UKDIVING
Clear as mud
Variation of concentration of inorganic particles around the UK for May 2025
It’ s one thing to map visibility, but it’ s quite another to explain the drivers behind the resulting variations. It doesn’ t take a seasoned marine scientist to suggest that suspended particles may be responsible, but does the data back that up? The map below shows how the sedimentary concentrations vary across the UK. A glance back to the visibility map will reveal an unmistakable correlation- not only validating our hypothesis but also unmasking a primary culprit for their source of the murk: river estuaries.
Britain’ s lowlands are dominated by soft, sedimentary materials; echoes of a time when the interior itself was a prehistoric seabed. As they meander for endless miles across the landscape, rivers in these regions gather vast quantities of particulate matter, eventually dumping them into the sea where they settle to form a thick, silty seabed. As any diver who has inadvertently‘ kicked up the crud’ will agree, disturbing the silt destroys visibility instantly, and because the particles are so fine it can take a prolonged spell of calm weather for them to settle.
Because of this constant replenishment of sediments, the seas here tend to be shallower. Add to this the natural funnelling of the Severn or the squeeze of tidal flows through the English channel and you have a recipe for huge daily churning of the deposited sediment.
Conversely, rivers in the south west of England and throughout Scotland traverse much harder, rocky substrates and therefore suffer much less from these sedimentary conveyor belts. While not immune to poorer visibility in bad conditions, they clear much more rapidly.
This is why these regions are known for having the best conditions when it comes to visibility: they lack the fine, muddy river outflows that dominate other stretches of our coastline.
Mussels and barnacles growing on the pilings of Eastboard Pier, note the streak of suspended inorganic particles
Prolific plankton
While a silty seabed can be stirred up at any moment by strong currents or a passing storm, there is another visibility-destroying phenomenon that is strictly seasonal: the dreaded‘ May Bloom’. The next map shows the distribution of phytoplankton during this peak month.
Phytoplankton are single-celled algae that sit at the base of the marine food chain, forming a vital food source for microscopic zooplankton, which are in turn
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Post-storm murk on Ringstead Reef as a snorkeller tries to make the best of the conditions