Sunny Scotland brings on plankton blooms - MOWI - Scotland

Sunny Scotland brings on plankton blooms

June 17, 2023

Much of Scotland has seen record temperatures and sunny days for the past fortnight, which has also brought about visible plankton blooms in lochs around the country. Loch Ailort, in particular, has been turned bright green and has passersby asking “What is that?”

“These nice warm and sunny days are wonderful for people, but they can be tough on fish and other sea life,” says Ronnie Dyer, farm manager at Mowi’s Ardnish trials farm. “The water throughout Loch Ailort has been bright green these last few weeks because of a plankton known as Coccolithophores, and fortunately it appears to be relatively harmless to our fish. At the peak, we were getting 16,000 cells/ml near the surface, which is very intense and severely limits the fish’s ability to see underwater and to feed properly.”

Loch Ailort June 2023

Deborah Brennan, Fish Health Laboratory Manager, is Mowi’s resident expert on plankton: “Phytoplankton can be very localised, and everything is dependent on the environmental conditions including tide, water current, temperature, sunlight, PH, salinity and nutrient availability. Any fluctuations in these variables can cause what we term an ‘algal bloom’ which is essentially a rapid increase in algae. We do experience more algal blooms in the summer as sunlight and temperature increase.

“We are able to do both qualitative and quantitative analysis of plankton from water samples. For qualitative, the farmers pull a plankton net through the water column and take a water sample. This water sample is highly concentrated and you see a lot of different species of plankton in that.

“Then for qualitative analysis, water samples are taken at different depths – 10m, 5m and 1m. This enables us to count how many algae phytoplankton are present per millilitre of water and what depth they are present. Understanding the depth is extremely important. If it is at 10m, so low down in the pens, then the fish can continue to feed above that level. But if the algae is sitting at 5m it can be risky to feed the fish.”

Ronnie adds: “If a harmful plankton species is identified and at critically high levels, the first step is to stop feeding the fish so they remain deep in the water column and below the highest levels of plankton near the surface. The next step is to activate our aeration system. The air bubbles can help disperse the plankton blooms which then move away with the tides.”

What are Coccolithophores?

Coccolithophores are a single-celled organism that produce calcium carbonate plates called coccoliths that cover the cell in a structure called coccosphere – the reflective properties of these plates is why water colour is changed to bright turquoise. They are quite unique in the plankton world and have been an integral part of marine plankton communities since the Jurassic period and today they contribute ∼1–10% to the primary production in the ocean’s surface. They maintain an important and primary role in the global carbon cycle.

Visit our national website: