Collecting images from the seafloor.

By Henry Ruhl

We have completed sediment core sampling at our four main study sites. This is a key achievement for the trip. We are now sampling one other site that allows us to cross-reference our findings with those of other studies and a long-term study station with the nickname Candyfloss. This site is closer to the continental shelf edge and open ocean than we have been for most of the trip. Marine life spotting has been good and we even laid eyes on the RRS James Cook about ten miles from us. The James Cook is researching life in the canyons that extend just beyond the shelf edge. We will soon return northward to for the more AUV deployments, the fourth deployment of the NOC lander, as well as a few other remaining tasks.

We are using the Autsub3 autonomous underwater vehicle (AUV) during our cruise to collect photographic images of the seafloor, as well as sonar-based images of the shape and texture of the seafloor. We have been running the AUV in a ‘mowing the lawn’ pattern of parallel lines that are about ~5km long. After checking our seafloor shape and texture mapping for any obstacles, we ‘fly’ the AUV as close as 2.5 meters from the seafloor to collect colour photographs in the moderately cloudy waters of the Celtic Sea.

The images will be geo-referenced, which effectively turns the photo into a map like you see in Google Earth. As you can see below, that 2.5 m height above the seafloor still gives us images that are very useful for determining the identity size, and location of all the observed images. This can provide a landscape scale view of the seafloor and its inhabitants, which we can then use to improve estimates of ecological and biogeochemical patterns and processes.

The seafloor and its inhabitants

 

AUV photographs are particularly useful in estimating the distribution of biomass of larger animals that are not sampled well by trawls or sediment cores. Observed animals include crabs, shrimps, and anemones as well as fish. The images above and below come from a Celtic Sea site where the seafloor is dominated by sandy mud.

The seafloor and its inhabitants

Snow catching across the Celtic Sea

Alex Poulton, National Oceanography Centre

Picture 1. Snow Catcher going over the side of the ship. Photo: Jose Lozano.

Of particular interest during this cruise is the fate of the material that is produced in the upper part of the water column - this material sinks down through the water column as large particles called marine snow. Marine snow is formed in many different ways. Some is formed from phytoplankton sticking together to form large aggregates when growth conditions are not optimal in the surface ocean, for example when nutrients are limiting growth. Others are produced by zooplankton eating phytoplankton and then producing faecal pellets. These marine snow particles can sink through the water column at various speeds, with their sinking speeds linked to their composition and size. As they sink they act as a food source for zooplankton and other organisms that live in the lower depths of the water column.

Picture 2. Snow Catcher being deployed to 70 m. Photo: Jose Lozano.

Collecting marine snow is a challenging business. During this cruise we are using Marine Snow Catchers - large volume (100 L) water bottles which we send down to the depth of interest and then close, enclosing the sinking particles which we then bring back up onto the ship and allow to settle for an hour or two (pictures 1-4). After this settling period we can then remove the water from the Snow Catchers and examine the particles in the bottom of the Snow Catcher. 

Picture 3. Snow Catchers taking a rest. Photo: Jose Lozano.

These Snow Catchers have been used on multiple cruises from the Arctic to the Caribbean individually, but unique to the Celtic Sea is the deployment of not one or two, but four Snow Catchers twice - once in the upper 10 m and then again at 70 m. This is quite some operation, taking a large amount of organisation, (patience), timing and around five hours. Over the entire length of the cruise we will carry out this large-scale water collection and snow catching exercise at five different sites, including our Central Celtic Sea site (Candyfloss). Our hope is that as well as seeing changes in the surface community we will also see changes in the composition of the material leaving the upper sun lit ocean and sinking down to the seafloor.    

Picture 4. Team Snow Catcher celebrating success. Photo: Callum Whyte.

Exploring the shelf seas – hunting the spring bloom

Alex Poulton, National Oceanography Centre

After almost a week since arriving and saying farewell to the benthic (sea floor) scientists from DY021, the RRS Discovery sailed out of Southampton just after lunchtime on the 1^st April on the second of this year’s Shelf Sea Biogeochemistry cruises. This cruise has a slightly different flavour to the last one – our focus is on the organisms living in the upper water column (pelagic), not in or around the bottom sediments. We hope to sample the plankton, tiny marine organisms that live in the water column, during one of the key periods in the seasonal diary of life in the ocean: the spring bloom. During spring, as temperatures get warmer and days get longer, phytoplankton, the tiny plants that form the base of the marine food chain have a growth spurt. This rapid increase in biomass provides a ready meal for the myriad of grazers present, and in this way the spring bloom fuels the food chain up to fish and beyond. To fuel this rapid growth, nutrients are required and the spring bloom rapidly diminishes the nutrient levels that have been present through the cold and dark winter. 

Picture 1 : The position of the Candyfloss site is shown on top of a satellite image (courtesy of NEODAAS) of chlorophyll (a pigment used for photosynthesis by marine plants, or phytoplankton) from late March. Colour changes from deep purple to green and yellow are indicative of increasing biomass of phytoplankton. Eventually patches of red will appear indicating that the spring bloom is well underway. 

Across the four weeks of this cruise we will travel to various sites within the Celtic Sea in order to build up a time-series of observations of the spring bloom as it happens, in terms of how it changes the water chemistry, how its biological components (bacteria, plants and animals) interact with one another, and how the physical environment of a shelf sea influences its formation and structure. To do all this work takes a huge team of scientists and technicians, and a top of the range research ship manned by skilled and experienced crew. Onboard we have 30 scientists and technicians, from nine different research institutes and universities across the UK. 

Picture 2 : One of the key sampling instruments for the cruise – a CTD (Conductivity-Temperature-Depth) probe with large grey bottles attached for collecting sea water returning to the surface with the first of our samples. (Picture by Jose Lozano).

ince sailing from Southampton on the 1^st April we have been making our way out to one of our key sites for the next few weeks: ‘CANDYFLOSS’ or the ‘Central Celtic Sea’ (Picture 1). This evening we briefly stopped to test some of the sampling equipment we use to collect water – a CTD (oceanographic instrument used to determine the conductivity, temperature, and depth of the ocean, see Picture 2 below). Our ETA is around lunchtime tomorrow, after a short stop at first light to pick up one of the gliders that has been monitoring conditions out here for the last few weeks. When we arrive at CANDYFLOSS, work will begin in earnest as we recover the moored instruments which we left here last November, put new batteries in them, download the data they have recorded, and plonk them back in again. As always on a ship, tomorrow is going to be a long day.

Shrinking Styrofoam Cups

Louis Byrne, British Oceanographic Data Centre, NOC

We’ve done it!  A day of sampling at CANDYFLOSS was followed by a quick jaunt to the shelf edge to pick up some gilders and do a few CTDs and the data collection phase of the cruise is officially over. The final CTD of the day was performed off the shelf edge to a depth of 1000m, and a few of the scientists decorated polystyrene cups and attached them to the CTD rosette. This is a bit of a tradition on oceanography cruises, as the pressure at 1000m causes the cups to shrink in size, producing quaint miniature decorations like the one pictured.

Polystyrene cups post CTD. Designs by Dicky Deal.

We’re now steaming from the shelf edge back to Southampton, a journey which will take approximately 36 hours. The work doesn’t finish then however, as the data that has been collected during this cruise will need to be processed, analysed and then written up into scientific papers. These papers will then be peer reviewed and published in scientific journals, contributing to the advancement of our understanding of key biogeochemical processes in the shelf edge.

The data meanwhile will be sent to the British Oceanographic Data Centre (BODC) where they will be archived along with all of their associated metadata. In doing so the final datasets produced by this project will be preserved for future use, and will eventually be made available to the public free of charge (access to the data will be restricted for a few years to give the researchers responsible for collecting the data the chance to write their papers before the data are made publicly available).