Thursday, 10 January 2013

Carpe solem

The weather deteriorated yesterday, becoming more typical of this time of year. The wind increased to 40 knots and the increasing swell meant that the camera sledge was moving up and down in the water column as it was deployed. It was not possible to settle the sledge on the bottom whilst getting a clear picture, so we were forced to abandon camera work until the weather improved and focus on sediment sampling.

To give you a picture of what we have been up to, here is a quick run through of what is involved in sediment sampling.

The ship’s captain, or officer in charge of navigating the ship at the time, tells us  when we are at the correct location selected for sampling. We can track the ships course using GPS and view it on a screen so we can see where the ship is in relation to proposed sample locations.

Navigation screen
Three ABs (able-bodied seamen) then assist by winching the Day grab over the side of the vessel to the seabed. It is possible to tell when the grab reaches the bottom, when the winch wire goes slack. The ABs then call through to the survey lab and a scientist takes a fix of the coordinates where the sample was taken so it can be later plotted on maps. Before deployment, the Day grab is set up with the jaws held open by a bar fitted between hooks on the jaws. When the grab hits the seabed pressure is placed on two plates at the base which push up the bar and release the jaws. As the grab is winched back up the grab wires are pulled taut, closing the jaws of the grab so a sample is retained inside.

Grab sampler set for deployment

When the grab is back on deck, the scientists check the depth of the sample which must be at least 8cm to ensure a sufficient proportion of the infaunal community is sampled. Infauna generally live within the top 5cm of the seabed. If the depth is too shallow, or the jaws of the grab have been held open by shells or stones, the grab will be deployed again. If a good sample is retained, scientists take a small core for particle size analysis which gives information on the size of sediment present and a description of the sediment type. For example, sandy mud or gravely sand, is noted using a standard classification system. A photograph of the sample is also taken as a record.

Depth measure and sediment corer

The sample is then dropped into a crate and taken for processing for macrofaunal analysis. The sample is first gently washed through a large 5mm sieve to remove the sediment and large organisms such as sea potatoes, crabs and brittlestars. The sediment and smaller fauna are washed through to a smaller sieve below where fauna larger than 1mm in size are retained. All fauna are then washed into a sample bottle using a funnel and water bottle and then fixed in formalin. Care is taken to ensure no fauna are left behind, and any small organisms caught in the sieve are painstakingly removed using tweezers.

Scientist Dr Paul Whomersley washing sampled fauna from sieve to sample bottle

Some interesting fauna retained in the sieve last shift including several starfish which were tentatively identified as Astropecten irregularis. We even found a mutant individual with only four legs which we took to be good luck.

Retained macrofauna including a sea potato and a starfish, Astropecten irregularis
Grab sample with seapen Pennatula phosphorea

The Scientist in Charge set scientists an imaginative challenge to keep them entertained during down-time. Each shift was charged with creating their own coats of arms and moto. A considerable amount of artistic flair and imagination went into both entries, and the day shift was unanimously victorious with the highly detailed design below (left).

Day shift's coat of arms
Night shift's coat of arms

 The latin motto ‘Carpe Solum’ can be translated (with some artistic license) as ‘grab the earth/bottom’. The seagull and crab are reference to an amusing incident on day shift when we caught a small crab in the grab. Just as we were taking a photograph for records a sharp-eyed gull swept down intent on an easy meal almost taking out an unsuspecting scientist in the process.

Crab found in grab sample
Hungry seagull making a quick getaway