The day of dolphins and trace elements

Louis Byrne, British Oceanographic Data Centre, NOC

Wednesday was our last day at Station A before heading to station G.  Half way through our third CTD of the day we were ambushed by a pod of common dolphins. The dolphins stayed around the boat for most of the morning and into the afternoon, with one theory being that they like the way the waves break around the ship. Aside from all that cetacean excitement some science also got squeezed in to the day’s events.

Common Dolphins around Site A in the Celtic Sea

Today was a day of trace elements, complete with their ultra-clean CTD, ultra-clean labs and ultra-tired scientists! For some background to the marine study of Iron, including why we are looking for it and why it is so hard to measure, there is an excellent summary written for this blog by Jonathan Sharples during one of the previous cruises – see post titled ‘Sampling Iron’ written on 15th November! 

Trace metal scientists at work in their ultra-clean lab.

As mentioned in Jonathan’s blog post we believe that one major source of Iron is resuspension from sediments on the continental shelf. The edge of the continental shelf can be thought of as similar to a vast desert on the edge of a gigantic cliff face, with the water depth increasing from just a few hundred metres to distances measured in kilometres as you move from the shelf edge towards the open ocean.
   

The broad, gentle pitch of the continental shelf gives way to the relatively steep continental slope.
continental shelf. 2015. Encyclopædia Britannica Online. Retrieved 13 April, 2015, from https://www.britannica.com/EBchecked/topic/134970/continental-shelf/285032/Origin

Currents and waves cause particles of sand and mud on the sea floor to be lifted off the seabed and mixed into the water column above, and these can then be transported off the shelf edge in giant plumes of resuspended particles. The last cruise found evidence of currents along the sea floor of the continental slope which were pulling sediment off the seabed and causing it to mix in the water column above.  

On this cruise one thing we are measuring is the concentration of Iron in the shelf sediments, which can be compared to Iron concentrations in sea water above to work out how much Iron the sediment is supplying the water column each year. As phytoplankton growth (and thus, primary productivity) is limited by Iron in 25% of the open ocean, a better understanding of the processes which supply Iron to ocean waters is important to understand how primary productivity in the open may change in response to climate change.

Charlie Thompson, Natalie Hicks and a
man in a hard hat pointing at the location of Site A.

Science begins in earnest - CTD and NIOZ corer

Louis Byrne, British Oceanographic Data Centre, NOC

Early start on Tuesday with the first sampling of the cruise scheduled to start at 0600. A few winch jitters before we finally got to witness the CTD disappearing beneath the waves at 0718, and there was much rejoicing! Apart from three CTD casts for the majority of today we have been using a NIOZ Corer to dig up sediment from the sea-bed, and it seems like an appropriate time to introduce two instruments which will be mentioned a lot of in this blog.

A ‘CTD’ is a metal frame consisting of numerous marine sensors encircled by a ring of 24 water bottles. CTDs are the bread and butter of marine research and are very useful for getting a picture of how the water column changes with depth.  The CTD is attached to a wire and lowered through the water column until it reaches 5-10 metres above the sea-bed. During its transit to the sea bed the CTD produces live output to a computer screen for Temperature, Salinity, Pressure, Fluorescence (essentially the chlorophyll in the water),
Transmission (particles in the water) and other variables. Scientists on deck then use this information to decide at which depths to fire (close) the water bottles as the CTD is raised back through the water column, in order to catch water from specific depths for analysis back on deck.
 

A 'CTD' as pictured as it is being lowered into the Celtic Sea.

The term corer is a general term used to describe instruments designed to collect samples of sediment from the seabed. The samples are then brought up to the surface where they may be analysed, or used in experiments. This could happen in one of the several laboratories on the ship, or they may be stored for analysis back home.

A NIOZ corer being setup on deck.

One of the scientists using water collected by the CTD and sediment from the NIOZ corer is Briony Silburn (from Cefas), who is running an experiment to investigate the impact of different types of fishing trawls on the seabed. When trawling, the heavy weights attached to the trawl nets are dragged along the sea floor; disturbing the sediment causing it to be mixed into the water column above (this is called resuspension). When this happens the nutrients contained within the sediments also get released.

Briony Silburn and her sediment resuspension experiment

Briony is using mud from the core which is added to a large container containing water from the Celtic Sea, and spinning magnets are used to continuously mix the water so that the mud doesn’t fall to the bottom. This experiment is repeated three times, using mud from the first two, four and six centimetres of the seabed. Over a 24 hour period the nutrient concentrations of the water are measured, and by comparing the measurements from all three experiments it is possible to get an idea of how the concentration of nutrients resuspended varies as the trawl weights penetrate deeper into the seabed. These nutrients are then measured on board the ship (more about them later).

This research can then be used to influence the design of trawl weights so that they are less harmful to the marine environment.