Dive Report: Mairangi Bay Rounded Depressions

Yesterday Alex Rogers sent me this really interesting report on some multibeam echosounder (MBES) data recorded by NIWA https://storymaps.arcgis.com/stories/7b0c9ab9fd7141abb14884f8a1f3efa4 It includes several interesting features. The most accessible were:

"A cluster of rounded depressions may be indicative of relict or active seabed seepage and potential sites of sensitive ecosystems. Seafloor backscatter reveals the sloping rim of these features may have a harder substrate compared."

They were only 10 m deep, and 1 km off shore. I added some chain to my kayak anchor rope and decided to head out the next morning.

Tue 7 Mar
Sunny
4 knots
HT 8:30AM
Wind N
Wave Hight 0.7

I kayaked from Mairangi Bay to the what I thought was an area of complexity in the middle of the largest depression (-36.736195, 174.767237) and dropped anchor. The sun had just come up but the visibility on the surface was 4-5m. The tide was slack with hardly any current. I descended the anchor line, visibility dropped sharply near the bottom and I had about 70cm vis on the seafloor. It was also very dark due to the low angle of the sun.

I was pleased to see some shells around and noticed interesting nodules protruding from the firm mud. I had decided on a Northern transect and stuck to it for 35 minutes. The substrate changed from firm to very soft mud then a bit more firm towards the end. I saw 10's of dead scallop shells and four cushion stars on the surface. The wandering anemone was by far the highlight of the dive and I wish I had bought my big camera. I was surprised to not see a single Mediterranean fanworm.

Where the mud was soft the visibility dropped to less than half a meter. I dug into the soft mud three times. The first time I could only find a fragment of scallop shell, the second one a scallop shell and a bit of green-lipped mussel shell. The last one had nothing at all. I am used to finding a lot more shell in the mud than this. I did not notice any change in the incline and my depth remained around 12.3m throughout the dive. I wonder if the MBES was detecting and old geological structure that has been since filled with mud.

I ascended with a 100-150m surface swim back to my kayak. On the way back I counted two juvenile southern black-backed gulls and two red-billed gulls on the water. There were also five white-fronted tern feeding close to shore. There were two little penguins feeding near where I dived, I wondered if they fed on the bottom.

Posted on March 6, 2023 11:31 PM by