Thursday 4th Jan 2018

The captain says we are going to run up to the south coast and shelter in the lee of the UK mainland, rather than go down the middle of the Channel as we usually would. He also said he is booking an early parking slot at Southampton, in case we get home early, so we can, as he put it, ‘Get a good night’s sleep’ before we disembark. Not sure how I feel about that, to be honest. Firstly, I sleep better when it’s moving, and, secondly, if I wanted to sleep in Southampton, I’d book a hotel. Meh, we’ll see what happens.

At least this gives me time for the sea colour update.

Water scientists have a scale for sea colour description, named after Francois Alphonse Forel and Willi Ule (Swiss and German respectively) and is, perhaps a little unfortunately, called the Florel-Ule Scale or FU for short. No, really. Google it if you don’t believe me.

The colour of the water is assessed by lowering a white disc called a Secchi disc into the water until it disappears completely, noting the depth (which is, perhaps unsurprisingly, the Secchi depth)(Secchi discs are all white for marine water and black and white quadrants for freshwater and were originally designed to measure water transparency), and then raising it to precisely half that depth. The colour seen is then compared to a standardised set of liquids of different colours and the closest number assigned. Apparently, we humans are remarkably consistent with our appreciation of water colours and it is all very regular and orderly – as one might expect from a Swiss/German invention!

Some natural phenomena can change water colour but it does not necessarily mean that the water is of bad quality. The different colour numbers correspond mainly to these types of water bodies:

  • Indigo blue to greenish blue with high light penetration (1-5 FU scale). These waters have often low nutrient levels and low production of biomass. The colour is dominated by microscopic algae (phytoplankton).
  • Greenish blue to bluish green (6-9 FU scale). The colour is still dominated by algae, but also increased dissolved matter and some sediment may be present. Typical for areas towards the open sea.
  • Greenish (10-13 FU scale). Often coastal waters which usually display increased nutrient and phytoplankton levels, but also contain minerals and dissolved organic material.
  • Greenish brown to brownish green (14-17 FU scale). Usually with high nutrient and phytoplankton concentrations, but also increased sediment and dissolved organic matter. Typical for near-shore areas and tidal flats.
  • Brownish green to cola brown( 18-21 FU scale). Waters with an extremely high concentration of humic acids, which are typical for rivers and estuaries.

So now you know.

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