Life between the tides

What could be more reminiscent of a happy childhood than paddling about in rockpools with a net to catch the shrimps and fish that are living there. We are so lucky to have the Chit Rocks exposed every time the tide goes out, but living on a rock shelf can be challenging.

Things that live in the sea have a reasonably constant environment, but those that live on the seashore between low and high tide have to contend with big changes twice a day. When the tide goes out, any plant or animal on Chit Rocks is exposed to merciless drying especially on a sunny day. When the tide comes in, they can be pounded by waves powerful enough to rip the sea wall apart as happened in 1924.

The only plants that live on the rocks and in the rock pools are seaweeds, primitive plants that have not developed a vascular system or flowers. They are adapted to life between the tides with strong holdfasts to grip onto the rocks, flexibility to ride out the pounding of the waves, and coatings of mucilage to survive short periods of drying.

Like most plants, seaweeds use light to produce food by photosynthesis but, the deeper you go in the sea, the less light there is, particularly the red light that is the main power source for chlorophyll. This means bright green seaweeds can only survive in shallow water. Two that can be found on Chit Rocks are the stringy Gut Weed and the filmy sheets of Sea lettuce. Like many seaweeds, both are edible, very nutritious and delicious if prepared properly.

Seaweeds in deeper water use other pigments called carotenoids which can photosynthesise using light from the more powerful blue end of the spectrum. These are red pigments and there are some bright red seaweeds further down the beach. Many seaweeds have a mix of chlorophyll and carotenoids making a range of browns. The main brown seaweed on the area of Chit Rocks where you can walk at low tide is the Serrated Wrack which has strap-like fronds with a serrated edge like a bread knife.

Around the lowest tide mark you find larger seaweeds, the Kelps. There are the crinkly straps of Sugar Kelp which is, as the name suggests, quite sweet. There are the large fronds of Oarweed like huge fingered hands on the end of a long stalk called a stipe. You will also find a mix of native Forest Kelps, or you used to.

Forest Kelps are our largest seaweeds with fronds up to 3m long. They crowd together and buoyant gas pockets in the fronds raise them up to form an underwater forest when the tide is in. Our Kelp forests are among the most productive and diverse habitats on Earth, home to a myriad of other sea life. They also do a wonderful job of absorbing the energy of large waves and help defend our coast from the worst storms.

Our native Kelps are very sensitive to changes in sea temperature. The average temperature of our local sea area has risen by one degree in the last forty years. This doesn’t sound much, but it is enough to drive out the native Kelps and they are migrating northwards. In their place, Golden Kelp has moved in from the Mediterranean and the Bay of Biscay.

The problem with this is that the other sea life does not recognise Golden Kelp as a natural home. Our Forest Kelp is shaggy with epiphytes and alive with a wide range of animals. Golden Kelp is smooth with no epiphytes, and there is a much smaller animal population among the fingered holdfasts. Eventually, nature will catch up and other species from further south will move in, but our new Golden Kelp forests are far poorer places than the ones they are replacing.