Anoxic seafloor bottom

Here’s a tricky problem to ponder.

How do you take samples of the seabed in a marine park, where you are not allowed to take any actual physical samples?

Coming up with the answer to this tricky problem was one of the exciting challenges the iXblue team faced during our recent 7-week survey of the spectacular Fiordland in the south western pocket of New Zealand.

The solution to the problem of not taking physical samples

What we decided to do for the collection of bottom samples was use a remotely operated vehicle (ROV).  It’s an innovative way to visit the seafloor without getting wet or causing any disturbance.

iXblue ROV Thalassa
The ROV called Thalassa

As we found out, doing it this way really opened our eyes to the complexities of the seabed.

When we launched our little ROV named Thalassa, we guided her downward until we could see the bottom.  By keeping her steady, we could look around and get a picture of what is at depth with real-time data feeds.  Depth and direction gauges helped us to keep track of our location, relative to the vessel.

A better solution than the normal technique

As it turned out, this improvisation proved to be much more useful than a simple sediment grab, which is the normal technique for bottom sampling.

When we use a traditional grab sampler, we don’t see the big picture.  With the video method, we image on our way down, getting context, not just of the seafloor sediments, but also the habitat and local ecosystem.

For example, if we acquired a grab sample, we might just pull up a handful of medium grain sand.  If we had of done this, we would never have known that the sand was in a small pocket surrounded by hardgrounds and kelp.  There may be additional information gathered regarding fish species and abundance that would remain a mystery, if just collecting a sand sample.

Our surveyors believe using video has opened their eyes about the composition of the seafloor and the variety of seabed types.  When surveying in existing or potential anchorage areas, the video helps show possible obstructions or dangerous rocky crevices that may pose a risk to mariners.

From a scientific perspective, the data acquired by video is essential for ground truthing interpreted seabed characterisation from our seafloor survey.  When we look at digital products, the video provides helpful information about the sediment colour, texture and size.  It can also provide us insight about the local environment and even some clues about the currents.

During our survey of Fiordland we were surprised to find quite a variety of seafloor types in the various channels within the Sounds.  We also spotted groundwater seepage on the seafloor where freshwater was being expelled and creating unique habitats.

We even met a very curious shark that wanted to check out our ROV and say hello.

Using the ROV showed all of us that not only is it environmentally safe, but a far more revealing way to see the seafloor ecosystem in all it’s complexity and entirety.


Elizabeth Johnstone

Dr. Elizabeth Johnstone is a marine geologist specializing in bathymetric surveying and sediment dynamics. She has over 15 years of experience using geophysical tools to conduct seafloor mapping around the world. She develops novel approaches to produce four-dimensional data products for integration into marine geological studies.

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