In what is quite possibly the first study of its kind, PhD candidate Brandy Biggar is looking into how sudden sediment burial impacts the survival, growth, and overall behaviour of kūtai spat (juvenile green-lipped mussels). While previous studies have tested the effects of sediment burial on other species, very few have focused specifically on the impact to juvenile bivalves, in particular mussels.
We know from Brandy’s previous research that spat can withstand relatively high levels of suspended sediment, likely because they can move out of the way (such as on naturally occurring kelp or seaweed). But what happens when sediment enters the water and quickly settles, burying kūtai spat? With an increase in extreme rainfall events washing more sediment into our coastal environments, understanding the impact of sudden sediment deposits is becoming more important.
Brandy is completing her PhD at Waipapa Taumata Rau / the University of Auckland, supervised by Professor Andrew Jeffs and Dr Jenny Hillman, investigating the impact of sediment on kūtai spat. This research builds on what many already observe in the moana – that increasing sedimentation is affecting the survival of young kūtai and other shellfish. The research helps build our understanding of how increased sedimentation will impact the health and survival of young kūtai by helping us understand more precisely how and when those impacts occur, as well as helping us to identify possible barriers that need to be addressed to support mussel reef restoration.
16 glass beakers were set up as test environments, each with a mud substrate at the bottom to simulate the natural seafloor. Spat (between 1-2 mm long) that had attached to a mesh surface were placed on top of the mud substrate. The beakers were then split into two groups, Burial and Non-burial, with 8 beakers in each group. For the Burial group, 7g of fine sediment was added to the 1L filtered, sterilised seawater to create a 1mm layer of sediment burial on top of the spat. For the Non-Burial group, no sediment was added. The beakers were then observed over 8 days to see how the spat responded.
On day 4 of the experiment – the halfway point – 4 Burial and 4 Non-Burial beakers were removed, with the mussels measured and checked for survival. Information was also recorded on where in the beaker the spat were found: on the mesh (their starting location), in the mud substrate surrounding the mesh, or climbing the inner walls of the beaker. The same measurements and data collection were repeated for the remaining 8 beakers at the end of the experiment on day 8.
In short, we’re seeing that while spat can cope with sediment suspended in the water, when sediment quickly settles and buries them, survival drops. Even a light layer (around 1 mm) can be enough to cause harm.
The survival, growth, and overall behaviour of the spat varied greatly, with each factor influencing another, creating a number of useful insights. For example, some spat attempted to escape sediment burial. These were significantly smaller than those that remained, and yet attempting to escape from sediment increased their overall mortality (from 36% among those that remained in their starting position, to 58% among those that attempted to escape).
Typically, spat of a similar size are highly mobile, readily moving to more favourable growing conditions, however, there was low overall movement of spat in both the Burial and Non-Burial treatments in this experiment, suggesting they were either unable or were not triggered to move away. These findings of this study demonstrate the sensitivity of spat to sediment burial, particularly for juvenile mussels.
What is clear from this research is that when sediment settles on the seafloor, it can smother young kūtai and reduce their survival. In this controlled study, even a thin layer of sediment increased mortality in spat, highlighting how sensitive this life stage can be.
This reinforces first the importance of reducing sediment entering our moana, while continuing to work together on restoration approaches that support kūtai to establish and thrive. And, trialing restoration methods that help support spat and juvenile kūtai through this sensitive life stage. We also need central and local governments to work together and put in place policies that reduce the amount of sediment flowing into our oceans.
There’s still a lot we have to learn about sedimentation and other environmental pressures in the Hauraki Gulf, particularly when it comes to understanding how these pressures impact kūtai recruitment, settlement and growth. That’s why our work at Revive Our Gulf explores multiple approaches for the restoration of mussel reefs: testing and researching how different sizes and locations of live mussel plots survive under current conditions, whether using shell to raise translocated kutai off the seabed impacts mortality rates, and how structures might encourage spat recruitment.
You can learn more about our restoration and research projects here, and you can access Brandy’s full research paper here.
