Shelf Sea Biogeochemistry blog

Showing posts with label nutrients. Show all posts
Showing posts with label nutrients. Show all posts

Tuesday 18 August 2015

36 years of working on Discovery

By Peter Statham
Ocean and Earth Science, University of Southampton

When I first set foot on the old Royal Research Ship Discovery in 1979 in Cape Town I had little idea that in 2015 I would be on the Discovery once again but now on the most recent version of the vessel to carry this famous name. 

I am interested in the chemistry of the ocean and how chemical processes affect the biology and other parts of the marine system. This aspect of oceanography is important in terms of understanding how the sea works and can be impacted by climate change. 

On this trip we are studying where the essential nutrient iron comes from on the shelf and how it may move away into the open ocean.  In some areas the element is at such low concentrations that it limits plant growth and thus impacts ecosystems, so it is important to know where it comes from, and one potentially important source are the edges of shelf seas. 

Launching a glider from Discovery. Gliders move up and down through the water by altering their density and “glide” on their wings from one location to another in the upper ocean, whilst collecting data that is sent by satellite to shore when it is at the surface. This new model has a small propeller to help it occasionally overcome strong currents.
Whilst frequently demanding with long working hours I always enjoy the times at sea with the wide range of people on board, the constant challenges to be dealt with and the buzz when a long planned experiment finally works out.  Whilst new techniques such as satellites and gliders are developing rapidly, ships are still essential tools in the study of the oceans. Discovery is a world-class research platform for UK marine science that will support our new generation of oceanographers into the future.

Wednesday 15 April 2015

Spring has sprung - here comes the bloom

Alex Poulton, National Oceanography Centre

After two weeks in the Celtic Sea we are seeing clear signs that the spring bloom has truly begun - nutrients are declining whilst levels of the pigment chlorophyll, used by phytoplankton for photosynthesis, are steadily rising. 

Just how green the water is at present (slightly cheating as this is a pigment extract rather than seawater). Photo: Chata Seguro.

The bloom appears to be patchy across the Celtic Sea; from the shelf edge where the bloom has not started to show strongly yet, to the central Celtic Sea (where our Candyfloss site is) where small phytoplankton are actively growing, to the northern Celtic Sea where we saw huge diatoms (images below) - a type of phytoplankton which often characterises blooms and productive waters - which were at least a hundred times larger than anything we have seen so far. 

Diatoms and zooplankton seen under the microscope. Photo: Chata Seguro.

A close up of one of the large diatoms we saw in the NE Celtic Sea. Photo: Chata Seguro. 

As the nutrient levels continue to decline we are keen to see what happens within the phytoplankton community: will there be a clear progression from large cells to smaller cells which needs less nutrients for growth, will the diatoms be succeeded by another phytoplankton group? How these changes are reflected in the rest of the ecosystem is a key question we will address over the next two weeks. For example, how will changes in which type of phytoplankton is present influence the different nutrients needed for their growth (nitrogen, phosphorus, silica), and will we see changes in the dominant types of zooplankton (tiny animals that eat the phytoplankton) across the Celtic Sea.

The ever present fog viewed from the bow of the RRS Discovery. Photo: Chata Seguro.

Though the bloom has arrived, we have lost the sun - a dense sea fog has descended on us over the last few days which means we can only see a hundred to two hundred metres in any direction (see image). The eerie silence that this has brought to the ship is broken up at regular intervals by the ear shattering sound of the ships horn announcing our presence. If the spring bloom didn’t know we were here before, you can be sure that it does now.

Monday 9 March 2015

Moving sediments at the bottom of the ocean

On Monday we got to see another sediment re-suspension experiment carried out by Charlie Thompson of the University of Southampton (UoS), and Sarah Reynolds of Portsmouth University. 

Charlie Thompson (UoS) has written the following about this experiment which was carried out using a piece of equipment called a benthic flume:
The seabed isn’t solid, but is instead made of mobile sediments (muds, sands and gravels), which are moved around by the action of waves and tides. Charlie and Sarah’s work looks at what happens to nutrients, carbon and the sediments themselves when they begin to be re-suspended and moved around by the tides and water currents.

Benthic flume sitting on the deck
 One way to do this is using a benthic flume (see photo above , which sits on the seabed and uses a motor driven paddle system to allow us to recreate strong currents onto the actual sediment at the bottom of the ocean, when and where we want them. By doing this, we are able to measure when a specific type of sediment (e.g. Sand or mud) starts to move – which in turn allows us to predict how often that sediment might be re-suspended by natural water flows over the seabed. We can also determine how the fluxes of nutrients and carbon into or out of the bed are changed compared to on a stationary bed from water samples taken at time intervals during the experiments.

Benthic flume being deployed over the side of the RRS Discovery