Plankton and Seaweed - Dr Charlotte Braungardt


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Plankton and Seaweed - Dr Charlotte Braungardt

Half of the oxygen that sustains life on Earth is produced in the ocean

That’s just one reason why exploring the base of the marine food web is important. Another is that it’s fascinating!

We collected some seaweeds on the coastline and brought it back to the ship for identification and finding out what’s living on it. Seaweeds are large algae, or macroalgae, which are an analogue for the plants on earth: they take up carbon dioxide and use the energy of sunlight to synthesise sugars in photosynthesis, which they then use to build their cells. Some seaweeds are sensitive to changes in temperature and the alkalinity of the water, and can be used as indicator species for changes to ecosystems that are related to the effects of increasing levels of greenhouse gases, such as carbon dioxide, in the atmosphere.

Our medic Dr Sophie Redlin was keen to learn with the trainees and volunteered to lead the session using a seaweed identification guide, hand lenses and trays to categorise brown, red and green seaweeds. With the excellent seaweed guide from the Field Studies Council, everyone soon got involved in distinguishing twisted from egg wrack, sea lettuce from Irish moss, and identify sugar kelp, ore weed and different coral weeds. One thing that stood out was how seaweeds live on other seaweeds, just as epiphytes on land live on trees and shrubs.

Meanwhile, I set up the microscope to examine the animals living on the seaweeds. A bladder wrack picked off the harbour wall in Dublin featured a cluster of eggs that turned out to enclose minute sea snails, their little shell whorls and eye spots clearly visible as they wriggled as if ready to hatch.

Bryozoa colonies settled on twisted wrack picked of Killiney beach. Bryozoans are small aquatic animals that live their lives as colonies that are settled on rocks, kelp and other solid substrates or in the sediment. They’ve been around on Earth for millions of years and each ‘individual’ lives in its own little box-shaped ‘sleeping bag’ (that’s my own very non-technical description) and looks like the polyp of a coral. The proper terminology is of course different: individual animals are lophophores that live in their autozooids, from which they extend their tentacles into the surrounding water. They feed by channelling microscopic algae and zooplankton and other suspended foods into their digestive tract. We were mesmerised by the intricacy of structure in the colony of autozooids and the tentacles moving and feeding.

Working at even smaller scales, we looked for plankton in the Liffey. With a plankton net we filtered particles and plankton smaller than 100 micrometer from the water. Under the microscope, we saw a surprisingly wide variety of green algae, mainly centric diatoms and diatom chains, which are single-cellular organisms that photosynthesise and produce much of the oxygen consumed by animals and bacteria in the entire ocean. Among the microalgae, sediment particles, colloidal organic matter and plastic fibres, we found a few barnacle larvae and copepods. Other zooplankton, including some tintinnids, were largely fragmented and had probably been washed into the Liffy on the tide. We are looking forward to inspect more diverse samples elsewhere.

To put all we saw into context, a general discussion about the marine food web rounded off our exploration of seaweed and plankton.

Featured Image: photograph of the Field Studies Council guide to common seaweeds.

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