Intertidal Zonation

By Nan Evans

The tide is out. It’s time to explore. Walk towards the low tide water
level on any shoreline, and there are wondrous things to see. As you
explore, you will notice that the biological communities change from
the high tide level down to the lowest levels. Marine scientists call
this “intertidal zonation.”  Ecologists group the zones into the
Splash, High Intertidal, Middle Intertidal, and Low Intertidal Zones.

Zonation is a complex result of substrate, tidal levels, predator-prey
interactions, and competition for space. Specifics differ between
rocky shores, sandy beaches, mud flats, a pillar under a pier, or
boulders strewn amongst sand and cobbles, but you will always see
changes in plant and animal communities.

At the highest levels, life is hard – too hot in the summer, too cold
in the winter, too dry most of the time, and too much fresh rainwater.
Isolated tidepools can become both too hot and too salty when exposed
to the sun for many hours. In the Salish Sea, the Splash Zone ranges
from about +7 to +9 feet above mean lower low water (which is “0” feet
in a tide table). Here we see only a few hardy species but often huge
numbers of individuals. You will find several lichen species. The
green algae sea lettuce (Ulva) makes its first appearance. A few land
plants that can tolerate the salt exposure may also be present. This
is the zone of barnacles – the small little brown barnacle and the
acorn barnacle. Look closely in cracks and crevices, and you might be
able to spot a few small marine snails and limpets. These hardy
animals feed only when briefly submerged. When the tide is out, they
basically close down tight to keep moisture inside their bodies –
clamping those barnacle plates together, or pulling into a snail shell
to shut the door (operculum), or just contracting those muscles that
allow a limpet to adhere tightly to the rock face. All of these
animals must have the ability to tolerate increasing levels of their
own biological waste products as the hours go by before they are
submerged again and are flushed by sea water.

Lottia digitalis (Ribbed Limpet)

Below this uppermost horizon, we encounter the High Intertidal Zone
that is submerged only during the highest tide levels. Life is
marginally easier in this zone, but its residents are still exposed to
the equivalent of a total of 145 to 70 total days of dryness over a
period of 6 months. The High Intertidal Zone in our area ranges from
about +7 feet to +4 feet above the mean lower low level. We see more
diverse life here. Consult a field guide; the number of species
explodes too much to describe briefly. Lots of algal species: brown,
green, and red. The brown algae Rockweed (Fucus) stands out on the
rocks. In addition to barnacles, there are more species of snails
(including carnivorous and algae-eating species) and limpets. Look
into small tidepools or under a few rocks (please replace them very
carefully!) to find hairy hermit crabs and the dark green and black
tidepool sculpin.

Fucus distichus (Rockweed)

Lower still, we find the ecological communities of the Middle
Intertidal. Communities here are submerged and exposed for more equal
periods of time. This is the area from about +4 feet down to 0 feet
(the mean lower low tidal level). Life really explodes here – crabs,
sea stars, more snail species, goose neck and blue mussels, chitons,
sea stars, and sea anemones. Curiously, the animals of this area are
rarely seen subtidally. They truly are residents of the in-between
world – part terrestrial and part marine.

Cancer productus (Red Rock Crab)

At last we enter the Low Intertidal Zone, which is exposed only during
the lowest tide levels. This is the realm of those marine creatures
who have adapted physiologically and/or behaviorally to survive some
amount of time exposed to air. We see the largest variety of species
in the Salish Sea in the Low Intertidal Zone, 0 to about -3.5 feet.
Some of the species also occur subtidally. Interestingly, although
there are many more different plants and animals, we do not see the
high numbers of individuals of the same species we saw in the upper
levels. Life for a marine creature is easier here, but then
competition for space and food is more intense as well.

Cucumaria miniata (Red Sea Cucumber)

Surfgrass and eelgrass (both flowering plants), plus vast numbers of
species of algae or seaweeds, are found here.  Abundant animal life
includes species of sponges, hydroids, anemones, flatworms, round
worms, segmented worms, sea stars, brittle stars, sea urchins, sea
cucumbers, brachiopods, snails, limpets, bivalve mollusks, chitons,
nudibranchs, shrimp, crabs, octopuses, clingfishes, sculpins,
pricklebacks, and gunnels – to name just a few!

If we use our intertidal visit well, we will explore all the different
zones, probably spending the most time at the lowest levels. But we
must be careful, the tide turns and comes back in quickly, and we may
have only a short time to get back to land before all these
communities return to the sea.