Marine Water Contamination Photo: Tom Allan
Marine Water Contamination Photo: Tom Allan

When the rain fell in Napier last November flooding the city – 242mm of it in 24 hours – satellite imaging captured the plumes of sediment flowing out from our rivers and estuaries into Hawke Bay. 

Hundreds of thousands of tonnes of precious soil some 20km from land, spreading like a Rorschach inkblot to the 12 nautical mile boundary of the inshore, causing untold devastation beneath the waves.

Land-based sediments, once dispersed, smother habitats, hinder fish from finding prey, and diminish plant-life, such as microscopic algae – a vital part of the food supply. What we know, from some of the latest research on estuaries and harbours in Aotearoa, is that when a storm like this occurs, coastal waters become more turbid (i.e. murky), nutrient levels increase, and as a result, biodiversity and ecosystem function declines. 

Hawke’s Bay’s main rivers already contribute an estimated 11 million tonnes of sediments per year to our coastal marine environment. With our highly erodible, denuded landscapes, it’s a massive, gnarly problem – many would argue, the greatest threat to our coastal marine environment by far, notwithstanding the loss of critical topsoils from our high-country land.

In the case of the Napier floods, however, the extent of the issue was made manifestly clear.

Following the plume

As it happened, Hawke’s Bay Regional Council has been funding a PhD student, Ted Conroy, to create a hydro-dynamic model coupled with sediment delivery, to better understand what’s coming out of the rivers, how it moves around our coastal system, and where it ends up. 

Hence Sentinel 2, which created the imaging Ted has been studying.

NIWA also ran an ocean glider (a self-driven underwater robot, or AUV) over the Hawk Bay, which autonomously samples the seawater, measuring temperatures, salinity, oxygen, light and turbidity over a period of time, and Ted has been using the empirical data to calibrate his model and satellite imaging. 

He’s yet to publish his conclusions, but they’re expected to corroborate some surprising findings off the Tasman/Golden Bays coast, as reported by Alison Balance in RNZ’s Our Changing World

Since 2016, NIWA’s two ocean gliders have been regularly sampling the Cook Straight, discovering freshwater plumes (but without the distinctive brown sediment signals) originating from the Motueka and Aorere rivers far outside the Tasman and Golden Bays they discharge into. By the rules of ocean processes, river water should have dispersed by then, through tidal and wind movements, and although these are only small eddies (having detached from the main plume) and short-lived (hours to days), they take the connection of land to ocean far further out to sea than we previously knew. 

The buoyant layer that floats on the top of the freshwater column and doesn’t mix straight away with seawater, travels beyond the visible reach of sediment plumes, and with it, potentially sediments, nutrients, contaminants, etc. The implications for a big weather event, where volume and reach is compounded, is the potential change to marine-water make-up.

Satellite photos showing sediment plumes on 10 November and two days after the Napier flood on 12 November. Photo. Planet Labs 

Ted’s also been studying the amount of erosion coming off the cliffs at Cape Kidnappers and the effects this is having on the marine environment, but that’s another story.

His research is an offshoot of a two-year, $300,000 joint research project, led locally by HBRC marine scientist, Anna Madarasz-Smith, in collaboration with NIWA scientists and the University of Waikato as part of the Sustainable Seas National Science Challenge. Trialling an ecosystems-based management (EBM) tool for managing two key overlapping marine stressors in Hawke’s Bay: land-based sediments and physical disturbance to the benthic seafloor. 

Benthic structure is important 

The soft marine sediments on the flat seafloors of our harbours, estuaries and open coastal environments, are home to a diverse array of organisms and play a pivotal role in marine ecosystem functioning, regulating so-called ‘fluxes’ of oxygen and nutrients through the bio-complex network of bacteria, microalgae and benthic invertebrates within the sediments. In fact, this network of interactions is so mindboggling intricate, a NIWA info sheet from 2004 warns “a lone scientist in a single disciple” will likely fail to grasp what’s happening. 

I speak to soft-sediment ecologist Conrad Pilditch, who explains how “The ecosystem itself is driven by these processes in the sediments.” If you lose that benthic structure through disturbance (for example, through contact bottom-trawling, or through nutrient overload, which causes eutrophication) or coat them in mud from sediment dumps, then you lose beneficial services you cannot replace. 

HBRC marine scientist Anna Madarasz Smith who is working on an ecosystems based management pilot as part of the Sustainable Seas National Science Challenge Photo Tom Allan

Horse mussel beds, for example, are highly vulnerable to disturbance, but once lost, the incredibly valuable filtration service and habitat they provide will never be as effectively recreated. If you lose the mussel beds, the efficacy of the system goes down, and then the ability to recolonise the space declines, and so on. And without a functioning benthic structure, you don’t have the worms and the microalgae seabirds live on, or a nurturing environment for fish to breed – the implications just ripple out.

The problem is not so much that benthic structure is disturbed – there’s a lot of capacity for recovery in a healthy system and there’s always going to be natural factors (such as a storms) that shift the balance – but it’s the magnitude, spatial extent and frequency of the disturbance, and that’s greatly increased when it come to anthropogenic activities. We’ve long lost the horse mussel (and other kuku) beds that were once prolific in Hawke Bay. 

It’s a question of defining the parameters of the disturbance, says Conrad: “How big, how often, and where is it coming from outside the system?” 

Cumulative effects and tipping points 

Marine stressors such as freshwater sediments can travel long distances, causing cumulative effects that extend over large areas and build up over time, making them extremely challenging to manage. But it’s the interactions between things, not a single factor in isolation that determines outcomes. 

For creatures struggling but surviving under particular conditions, though, it can take just one more adverse influence to ‘push them over the edge’. As was the case several years ago with a big shellfish die-off in Whangateau Harbour: there had been a series of low king tides, which coincided with high temperatures (so the inshore was warmer, drier), but the shellfish were also carrying a heavy parasite load. Either condition might have been survivable in isolation, but in combination, it wiped most out of the population. 

And then there are background stressors, such as turbidity (a measure of ‘murkiness’ – the suspended particulates, coliforms, tannins, etc, in the water column), which affects the way light penetrates the water, in turn affecting marine organisms, many of whom are sensitive to different wave-lengths of light. 

Marine life can cope with turbid conditions but it does create stress, which weakens resilience (the ability to respond to change and recover from disturbance). If turbidity is then coupled with rising temperatures, the extra effort (of dealing with turbidity) plus a warming event can lead to catastrophic collapse. 

Thus, stressors can lead to a ‘tipping point’ where an ecosystem suddenly loses its capacity to cope with change and rapidly degrades. 

The ability to detect the early warning signs in an ecosystem, or to even know when a tipping point is being approached, is very difficult, says Conrad, and we don’t monitor our marine environments in a way that we can detect them. Namely, because we don’t monitor or understand multi-stressor interactions, and the sampling duration and frequency of many marine monitoring programmes are largely inadequate. 

In New Zealand, we manage our marine space through setting limits in isolation (catch allotments for individual fish stocks, different parametres for different contaminants, maximum nutrient loads) and then we tend to operate at that limit, he explains. “From a practical point of view, setting limits is a useful tool, but if you don’t get it right in a multi-stressor world or you don’t know what the actual limit is, there are consequences.” 

A collective problem 

The ocean is a hyperconnected world, and we can’t contain impacts to one location or ringfence our actions, as we do on land (and even what we do on land finds its way to the sea). 

Moreover, “without understanding how things function, and their interconnections, you’re managing in a vacuum,” which is largely the case with our oceans and estuaries.

Part of the problem is we don’t value the full suite of ecosystem services the ocean provides. We only see the marine space in terms of food production, but it holds huge potential for carbon sequestration, for instance – if we’re serious about offsetting carbon as a country, says Conrad, then it’s not just forests we should be focusing on, but the rocky reefs and soft sediments in our extensive coastal areas, which actively capture and store carbon.

It’s about expanding the knowledge lens to gain a fuller picture, which means drawing not just on western science and neoliberal economics but on indigenous knowledge – mātauranga Māori – and on the different social and vested understandings we bring collectively to the table. 

When it comes to managing the marine space, what’s required is a much more bespoke management system, that is place-based rather than defined by set national limits, that’s more quickly adaptative, and that cuts across the different, intersecting legislative arenas – the RMA (which is now in process of being scrapped), the Fisheries Act, the Māori Fisheries Act, Biosecurity Act, territorial bylaws, etc. 

To this end, in 2018, the regional council and Hawke’s Bay Marine and Coastal stakeholder group (HBMaC) co-created a Research Roadmap for our coastal marine environment, with the vision to “achieve a healthy and functioning marine ecosystem in Hawke’s Bay that supports an abundant and sustainable fishery.” It’s this piece of work that’s laid the foundation for the Sustainable Seas EBM pilot.

Systems-mapping as a tool

When problems are seen in isolation, you can only react to events as they occur. To manage risk, you need to see the trends and understand the interconnected causes and drivers (the systemic structure) and why these are occurring, and, further, to transform the (often unexamined) perceptions and values that underpin them.

That’s the philosophy behind the extraordinarily complex systems-mapping tool HBRC has now co-developed with HBMaC to understand the dynamic multi-sector interactions of multiple stressors in our marine environment (namely sediment inputs and bottom-trawl fishing) and to explore different pathways for intervention. That’s phase 1 of the regional Challenge.

It takes a webinar presentation and supporting documents to be able to make sense of the map if you weren’t at that table yourself (and critics say that’s ridiculous). But the objective is in the exercise itself – it’s intended as a tool for participatory engagement, with the potential to build consensus and support for tangible, evidence-based, future-oriented ‘interventions’. 

Using simulation, participants are able to gain insight into the processes involved in “fixing the problem”, from the indirect influences of seemingly isolated factors, to grasping the inherent time-lags whereby changes in the physical structure lag behind the changes in the phenomenon causing it, to realising that “the highest-level intervention area is not necessarily the most proximate to the issue”. 

In the obvious case of freshwater sedimentation, for example, the largest gains to be made are in land-based practices far from shore that reduce farm (and forestry) soils from being washed off land and down rivers in the first place.

NIWA’s Carolyn Lundquist, who heads the project, says the systems-mapping exercise helps people realise that “decisions made now won’t probably have this fantastic result in a year; that it might take a decade, or multiple decades. But the decisions made now really need to happen in order to create that change for the future – we can’t just keep waiting and piling things up.”

This year is all about putting the EBM tools into action (phase 2): co-developing potential management interventions; running these, along with information gleaned (e.g., the work Ted Conroy is busy with) and data collation, through computer modelling and a damage and recovery model; exploring the outputs and implications; and assessing how to navigate the pathway forward for change.


While contaminants have not been included at this stage into the EBM mapping tool, with Ahuriri Estuary in such close proximity to industrial and urban environs, the issue of contaminants in our marine space is of concern to many, particularly as it’s an area that’s well within our purview of local-level action.

With Napier’s ageing municipal water infrastructure unable to cope with heavy rain, the occurrence of raw sewerage and contaminated stormwater flowing directly into the estuary is not a once-in-120-year event but an almost annual ‘misfortune’, while industrial spills into waterways are not uncommon. Both Napier City and Hastings District Councils continue to pipe their municipal wastewater (which includes consented and in-breach-of-consent trade waste, urban stormwater and both treated and raw sewerage) into the sea. 

As regional councillor Neil Kirton (who holds the marine portfolio) points out, “There seems to be a legitimising of what is a main source of contaminants” without any understanding or due investigation into the impact these may be having on our marine environment. 

He believes the recent industrial hydrochloric acid spill into the Thyne St waterway (which led to ‘distressing’ fishkill in the estuary and set back years of conservation efforts) is just the tip of the iceberg – “It’s a wake-up call for a serious conversation with industry in Ahuriri” – but is dubious that a fine is the way to go.

“We’ve been issuing abatement notices for years and it’s not working. The emphasis on a regulatory body has clearly failed, so we have to move to a more collaborative approach,” recognising that there’s been a shift in the social licence “for industries to keep doing what they’re doing” and that what we’re faced with is also a legacy of disregard.

The situation in Ahuriri is “an ongoing disaster as a result of historic planning decisions driven by greed and money, wilful negligence of municipal wastewater infrastructure on the part of NCC over the decades, HBRC’s laissez-faire approach to consenting in the past, and a moribund approach to compliance.” 

But it’s too easy to palm off blame – galaxalides (synthetic fragrances found in common household products) have now being found to weaken shellfish formation in the Ahuriri Estuary, for instance, and that’s down to chemicals we unwittingly use in our homes. 

Let’s stop stalling 

Current councils are dealing to some extent with the marine issues – it’s not that nothing is happening – but need to accelerate the process, says Kirton. While he supports the Sustainable Seas project and the need for an EBM approach, he fears “we’re defaulting action to it; prevaricating.”

“We need to accelerate our understanding of the issues and get the answers on the table; we need to accelerate the science.” In the meantime, we should be taking all the steps we can, “not just focus on one or two big ones”. 

While there are complex charges for land-based discharges there’s nothing for oceans, for instance, and he’s of the view that has to change. “We need to implement a science charge, a ‘toilet levy’, if you like, which can help fund the science, at the very least, to investigate the effects [of piping waste-water out to sea].”

Neil’s biggest frustration is that “we’re not prepared to put our money where our mouth is”. Council’s constrained by lack of investment and there’s little community momentum for action. As he points out, Hawke’s Bay’s marine space makes up a third of HBRC’s territorial area but only $200-300k is spent on it per year, a marginal sum comparative to size – “It’s wholly insufficient.” A good chunk of that is spent on land-based mitigation, such as riparian planting, with a small allocation to the inshore itself.

Where we are with the marine space in Hawke’s Bay is essentially where we were for freshwater 20 years ago, Neil believes, and says he feels a sense of sadness for “the lack of diligence and attention” given to our oceans. We’re facing a critical point, but are choosing to look away because the issues are not directly visible, “We’ve become ocean-collapse deniers”.

Conrad Pilditch, however, is less pessimistic and believes we can turn things around with the science and knowledge we already have on hand, but people need the opportunity to understand the linkages and consensus-building will be key. 

“We can achieve quite positive outcomes in a short time when we take collective action. But the capacity to recover won’t be there forever, we do have to move now.” 

In this article, you’ll note there’s little mention of fish – fish have been fulsomely covered in my marine feature last issue (Jan/Feb 2021), ‘Who Speaks for the Fish?’ and in my 2019 feature on the quota management system, ‘Save Our Fishery’, both of which can be found at

For more on the Sustainable Seas National Science Challenge, ecosystem-based management, and the Hawke’s Bay regional study, see 



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1 Comment

  1. Brilliant article, thanks.
    One thought I had, a couple of summers ago when the water temps at Cape Kidnappers were reported as high, I wonder what the accumulative affect is of PanPac pouring hot water into the sea 24 hours a day on-going? Might it travel on down to the Cape?

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