13/02/2025 by Francois Thoral
Cyclone Gabrielle has tragically showed us how extreme events not only impact us on land, but also how they can affect coastal ecosystems, for example with enormous delivery of forestry slash (see here, or here) or suspended sediments that end up linger along the coastline and create murky conditions.
These murky conditions pose several issues for coastal ecosystems, notably for kelp forests. For example, when the water is not clear, light does not reach the seaweed that live on the seabed, making them less or not productive at all. Visual predators like fish or marine mammals can also be affected as murky conditions make it harder find food.
These periods of underwater darkness are potentially one of the biggest issues for coastal ecosystems across the motu, but not much is known about them. How long and frequent are they? What are their ecological impacts? What are causing them? The satellite images below show that such events of underwater darkness come and go in time.
1. Sediment runoff from rivers creates periods of underwater darkness.
In a global context, this is an issue that is also observed in many other countries and ecosystems. However, there is currently no widely accepted term and definition to characterise such events of underwater darkness. Having a coherent framework would really help understanding the nature, impacts, and sources of these events. Research within the Toka Ākau Toitū Kaitiakitanga project is proposing to call such events “marine darkwaves” with a consistent way to detect and measure them.
2. The marine darkwave definition and framework aim at increasing the awareness and understanding of extreme periods of underwater darkness.
The research we are conducting within Te Whānau-ā-Apanui rohe is helping towards increasing our understanding of such marine darkwave events. We are trying to measure and understand how frequent and intense these events are, especially around awa Raukokore, and how the coastal ecosystems are responding to these events. People around the rohe seemed to be particularly concerned about the role of forestry in creating sediment runoff, which in turns create marine darkwaves.
To measure marine darkwaves, we are currently deploying devices along the shoreline that measure light intensity underwater. From these measurements, we can also estimate water clarity, and compare how this changes along the coastline. Ultimately, we will get a better understanding of the duration and intensity of the marine darkwaves that we can capture with our instruments.
3. The instruments and plan to measure marine darkwaves along the coast.
Stay tuned for the incoming results!