Cumulative effects are one of the most urgent and complex problems facing our marine environment. In some instances, when effects overlap they can offset each other and reduce the overall impact, but more often effects compound, or result in unexpected impacts (Davies et al, 2018).

A overhead shot of Porirua Harbour and Kapiti Island
Paremata Mana Plimmerton Porirua Harbour and Kapiti Island

Image: Colin Monteath, Alamy Australia

This report covers four priority issues to give a deeper understanding of the complexity within each issue. However, in reality, the effects of our activities and natural stressors overlap. Issues therefore need to be considered together rather than in isolation.

Effects overlap and build over time

The consequences of cumulative effects are highly variable. They are determined by:

  • the range and type of pressures in a place 
    • for example, pressures from excess sediment and nutrient inputs are most relevant in estuaries but pressures from climate change occur everywhere
  • the intensity and extent of effects over time 
    • for example, effects could be seasonal or localised, like from finfish aquaculture, or small incremental changes over time, like rising sea-surface temperature
  • historical pressures on the local environment 
    • for example, places that have been trawled or have accumulated sediment from modified catchments, may be more vulnerable to the adverse effects of sedimentation even after the activity has stopped
  • current and past national or local protection and restoration efforts
    • for example, marine protection (like marine-protected areas and areas over which rāhui have been established) can provide refuge for species and potentially increase their resilience to future pressures.

Data on cumulative effects is limited but increasing

The complexity of our environment and patchy, long-term observations of parts of the system mean a full understanding of the impacts of cumulative effects is lacking (Larned et al, 2018). National data on the impacts from cumulative effects is scarce.

Understanding the scale and characteristics of these effects is complicated by the natural capacity of a species or habitat to respond – for example, they may respond or recover at different rates or be impacted by cumulative effects (Davies et al, 2018). Measuring the impact of cumulative effects requires an ability to detect change and understand tipping points.

New Zealand is making progress in understanding cumulative effects. Models are being developed that allow the effects of several pressures (such as catchment changes and fishing pressure) to be considered together. New technology, including continuous fixed-monitoring buoys, drones and cameras, along with coordinated national monitoring, have the potential to monitor the marine environment more cost effectively.

Integrating this data with local information and mātauranga Māori can provide holistic, place-based knowledge that is crucial to understanding cumulative effects. Connections between pressures are not new from a te ao Māori perspective, where even small shifts in the mauri of any part of the environment would cause shifts in the mauri of related parts of the environment, and eventually the wider environmental system (Environs Holdings Ltd, 2011).

The final image (below) in the story of kuku, the green-lipped mussel, shows that while we can measure individual effects, an understanding of cumulative effects when pressures overlap is limited. This considerable knowledge gap constrains our understanding of further impacts and where thresholds and tipping points for kuku may occur. The example from this single species illustrates the complexity of cumulative effects that occur more widely in our marine environment.

All pressures that affect kuku

All pressures that affect kuku. Infographic.
All pressures that affect kuku. Infographic.

Read the long description for All pressures that affect kuku

Most of our impacts in the sea and many on land have impacts on kuku that overlap and interact, resulting in cumulative effects.

Pressures on kuku

  • Pollution
  • Marine activities
  • Climate change

Impacts between many pressures or impacts can occur at the same time.

Smothered by sediment; Loss of biological diversity and reduced water quality; can’t bind to surfaces; ingestion of plastics; accumulation of toxins; pressure from non-native species; habitat destruction; reduced habitat for mussels to settle on; pressure from native species; fragile shells; loss of habitat productivity and associated benefits such as water filtration and nutrient cycling; increased wave action.

The interactions between past and current pressures have had long-lasting impacts on kuku. The resulting cumulative effects, in combination with future pressures, are variable across Aotearoa and are hard to predict.

The benefits of healthy, dense kuku beds are being degraded or lost

Dense kuku beds:

Provide economic and income gain; provide homes and food for other marine animals and plants; stabilise sediment; improve water quality for swimming and harvesting shellfish, filter out pollutants and increase water clarity; support fish population and fisheries.

Impact on our values

Loss of benefits and biodiversity erodes mauri and takes away opportunities to express kaitiakitanga, put kai moana on the table, and share intergenerational knowledge.