Scum and dead sea life on the Firth of Forth coastline

date26 May 2017

Updated 31 May 2017

There have been a number of reports this week from both east and west coasts of slicks or patches of scum, including along the Fife coastline and areas around Edinburgh. SEPA considers that these are likely to have formed by algae that grow naturally in the sea around Scotland.

Due to the particularly calm and sunny weather that we have now been experiencing for a prolonged spell, we are witnessing more algae than normal. As these algae die off, these scums form along the tide line. While these scums may be unpleasant to look at and may smell bad, they are not a health hazard to swimmers or dogs.

There have also been report of dead sea life, such as sandeels and sea mice.

Why has this happened?

In May there was a large bloom in the Firth of Forth of a species of algae called Phaeocystis. This is a single celled microscopic species that lives in ball like colonies. They are golden-brown in colour which leads to discolouration of water when they’re in high abundance. When these blooms run out of nutrients they need to grow, the cells die and the colonies break apart. The remnants of these colonies trap air and dead organic matter leading to patches of scum or foam along the tide line.

In the Firth of Forth the action of the wind against the tide normally serves to mix water and disperse these scums. However, because there has been a long spell of calm sunny weather, this hasn’t happened to the extent it usually does. That means quite a lot of organic matter has sunk and begun to be broken down by bacteria in a relatively small area.

When organic matter settles to the seabed it smothers the vicinity and bacteria breaking it down use up dissolved oxygen in sedimentsand the overlying water. Mobile species like fish will vacate the area, while animals that live in sediments will be suffocated.

In this case so far there have been reports of sea mice and sandeels washing up dead, which will have been caused by this bloom breaking down.

There is often a lag between the decline of an algal bloom in these conditions and the emergence of dead animals along the shore, because they tend to die in their burrows and are only dislodged when there’s some rougher weather to stir up sediments.

Other groups of animals may wash up dead as well. Phaeocystis blooms often happen at the same time as jellyfish swarms. One of these jellyfish species, the moon jellyfish Aurelia aurita has a shrimp like animal that lives inside it as a parasite, called Hyperia. If oxygen levels in the water are depleted by the breakdown of the bloom, both the jellyfish and their parasites can be killed. Dead Hyperia have previously washed up along the Scottish coast in their thousands in similar conditions to those experienced this month, so this might happen again.

These events are natural and not caused by pollution, however they tend to only happen when there are specific sets of conditions that only come along every few years, so they can be alarming to members of the public who often are witnessing this for the first time and are understandably worried by what they are seeing.

Any other unusual reports?

SEPA officers have also investigated reports of sewage discharged on Portobello Beach, but having investigated the issue and visited the area they found that rougher seas resulted in dead sea squirts washing up on Portobello Beach. Although the marine animals can look like sewage when floating in the water or washed up on the beach, they are harmless.

What do I do if I'm not sure?

If you’re not sure what you’re seeing, or are at all concerned, please do contact our 24 hour pollution line on 0800 70 60 50 so we can investigate.