Winter beneath the waves

21 December 2022  /  By Chris Taylor

Spare a thought for seagrass on the shortest day of the year.

We can see how plants on land react to colder temperatures and shorter days. But have you ever wondered what goes on when it’s winter beneath the waves? Marine specialist Chris takes a deep dive into how seagrasses survive the stresses of winter…

In winter, days are much shorter…

The sun sits lower in the sky, where its rays penetrate the sea surface at a more acute angle and therefore reach lesser depths. Storms become stronger and seas more turbulent, uplifting more sediment and reducing how much light reaches the seabed. This has a significant impact on the ‘photic zone’, the zone with adequate light for photosynthesis.

To prepare for winter, temperate seagrasses (Zostera sp.) build up stores of excess carbohydrates in their roots, a little like how a bear stores fat reserves for winter hibernation. Seagrasses become dependent on their carbohydrate reserves to survive the cold months of winter. Growth is kept to a minimum, as they must retain enough carbon to give them a boost of growth in early spring.
Sunbeams shining on seagrass

Credit: Fiona Crouch

Seagrasses have wonderful adaptable abilities. They have evolved to live in near-shore and estuarine environments for over 100 million years. Many species can adapt to different depths by growing longer or wider leaves to capture more light. This tends to happen in larger, tropical seagrasses (Posidonia sp. and Thalassia sp.). In winter, some species will lose their leaves to reduce the burden on their carbohydrate stores. Zostera marina has evolved to withstand extreme cold and limited light conditions and can even be found in icy Canadian waters.
So, spare a thought for seagrass as it hunkers down for winter. Like many of us land-dwellers, it’s getting ready to emerge again in spring.