that soil destabilizations will exacerbate seagrass losses in adjacent areas.   

In a survey of recreational users referenced during management planning in 2002, the top recreational activities reported for the lagoon were walking, relaxing, picnicking/BBQ, fishing from the shore, socializing, swimming, cycling and sightseeing.  The users seem to value the reserve for its quiet, tranquil atmosphere, scenic value and natural environment.  The importance of the site for boating and sailing were not found to be a priority.  However, in the same document, sailing and boating interests (dredging) are determined to be priorities of management and stakeholders, including the sailing and boating clubs. From my perspective, these objectives are succeeding in meeting the desires of a minority special interest stakeholder group and failing to respond to the requests and desires of the majority of the general public. 

Given the state of declining seagrass and the lack of interest in the majority of the public for dredging the lagoon for use by boats, it is unclear why dredging (injurious in itself) and increasing boating recreation on the lagoon (injurious in the future) is where the council is considering spending a such a great deal of money (in excess of 1 million dollars).  It seems to me that those kind of funds could be put to better use installing sediment controls, restoring lost habitat and protecting critical habitat for valued threatened and endangered species.

Still, if you decide that the money is best spent on dredging the lagoon, I hope you will select the least invasive option, which appears to be Option A in this case. 

Thank you again for the opportunity to comment on your plan to dredge the beautiful and unique Narrabeen Lagoon.  I look forward to finding out about the results of your public inquiry and hopefully your selection of a no-action alternative.  I can be reached at if you have any further questions or comments.

Best Regards,


Brooke Sullivan

Seagrass Biologist

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