Day 14 of Life Beneath the Keel

On Sunday 13 September, at 1800 BST, The Island Trust is live streamingLife Beneath the Keel’ on and YouTube from Plymouth Yacht Haven as part of the Ocean Institute’s virtual Maritime Festival. This is the fourteenth and final blog in the series about the marine life we hope to discover then.

Day 14:  13 September 2020 | Plankton

I’m finishing with plankton, or “Ocean Drifters” as Richard Kirby’s book of the same name describes them, because they drift at the mercy of currents and tides. Dr Kirby has also made some short videos about plankton narrated by Sir David Attenborough which you MUST see at PlanktonPundit.

OK, I know that “plankton” isn’t a valid taxonomic group, but it does include examples of all the phyla we have already looked at. And as we’ll be catching and looking at planktonic organisms with the same methods, and together they are hugely important for their impacts on the climate and the marine food web, I’m going ahead anyway.

  1. Plankton range from viruses (smaller than anything we can see with the sort of kit we’ll be using) to jellyfish bigger than two grown men such as the barrel jellyfish. We will collect some of the smaller plankton in plankton nets, to look at under the microscope.
    Phytoplankton are like plants, they photosynthesise. They may be small but there are so many they produce about half of earth’s oxygen. Most of it stays in the ocean where it used by the creatures there, from sponges to fishes.
  2. Among the Phytoplankton is Noctiluca scintillan, a dinoflagellate that exhibits bioluminescence – it glows in the dark when it’s disturbed. Its scientific name is Latin (not all are) and means sparkling nightlight.
    Zooplankton are animals. They eat phytoplankton and other zooplankton, are eaten by other zooplankton, and other animals. Some live their whole lives in plankton, others just early life stages.
  3. Arrow worms live their whole life in the plankton as voracious predators of anything, including other arrow worms.
  4. The larval form of sea mat settles out of the plankton and clones a whole colony of individual zooids, which I mentioned on the Bryozoans blog. We will meet them growing on weed growing on the pontoon.
  5. Barnacles shed the exoskeletons of their feeding appendages when they grow, which also drift about.
  6. There is dead stuff like this, and as all things must die, there’s an awful lot of dead stuff. Some of it starts small (dead plankton) and some of it starts big and breaks into smaller pieces. And as all living things must get rid of their waste, there’s a lot of poo too. As I said in earlier blogs, all this plankton, dead stuff and poo is food for many creatures. Unfortunately, so is much human rubbish, like plastic, which also breaks down into smaller particles. Those animals who have evolved to live on the plankton, dead stuff and poo can’t tell plastic is something different and is bad for them.
  7. We can collect some of the smaller plankton to look at using plankton nets. Plankton nets are normally towed behind a boat, but on the pontoon, if the tide is running you can let the water run past your net, or you can throw it out and pull it in. If the tide is not running or you want to avoid getting a net full of phytoplankton when it’s blooming, you can do a vertical trawl. Weight the net, drop it down then pull it up, and repeat a dozen or so times, or until you get fed up. Don’t forget to tie the line off before putting the net in the water.  This can be quite fun in a large traditionally rigged boat, which is reluctant to stop dead in the water. You have to drop near the bows, with the line led aft, wait until the boat has passed over the net and pull it up. Then pass it from hand to hand round all the rigging back to the bows for dropping again. Changing round gives everyone a chance to do everything.  The picture in image 7 shows “the drop” in Pegasus, our gaff cutter.

Now that you know just how diverse the marine life is around our marinas, and are perhaps inspired to find some of it yourself, you might want to get help with identification and to record your findings. There is a Wakelet’s Collection of Citizen Science projects for sail trainers. It is fairly UK based, but Secchi Disk and iNaturalist are world-wide.

John Hepburn, Ocean Discoverability Project Manager
13 September 2020