Flooding hurts estuaries and coasts
Over recent months there’s been more climate-related flooding – from tautahi/Christchurch to Tāmaki Makaurau/Auckland to Whakatū/Nelson and across the ditch in Sydney. And sea level rise, which exacerbates flooding, is already affecting many towns and cities.
The cost of onshore clean-up is painfully clear, but our Sustainable Seas research reveals unseen costs to our estuarine and coastal ecosystems, from the micro- to macroscopic scale.
The increased flooding and sea level rise set to come will push some estuaries to the brink of collapse. Some may never recover.
Flooding and sea level rise increases sediment and nutrients flowing into our estuaries, affecting the primary producers that are the basis of coastal food webs.
They can also modify species’ behaviour and decrease biodiversity. These stressors can build up over time, as well as interact with other stressors (such as over-harvesting and heavy metal contamination) to generate cumulative effects.
The more an estuary is flooded, the more turbid it becomes, reducing sunlight filtering through the water to the sea floor.
This, combined with sea level rise and nutrient run-off, reduces seafloor primary production and the nutrient cycling carried out by the microphytobenthos (microscopic plants on the seafloor), with knock-on effects on the entire food web.
Increased turbidity (in combination with other stressors) erodes an estuary’s ecosystem health and increases its vulnerability to a tipping point.
A tipping point occurs when an ecosystem loses its capacity to cope with change and it rapidly transforms to a new state. Tipping points often involve the loss of valuable marine resources and ecosystem services.
For example, loss of shellfish may result in a rapid decline in water quality – because filter feeding by shellfish reduces the concentration of suspended sediments in estuaries, lowering the turbidity. But the more their environments are altered by flooding and sea level rise, the less ability they have to filter sediment and process nutrients.
Changes to species that are the foundation of estuarine ecosystems may also affect marine life higher up the food chain. For example, international research shows kuaka (bar-tailed godwits) need the essential fatty acids produced by microphytobenthos to support their long migration from Aotearoa to western Alaska each year.
With the loss of the inter-tidal zone from sea level rise and more flooding, microphytobenthos are compromised because they can’t photosynthesise effectively and have less habitat to live in. Of course, kuaka may adapt to this change and seek a different food source, because seabirds are good at adapting. But microphytobenthos will continue to be negatively impacted by loss of water clarity.
Our research has also shown that what happens in estuaries affects ocean ecosystems on a macro scale. Sediment and nutrients are carried by river eddies over 100km out in the sea’s surface; while submarine canyons connect coastal and deep-sea ecosystems, transporting materials from the land into the deep.
This means our environmental footprint goes much further into our marine Exclusive Economic Zone than previously thought.
One vital solution that Sustainable Seas is working towards with co-developers is local, placed-based strategies using ecosystem-based management (EBM) to manage the amount of sediment and nutrients flowing into Aotearoa’s estuaries, giving them the best chance of surviving the impacts of climaterelated events.
Otherwise, we may stand to lose our precious estuary resources and species forever.