Springtime phytoplankton blooms in the Celtic Sea

Louis Byrne, British Oceanographic Data Centre, NOC

The seasonal changes in the Celtic Sea primarily revolve around the development of water column stratification in spring and when it breaks down in late summer to early autumn.  Right now in March, the Celtic Sea is fully mixed, however with the days getting longer and warmer (we hope), the surface of the Celtic Sea is also warming. As the surface warms its density decreases and the water becomes lighter compared to the colder waters below which don’t have access to the suns heat. (Fig 1.) To help watch for these changes we have a daily set of sea surface chlorophyll and temperature satellite images sent from the NEODAAS team at PML to the ship, and any developments of blooms and changes to the temperature can be seen as they occur.

Fig. 1: Temperature profiles in the mid latitudes in the ocean. Dashed (- - - -) line is for the winter and the continuous line for the summer season.Source: https://nptel.ac.in/

This will eventually result in the creation of two distinct bodies of water, with a warm surface layer resting above a colder layer below, much like a cocktail which often have two or three coloured layers sitting on top of one another.

As well as causing the onset in stratification, the increase in temperature and sunlight also causes a truly massive increase in the number of phytoplankton in an event known as a plankton bloom [many plankton blooms are so large they can be seen from space! (see Fig.2)]. This results in a feeding frenzy as zooplankton (Fig. 3) numbers surge and they are in turn eaten by other organisms, passing the energy down the food web.

Fig. 2: Plankton Bloom in the Celtic Sea. Captured by the Envisat's Medium Resolution Imaging Spectrometer (MERIS) on 23 May 2010. Credits ESA

The phytoplankton bloom starts just before the onset of stratification, and then continues in the surface layer as the water there is warmer and receives much more sunlight. Eventually the phytoplankton will use all of the nutrients available in the surface layer and most of the plankton will die off. When this happens their cells will fall through the water column, causing a large increase in the biological material available on the seabed.

"Copepodkils". Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Copepodkils.jpg#/media/File:Copepodkils.jpg

 

When stratification breaks down at the end of summer, the water column in the Celtic Sea is again fully mixed. The bottom layer of water is still nutrient rich and these nutrients are also mixed into the surface of the water column, and become available for photosynthesis. This causes a smaller phytoplankton bloom at the end of summer before the days darken, and the cycle is complete.

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

What is happening in the benthos?

Louis Byrne, British Oceanographic Data Centre, NOC

The seas picked up again on Saturday and unfortunately again a few members of the science crew have been feeling a bit green, however the strong winds left us on Sunday and we had our first days’ proper sunshine of the cruise, complete with the obligatory sunset photograph, but not the fabled green flash!

The focus on this cruise is on processes that are happening in the benthos – meaning the environment above and within the seabed – and how these processes change as the seabed moves from sandy sediment to muddy sediment. To do this we are investigating four sites which are characterised as sand, sandy mud, muddy sand and mud. We managed to complete the NIOZ coring of the sandy site (Site G) over Sunday night, giving our sedimentologists some sandy samples to analyse along with the muddy sediment collected from site A.

A sample of sandy sediment ready to be sliced

Natalie Hicks (SAMS) is using the samples collected by the NIOZ corer to investigate the dynamics of benthic carbon cycling, including how deep and for how long carbon is stored in the different types of marine sediments, and how much is released back into the water column. She is collecting sediment samples from the seabed to a depth of 25 cm and then slicing it into cross sections, with each slice containing sediment from a different depth. These will then be stored in a freezer until the end of the cruise, when they are taken back to the laboratories in the Scottish Association for Marine Science (SAMS) to be analysed.
 

Natalie slicing her core into cross sections

Back at SAMS, the sediments will be analysed for their porosity, which refers to how much space there is for water between the grains in the sediment. Muddy sediment has smaller grains which fit together more tightly than sandy sediment, leaving less space for water between them. This makes it easier for pockets of water deeper in the sediment to be cut off from the sea water above.

Once this happens the water in the pocket will quickly run out of oxygen (there will be more about this later in the blog), making it impossible for aerobic bacteria (they are the ones requiring oxygen for respiration) to consume the organic matter in the water. This organic matter will then be stored in the sediments, unless it is resuspended through physical water movement or animal activity.
 

A NIOZ core about to be dropped into the Celtic Sea

Apart from the porosity and grain size, the carbon (both inorganic and organic) is measured as well as the amount of the lead isotope, ²¹⁰Pb. The carbon is measured so that we can have an idea of how much biological material is buried within the different sediment types.  ²¹⁰Pb is measured as it can be used as an indicator for how often the sediment at each depth is being resuspended or accumulated. Putting all of these measurements together, can give you a better understanding of benthic carbon cycling, and how this differs between the different sediment types

All of this is important so that we can determine whether each sediment type is a source or a sink of carbon. If we can understand better how deep carbon needs to be buried in the different sediments before it is sequestered (stored permanently in the seabed sediments) and how long it will stay the sediments for, then we can know how much carbon they will absorb over time. This will help us predict how much atmospheric CO₂ may be buried in our marine sediments over a certain timescale.

The rains and rough weather returned on Sunday, however I think most of us have our sea legs now!
 

Sunset on Saturday