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The Seaweed Foragers Guides: Understanding the intertidal zone.

The Seaweed Foragers Guides: Understanding the intertidal zone.


If you are anything like us, you so desperately want to proclaim the end of Winter. But the cold dark soil is not releasing its grip and we wake each morning to frost and ice.

But while we wait, we plan and we plan some more. Because that's how we push through the hard days.

We propose you use this time to get to understand foreshore. That way, when the seaweed starts to come alive in the coming weeks…You will be poised and ready to take full advantage of the very best of the seaweed foraging season.

Welcome to the Foraging Club. A place not for experts, but enthusiasts. A space where we research, plan, and share as we learn.


It is February. And I know that the best time to forage for seaweed is between March and October. And, yet here I am. An early bird… down at the beach, scanning the rocks for the first signs of spring growth. I can’t help myself, I’m enthusiastic. It is never too early to start planning I tell my one-year-old companion. He's strapped onto me in his sling and he is fast asleep.

One of the real joys of wild food foraging is the intimate relationship between wild food and the cycle of the seasons. Each foraging season I am transported back in time 12 months, to where I was and what I was doing the year previously and the year before that again.


The tide is quite far out already and there is a large area of sandy beach

There are also areas of exposed rockpools, a small area of cliff face, and some sections of exposed rocky shore that are accessible on foot. 


“This time last year he was merely days old... we’ve got some catching up to do ”


On this day, in mid-February, we arrive at the beach about 1 hour before the low tide of the day. The tide is far out already and there are areas of sandy beaches, areas of exposed rockpools, a small area of cliff face, and some sections of exposed rocky shore that are accessible on foot. It is not long before I uncover some pepper dulse on a rocky outcrop, growing in its usual spot, where I find it every year. I nibble on a small piece, look down at the newly one-year-old baby in his sling, and think about all that has happened in the year that has passed.

As I chat to the baby I explain to him that this time last year he was merely days old. Last year we missed all of the best foraging days. "You know, we’ve got some catching up to do," I tell him as he snores gently. It feels important to me to be back here. I love this little beach and I can’t wait to share it with him...


Pepper dulse grows only on rocks and is thriving here in a north-facing rock crevice underneath a still small crop of bladderwrack in the middle intertidal zone.


“Good places to find seaweeds in the intertidal zone are on shaded (north-facing) rocks and in small crevices and runnels.”


Spiralled wrack, bladderwrack, and eggwrack grow in the upper areas of the shoreline. When we find these “wracks” growing abundantly it is an indicator of a sheltered area of shoreline.


In a more exposed area of rock face, on the same beach, we find much less seaweed cover.


One of the easiest things to do when you arrive at a beach is to get an understanding of where the high tide line is…


The High Tide and Low Tide Lines

The high tide line is the highest point on the shore that is covered by water at high tide. It is the level on a beach where the high tide reaches each time the tide comes in on a Spring Tide. You can usually see this clearly whenever you take a walk along any beach as you will find a line of debris. Tide lines used to be mainly fragments of dead seaweed, but sadly there will be a lot of human-created plastic rubbish and ropes as well. 

The low tide line on the other hand is only exposed momentarily for a few minutes at a particular time of the monthly tidal cycle. The low tide line is mostly submerged - it is only exposed at the point of low tide and for a longer period during extremely low tides.


“Anything living in the intertidal zone must be able to survive sometimes extreme changes in environment. Consider that the upper shore is immersed in water 20% of the time, while the lower shore is immersed in water 80% of the time. ”


As the tide continues to go out we walk across the wet sand to an area of seaweed-covered rocks. An area that an hour ago was underwater. We are standing in the intertidal zone.


Getting to know the intertidal zone.
The intertidal zone is the name given to the area that lies between the high tide and low tide lines. Also sometimes referred to as “the foreshore” this is the area above water level at low tide and underwater at high tide. The “upper shore’ of the intertidal zone is immersed in water 20% of the time, while the ‘lower shore’ is immersed in water 80% of the time. Sandy beaches, shingle strands, plateaus, rockpools, and cliff faces are all different examples of intertidal zones.

This intertidal zone is an extreme ecosystem or habitat. Anything living in the intertidal zone must be able to survive sometimes extreme changes in moisture, temperature, and salt levels as well as withstand strong waves. Consider that across a 12hour period the intertidal zone experiences a shift between two very different states: at low tide when the majority of the intertidal zone is exposed to the air and direct sunlight and the opposite state at high tide when most of the intertidal zone is submerged in seawater. Seaweeds are one of the most robust species of plants, animals, and algae that thrive in this harsh intertidal zone.

The size of the area within the intertidal zone is dependent on what is known as the tidal range. The tidal range is the difference in the height of low tide and high tide. Coastal tidal ranges vary globally and can differ anywhere from near zero to over 11 m (36 ft). Around the Irish coast, tidal ranges vary from around 1.75 metres on the southeast coast of Ireland to an average of 4.5 meters on the west coast. 


At low tide, you might notice stripes of colour across a cliff face or rocks.


This can be referred to as inter-tidal zonation. Intertidal zonation marks like those in the image above are a reminder of the changing sea levels across the tide and across the month.  


The zones within the Intertidal Zones 

The intertidal zone is then broken down further into 3 elevation gradients - the upper, middle and lower tidal zones. Below the lower tidal zone lies the subtidal zone. Above the upper zone lies the splash zone.

Different species of seaweeds have adapted to situating themselves in different parts of the intertidal zone. Channeled Wrack, for example, lives in the upper intertidal zone and so spends the majority of each day uncovered and exposed to the harsh winds. In response, it has specially adapted small channels to capture water on its surface.  At the opposite end of the tide lies the shallow sub-tidal kelp forests which have adapted to spending almost all of the day underwater. Kelp forests are sometimes referred to as “ecosystem engineers” because they create an important habitat that supports other marine life. The thick dense kelp forests provide protection, food, and hunting grounds for invertebrates, fish, and marine animals including seals, otters, and seabirds.


Above the upper shore lies the splash zone. This is the area on exposed shorelines where the splash of the wave extends beyond the intertidal zone. Note in the upper part of this image is the 'splash zone’ - here we can see some lichens alongside some dried out sea campions. Late in the year this section of the rock face will be fluttering with flowers.


The Spray Zone: The spray zone is the upper part of the beach. This area is dampened by ocean spray and high waves and is submerged only during very high tides or severe storms.

The Upper Tidal Zone: The Upper tidal zone floods during the peaks of daily high tides but remains dry for long stretches between high tides. It is inhabited by hardy sea life that can withstand pounding waves, such as barnacles, marine snails, mussels, limpets, shore crabs, and hermit crabs. Here you will find Channelled Wrack. Channelled wrack lives on rocks at the top of the tidal zone. It is highly adapted to survive out of the water for days at a time.


The distinctive chanelled shape of channelled wrack is said to allow this seaweed to trap water for longer and so allow it to thrive in the dryer upper reaches of the intertidal zone.


Middle Intertidal Zone: Middle intertidal zone: over which the tides ebb and flow twice a day, and which is inhabited by a greater variety of both plants and animals, including sea stars and anemones. Here you will find sea grass, bladderwrack, pepper dulse and sea lettuce



The Lower Tidal Zone: The Lower tidal zone is virtually always underwater except during low tides. Here you will find dulse, nori, sea velvet, and carrageen in the upper reaches and in the lower sections accessible at a Spring tide you will find sugar kelp, oarweed, and alaria (Atlantic wakame).


Around the Irish coast tidal ranges vary from around 1.75 metres on the southeast coast of Ireland to an average of 4.5 metres on the west coast.”



The Sub Tidal Zone: The subtidal zone is below mean low tide and is rarely, if ever, exposed.


“A wild species will grow where the conditions are right for it, even if it is not its typical habitat.”



Now that we have an understanding of the intertidal zones and the different levels within it we are ready for seaweed foraging right?!

Well, not exactly. It is a constant enjoyable puzzle - searching and trying to anticipate where we might find a particular seaweed. This is because while there are some typical habitat types where we might find particular seaweeds it is important to understand that there are no strict rules as to what species will be growing in a particular habitat. A wild species will grow where the conditions are right for it, even if it is not its typical habitat. Perhaps the best way to understand this is to remember that different seaweed species are characteristic of different types of seashores. By this, we mean that it is better to look at a species and try to understand what this tells us about the area where it is growing rather than the reverse -  to arrive at an area and expect to find a certain species of seaweed. For example - Spiralled wrack, bladderwrack and eggwrack grow in the upper areas of the shoreline. When we find these “wracks” growing abundantly it is an indicator of a sheltered area of shoreline. In a more exposed area, we will find less much less seaweed cover.

Some simple things to keep in mind when searching seaweed habitats.

  • Seaweeds do not have roots like plants. The gnarly part where a seaweed attaches to a rock or hard surface is called the holdfast.

  • So if seaweeds mostly grow attached to hard things we are more likely to find seaweeds on beaches with “hard things” in the intertidal zone. Think rocky outcrops, cliff faces, large boulders, and rock plateaus. That said, even soft muddy shores have seaweeds as there will inevitably be some shells or pieces of gravel to attach to.

  • Seaweeds have slippery coatings that allow them to move freely in the waves without getting too damaged. So remember fresh seaweeds are slippery. Take care.

  • There are three main groups of seaweeds - red seaweeds, brown seaweeds, and green seaweeds. They all make their energy by photosynthesis using light-harvesting pigments which give each group their distinctive colour. 

  • There is a major relationship between the amount of time an area (eg. a section of rock) is submerged in water (where it lies within the intertidal or subtidal zone) and the species of seaweed and marine life that will be found there. However, other factors such as competition for space and food are also influencing factors.

  • There are thought to be in the region of 7,000 species of red, 1,700 species of green, and 2,000 brown seaweeds in the world. So the pressure is off - don’t expect to get to know them all. Also, most seaweeds do not have “common names” and are primarily referred to by their Latin names by seaweed experts. Luckily for us the seaweeds that are traditionally used in food mostly have common names. I find common names infinitely easier to remember - though there is a complication in that common names vary greatly from place to place.

  • A simple rule of thumb. A good place to find seaweed in the intertidal zone is on shaded (north-facing) rocks and in small crevices and runnels.

  • The ideal time of the day to go out exploring the seaweed foraging is about 90 and 60 minutes before low tide (screenshot). As the tide goes out, different habitats and different seaweeds are revealed. This gives you time to look for different seaweeds in the upper, middle, and lower tidal zones, and to return to shore as the tide turns.

Coming up next in the series :

The Foraging Club - A guide to understanding tides for the Seaweed Forager.

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