Shelf Sea Biogeochemistry blog

Tuesday, 3 March 2015

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.

Monday, 2 March 2015

Discovery leaving Southampton for cruise DY021

By Louis Byrne, British Oceanographic Data Centre, NOC

RRS Discovery docked in Southampton. Picture taken by Amber Annett

After a week of setting up where all manner of frames, sensors, analysers, buoys, containers, chemicals, supermassive autonomous vehicles and an array of bedraggled looking scientists have boarded the RRS Discovery it was finally time to leave the port of Southampton for the open sea!

Breakfast on the morning of departure is at 7:30 and is followed by a safety briefing and familiarisation before departure at 1015. Rumour had it that we were leaving port into fairly rough seas, and once we have escaped the shelter of Southampton docks those rumours turn out to be correct.  It is a bit of a baptism of fire for some of the scientists on board and I think many are feeling a little queasy. 

Before our stomachs have had time to settle it is time to practice the muster, which is similar to a fire drill, however once at the assembly point everyone is required to put on a life jacket, enter an orange life boat and consider how rubbish it would be if we actually had to use it,  52 people in a small orange box with no toilet, one small hatch for air and fishing rods which aren’t actually provided for us to catch fish, but to give us the psychological illusion that there actually something we can actually do ourselves to improve our chances of survival as we get tossed around like an orange cork in a gigantic tumble dryer (if today’s  seas are anything to go by). 

Mini Stable (left) and autosub (right) on deck departing Southampton. Picture taken by Richard Cooke, National Oceanography Centre, Liverpool.
The original plan was to conduct some equipment trials at a long term observation station near Plymouth called E1, however due to the state of the seas we’ll be skipping the trials station and heading straight out to the Celtic Sea and to site A, which we are expected to reach at 9PM tomorrow (Monday). As there is little left to do today a few of us hit the bar and tv room, just in time to watch England get mauled by Ireland in the 6 nations. All in all it’s been a pretty rough day.  Just before signing off there is time for a bonus question:

Who set off the Discovery’s fire alarm the night before departure by spraying deodorant in their cabin? Answer in the next blog.

Shelf Seas Biogeochemistry – A short introduction

By Louis Byrne, British Oceanographic Data Centre, NOC

We woke up on Monday to a sea which was perhaps even worse than Sunday. We were still a fair distance away from site A and were not scheduled to reach site A till approximately 2100 Monday evening. Due to the rough seas I spent the majority of the day hugging my toilet bowl, but not before making the rookie mistake of blocking my sink with the remains of my breakfast, which Geoff the Steward was not too happy about.  Due to a day spent in transit not much happened, and due to my sea-sickness I was not around to see what did, therefore I thought it would be a good time to introduce the reason why we’re rushing towards the Celtic Sea at a slow and steady speed of seven knots.

Although shelf seas make up only 5% of the ocean surface, they have been estimated to be the most valuable biome on earth, with high levels of primary productivity supporting diverse ecosystems. High concentrations of nutrients support the growth of phytoplankton, which are single celled marine organisms that photosynthesise like plants on land. Like plants on land, Phytoplankton are the base of the marine food web and they provide a diverse food source for many marine creatures, such as zooplankton.

Phytoplankton are the foundation of the oceanic food chain.

Zooplankton are tiny marine animals which are food for fish and countless other marine organisms, that are then in turn eaten by others. It is in this way that the sun’s energy fixed by phytoplankton on the surface of the water column is distributed throughout the marine ecosystem, underpinning more than 90% of global fisheries and offering many other important ecosystem services.

In addition to supporting the entire marine food web, the photosynthesis carried out by phytoplankton also removes significant amounts of carbon dioxide from our atmosphere.  Although tiny, phytoplankton have a disproportionately massive effect on our atmosphere, and are responsible for creating as much as half of the oxygen that we breathe, removing an equally large amount of carbon dioxide as they do it. Some of the carbon extracted by the phytoplankton will sink to the sea floor and be stored in the sediments (often for thousands of years!), reducing the overall concentration of carbon dioxide in our atmosphere.

In order for the shelf seas to sustain these high levels of production, the phytoplankton must be supplied with nutrients, but where do these nutrients come from? It is the need for us to better understand the role of shelf seas in the global nutrient cycle, how this supply of nutrients determines the shelf’s primary and secondary production and how this affects other processes such as carbon storage which has led to the Shelf Seas Biogeochemistry programme.

At 2100 on Monday night we reached site A and decided that the seas were too rough to sample that night. Therefore, an 0600 hours CTD cast was scheduled for the following morning, and we were hopeful that our cruise was about to get its first piece of data.

For those of you wishing to see the answer to yesterday’s question, the answer is Richard Cooke of the National Oceanography Centre, Liverpool.

Tuesday, 27 January 2015

Expeditions of Discovery find the secrets of the shelf seas

Shelf Sea Biogeochemistry and the recent pelagic cruise (DY018) were featured in an article 'The secrets of the shelf seas – one of Earth’s most important ecosystems' by The Observer newspaper. 

The sea off our coasts teems with microscopic life that breaks down the carbon dioxide we pump into the air. Now a series of expeditions aims to find out more.

Colony of salps floating under the RRS Discovery during DY018. Underwater photography courtesy of Claire Ostle (University of East Anglia).