Measuring the metabolism of the seafloor

By  Megan Williams, National Oceanography Centre

Today we recovered our benthic lander. The frame had been deployed for two days and has nine instruments measuring a range of parameters including water velocity, nutrients, suspended sediment, sediment particle sizes, and benthic oxygen consumption. Our first deployment was at a site with sandy sediments.

Recovery of the benthic lander 

 The steps toward our first recovery were many (see pictures): after driving the instruments and frame down from the National Oceanography Centre in Liverpool to our sister location in Southampton, we built the frame and started attaching instruments, batteries, and routing cables. When the frame was in a state it could be moved (with fragile instruments not yet installed), the frame was driven to the mobilization dock and loaded onto the RRS Discovery. Once on the ship, we could install the fragile water sampler (which will be used for nutrients and suspended sediment measurements) and the eddy correlation system (which makes fast oxygen and velocity measurements near the bed). The eddy correlation system measures subtle turbulent currents (eddies) just above the seafloor with both up and downward elements as they move past the sensor as swirls of water 'rolling' over the seabed. The sum of the upward (positive) and downward (negative) movement of dissolved oxygen gives a measure of how much oxygen the seafloor is using (i.e. the metabolism of the seafloor).

With a planned deployment time, we programmed instruments to start, did last minute calibrations, and set up the mooring. The frame was then slowly lowered 100 meters (m) to the sea bed, a ground line was set out, and a weight and buoy are connected 300 m away so as to not interfere with measurements.

All has gone well so far! We have the frame back on the ship this afternoon. We have now started to collect all the data off the lander, changing batteries, and preparing for another deployment of the instruments at a site with muddy sediment.

Calm Seas

By Gary Fones 

Calm seas: Credit: Gary Fones

12^th of May saw some much appreciated calm weather and lots of science activity aboard the RRS Discovery. Lunchtime saw the deployment of the PML Buoy Profiler, which is a SSB PhD project (more of this in a latter blog from Rich Sims, PML).

 Picture of PML Buoy. Credit: Gary Fones

Sediment coring followed this, this is a key activity of any benthic cruise. We are using a number of coring devices to collect sediment from the seabed beneath us, which is 100m down. On this research cruise we are using a NIOZ corer which is used to collect sediment (mud) from the ocean floor,  a mega-corer (able to take up to 12 undisturbed samples in clear plastic tubes),  and a large SMBA box corer which is designed to take a 600mm square, undisturbed sediment sample up to a maximum depth of around 450mm.

 NIOZ corer recovered to deck. Credit: Gary Fones

Wednesday 13^th of May started with calm seas and a lovely sunrise. This was followed by a very successful recovery of the NOCL mini-stable lander that has been on the seabed the last few days gathering in-situ data (more of this in a latter blog) which will be used by the scientists to understand processes happening at the boundary between the sea bed and water column.

Recovery of Lander: Credit:Richard Cooke

The Eclipse and a Rescue Attempt

Louis Byrne, British Oceanographic Data Centre, NOC

The eclipse viewed from the RRS Discovery (photo by Helen Smith)

Friday started much the same as the previous two days with a 0600 shift  changeover followed by some clear blue skies (with a few white clouds) and more spatial survey. At 0800 we took a break from the spatial survey for about an hour to watch the eclipse as we moved between sampling sites, and a couple of the pictures we took have been provided in the blog. Following the eclipse we continued surveying till one, at which point there was a break to attempt to rescue the NOC-L (National Oceanography Centre, Liverpool) Lander which we had lost on recovery a few days ago.

Steve and Owain attempting to get the Lander back on deck

When deployed the Lander is dropped first, followed by approximately 50 metres of wire and then a heavy weight, which is attached to a surface buoy to mark the position of the lander and make it easier to retrieve. Unfortunately the rope to the surface buoy was detached from the Lander, meaning that to retrieve it we first had to trawl for the wire connecting the Lander to the anchor weight, and then somehow get the tangled mess back on deck.

The Lander is recovered!

The rescue attempt took a few hours, but after some fantastic work by CPO (Chief Petty Officer) Steve, Owain (National Marine Facilities) and the rest of the deck crew the Lander was rescued from the bottom of the Celtic sea and back on deck. If the rescue of the Lander wasn’t impressive enough, we were in for another surprise as somehow, the ultra-fragile oxygen sensor attached to the mooring managed to survive the rescue attempt and made it back to deck in one piece. I am told you really have to work with these things to understand just how unlikely it was that this happened. Following the Lander recovery it was back to the spatial survey, with approximately 12 stations to go before we are free of our sampling shackles and can head to CANDYFLOSS.

Deploying the large yellow torpedo!

Louis Byrne, British Oceanographic Data Centre, NOC

Thursday was an exciting day for this cruise as we were finally able to deploy Autosub3, an autonomous underwater vehicle (AUV) which looks like a large yellow torpedo. Autosub3 was developed at the National Oceanography Centre in Southampton, and can be pre-programmed to survey a site for over 24 hours at a time. For this cruise the main objective of Autosub3 was to collect images of the seabed at the 4 sampling stations to look for what animals are living on the different sediment types.

This is done by pre-programming the vehicle to complete a mission in a ‘lawn mower’ style pattern where images are taken along 5km tracks at more than one per second!! Meaning thousands of images are collected in one mission. We are also able to collect information on the seabed morphology using two different scientific methods (bathymetry and sidescan) allowing the creation of biological map. This is a new method being used for monitoring of Marine protected areas and thus is at the cutting edge of science.

 Mini-STABLE deployment (photo by Richard Cooke)

Autosub was the first instrument deployed at Site G, which is approximately 26 miles west of site A. Once in the water we returned to Site A to deploy a Lander called ‘Mini-STABLE’.  Landers are pieces are frames which sit on the seabed at a given location and dependent on the needs of the study have different instruments attached. The instruments attached to this particular instrument are being used to measure sediment transport. Autosub and Mini-STABLE are two high tech pieces of equipment, and illustrate how the ocean can be investigated in different ways dependent on what you are trying to find out.

Recovery of Autosub3 (photo by Richard Cooke)

After deploying Mini-STABLE we travelled back to Site G to pick up Autosub3 after its mission.