A summary of Biggar et al., 2025
The amount of sediment entering coastal environments is increased through activities such as agriculture, deforestation, mining, and construction, as well as increases in extreme weather events because of climate change. Sediment moving from land into the ocean increases suspended sediment, which is sediment in the water column that can make the water murky.
Increased sediment in coastal environments is thought to be a contributing factor in the bottlenecks to the recovery of kūtai beds. Not only can kūtai be smothered by sediment, but, as they are filter feeders, too much sediment in the water column can clog or damage their feeding appendages.
Suspended sediment may also impact where kūtai spat (baby kūtai) choose to settle. An important part of the kūtai life cycle is the spat settling onto substrate. Once kūtai spat find a suitable surface, they will attach themselves and continue to grow. They can settle on hard structures like rocks or adult kūtai on the seafloor, or surfaces like seaweed.
Brandy Biggar is completing her PhD at Waipapa Taumata Rau / University of Auckland supervised by Professor Andrew Jeffs and Dr Jenny Hillman, investigating the impact of suspended sediment on kūtai spat. This research is important for understanding how increased sediment entering coastal areas will impact the health and survival of kūtai, as well as helping us to identify possible barriers to restoration.
Experiment
Brandy designed a laboratory experiment where she measured the effect of a range of suspended sediment concentrations (between 0 to 1250 mg / L) on the survival, growth and nutritional condition of kūtai spat. This involved setting up multiple tanks using the seawater facilities at the Waipapa Taumata Rau / University of Auckland’s School of Biological Sciences and sediment collected from the Whangateau Harbour. Small pieces of mesh were suspended in tanks to offer kūtai spat different settlement surfaces at different heights in tanks. The kūtai used in this experiment had a shell length of approximately 1 – 2mm, and were provided by SPATnz Ltd.
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Results
Suspended sediment concentration impacted where kūtai spat settled in the tanks. At low concentrations of suspended sediment, there were more kūtai spat settled at the bottom of the tank, and at high concentrations of suspended sediment, kūtai spat settled higher on the mesh. This suggests that high suspended sediment may cause spat to settle higher up. If those higher up settlement surfaces don’t exist in areas with high suspended settlement, spat may not survive.
The kūtai spat had high survival and growth in high suspended sediment and there was no difference in nutritional condition at the different suspended sediment concentrations. This indicates that kūtai spat can withstand relatively high levels of suspended sediment.
Conclusions
While kūtai spat are robust and seem to be able to survive and grow in the short term in high suspended sediment concentrations, these concentrations may be disrupting settlement cues for kūtai spat, which will prevent them from settling and repopulating adult kūtai beds. This could be one of the factors that is limiting recruitment, as there is too much suspended sediment, signalling to kūtai spat that they need to find somewhere else to settle.
Looking forward, there needs to be a sustained focus on reducing sediment from washing into our coastal environments and encouraging our decision makers to pull the levers to help address this complex and long-term issue. For kūtai restoration, in places where there are high amounts of suspended sediment we may need to consider introducing structure that sits higher in the water column to provide a suitable place for kūtai spat to settle. This may give kūtai spat a better chance to settle and grow, even when there’s a lot of sediment floating around.
To see the full results of this experiment, the journal article is:
Biggar, B. S., Jeffs, A. and Hillman, J. R. (2025). “Effects of suspended sediment on survival, growth, and nutritional condition of green-lipped mussel spat (Perna canaliculus, Gmelin, 1791).” Journal of Experimental Marine Biology and Ecology 582: 152074.