Cyclone Gita in Tasman Bay, it’s ecological impact.

Cyclone Gita in Tasman Bay, it’s ecological impact.

In the wake of cyclone Gita and Fehi,  coastal erosion, sea level rise, mudslides, logs and property damage, the community is uniting to help get everyone back on their feet and into their houses and businesses.  We have seen and heard many stories of the impact on the human community of our region.  Now that the initial survival stage is under control,  I thought it would be an appropriate time to highlight the effect that all of this mud is having on the fragile ecosystems of our coastal marine environment.  Primarily,  I must say that erosion and subsidence has been happening forever,  it is this process that has brought the fertile soils of the Marahau valley.

Through building and living on these dynamic environments,  it is human nature to want to stabilize the situation in order to protect our property,  however it is also human nature to make or save a buck where we can, council, communities and corporations must find the most appropriate solution to safeguard our precious environment,  if we drop the ball on this one,  we only have our grandkids to answer to.  We need to find ways to work together to solve these issues,  or at least have a go before we start laying blame.  Read on…

Cyclone Gita
Recently forested separation point granite hillsides above residential properties have subsided in Marahau during cyclone Gita. The damage has been widespread from Marahau to Motueka Valley. (Jude Heath)

Let’s take a journey starting in the streams.


Cyclone Gita
Mud from Cyclone Gita has filled streams and ditches in Marahau reducing the available habitat for native freshwater species. (Stew Robertson)

Tasman Bay’s rivers and streams are under pressure from a variety of land uses,  generally stream health is degraded when surrounded by intense residential or pastoral land.  See TDC’s latest report here.  In saying that however,  we have personally observed abundant populations of native freshwater fish species such as eels, inanga and kokopu surviving in relatively polluted watercourses such as the drains running into the Moutere inlet.  In Marahau,  we have observed abundant native freshwater fish species.  During the last two storm events,  these fish were witnessed by locals, washed up by the flood and left to die on the resulting mudflats.  Saving wildlife is not the priority when one is trying to save one’s own family.  The available habitat for these fish has diminished in the last two years since forest harvesting commenced in Marahau.

A deep pool in the Marahau river once held at least five species,  where now I only see one or two.  Since the mudslides,  many of these drains and streams can no longer hold water.  Every day,  we venture into the Abel Tasman National Park,  in this regenerating environment of the same geomorphology,  the ecosystem responds differently.  After rainfall, we experience the staining of the fresh-waters with tannins for the first few days,  some coarse sediment is washed down, these generally start running clear again quite quickly.  We don’t really see much fine sediment.



Cyclone Gita
Otuwhero Wetland salt marsh has collected large amounts of debris and mud, preventing them from entering the sea. (Stew Robertson)

An important part of the ecosystem,  with only 10% of New Zealand’s wetlands still intact,  there has been catastrophic impacts already to these vital areas.  Communities are starting to value the services that wetlands provide, such as habitat for bird species such as fernbird and banded rail,  spawning sites for galaxiids (whitebait), filtration of nutrients and sediment from runoff water, buffering flood impacts by absorbing water for slow release.  Here in the Otuwhero wetland, years of community work have been swamped by mud and debris, but from a certain perspective you could say that the work has actually paid off as it has done part of its job as a result.  Deforestation of the pine plantation on the unstable separation point granite hillsides has caused large landslides that have smothered the wetland with mud and logging debris.

A more mature restoration project (20 years +) at Marahau managed to halt the log debris after they washed as a powerful lahar down a hill,  across a paddock and a road and saved them spilling into the water body that they protect, see photo’s above.  Helen Lindsay of Otuwhero Trust states that plants that have been in the ground for 4 years or over have generally survived well,  whereas younger ones have perished.   Planting native trees is never a bad idea, especially with planning, eco-sourcing and maintenance.  Another great reason to plant more trees,  as soon as possible,  always.

Find out more about New Zealand’s Wetlands here.


Cyclone Gita
Deposit feeders such as mud snails (Amphibola) and mud crabs (Helice) picture here, appear to be thriving in the mud, suspension feeders such as cockles (Austrovenus), may struggle to breath however. (Stew Robertson)

Home to a large diversity of species, these areas have impacts on the wider coastal seas.  Filter feeding organisms such as cockles, can filter as much as 20l of water per animal per tide (See how cockles fit into the ecosystem here) they also process contaminants such as nutrients and toxins.  The deposit feeders process their waste, drawing it into the sediment for microbes to process.  By digging holes and microbial gardening, bioturbators such as mud crabs, aereate the system and keep the aerobic bacteria thriving, enabling the processing of more biological inputs from the seas and rivers.  The mud has smothered these systems in places, and hopefully the survivors will work together to heal themselves.  Fine sediments are more prone to re-suspension than course sediments and so this may mean that our estuaries are a bit more murky than usual for a while.  Time is a great healer,  and flushing will gradually occur.  But where do all these fine sediments end up?

Find out more about New Zealand’s Estuaries here

Coastal Marine Environment:

Cyclone Gita
Mounds of woody debris on Marahau Foreshore. (Stew Robertson)

After most significant rain events,  our rivers with catchments influenced by human activities such as roads, agriculture and forestry are prone to sedimentation,  this enters the sea in the form of mud plumes, which generally subside after a number of days.  This mud also contains contaminants such as E.coli, which also have impacts on the ecosystem as it enters the food-chain through filter feeding shellfish including farmed Greenlipped Mussels (see this Cawthron Paper for further details).  The Motueka river plume has been measured to covered  15,000 sq Km after a large event, and this sediment settles to the sea floor.  The sediment also detracts from the tourism experience and the debris in the Bay are a hazard to navigation.  Every day,  we travel this coastline,  we can measure the water clarity go from 0 – 2 m near Marahau to 10m + in the heart of the park.  This is a graphic representation of how to protect the sea we must protect the land.

Cyclone Gita
The Motueka River Plume 3 days after a large rain event, spreading columns of mud into the waters adjacent to the Abel Tasman National Park, seen here at Snapper Rock, Mad Mile. (Stew Robertson)

In future Blogs we will take you to the sea floor and show you first hand how the silt is affecting life there.  We are researching the rules and regulations around land use,  and discussing potential pathways to increasing resilience on the land.  We would like to know how others are dealing with this situation and how we can extract the positives from this.  Our group Tasman Bay Guardians are on the cusp of becoming a charitable trust,  we deliver education programs and will be leading projects that will enrich our environment into the future.  If you would like to be involved or keep track of us,  please follow our TBG Facebook Page.or send us a message through this page.  If you are interested in helping with the recovery of Marahau in the form of funds to plant trees, our group Abel Tasman Tree Collective can organize this for you.



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