Intertidal Zone & Associated Habitats
Did you know that Puget Sound has a ring around it just like a bath tub? The ring is easy to see on rocky shores like the one shown in the photograph. The ring is not caused by pollution, but rather by animals and plants growing below the high water mark.
Intertidal Zone means “between the tide lines” or the vertical dimension of a beach that is exposed when the tide recedes. In Puget Sound the tide moves in and out two times every 24 hours. The inhabitants are subjected to wide extremes of temperature and salinity and are a hardy breed.
South Puget Sound Intertidal Habitats
This map shows the distribution of a variety of intertidal habitats in south Puget Sound. Note that the mud beaches are within the bays while sandy beaches are in the more exposed areas outside of bays. This is the result of wave and tidal current distribution. High energy areas lack fine sediments on the beach which low energy areas have higher levels of fine sediments.
A term “flat” on this map means a wide beach.
The map was drawn by Tom Mumford for the South Sound Estuary Association and is based on ShoreZone 2000 data . It is provided here with the permission of Tom Mumford
Rocky Beach
Rainbow-leaf Seaweed, Sea Brush, Sea Cauliflower, Bleach Weed, Five-ribbed Kelp, Polysiphonia, Painted Anemone, Aggregating Anemone, Polyclad Flatworm, Slime Worm, Red-flecked Mopalia, Frilled Dogwinkle, Japanese Oyster Borer,Rock Oyster, Opalescent Nudibranch, Shaggy Mouse Nudibranch, Thickclaw Porcelain Crab, Purple Shore crab (Hemigrapsus nudis), Pigmy Rock Crab, Puget Dwarf Brittle Star, Mottled Sea Star, Orange Sea Cucumber, White Sea Cucumber, Mushroom Sea Squirt, Slimy Snailfish, Great Blue Heron, River Otter and Harbor Seal. In the sediment around cobbles can be found the Littleneck clam, Manila clam and Butter clam.
The vertical face of the rocky beach, as seen in the photograph, is divided into several areas or layers. The upper area is covered by upland plants. This area is rarely, if ever, covered by salt water so no salt water species grow there (however some may be salt tolerant). Below that is a bare area that is thin or wide depending on the exposure to the sun, amount of freshwater runoff, and exposure to salt spray. Further down is black material that looks like oil. In fact it is a lichen called Verrucaria which marks the upper zone of salt water tolerant animals and plants. Small hardy marine snails called Periwinkles also frequent the area. Just below this black layer is a thin, white layer of Barnacles. Continuing downward lies the first of the true marine plants, the brown seaweed called Rockweed or Fucus. Often found among the rockweed are several species of snails called Limpets with shells that look like little hats. The next lower layer is a black mat of the Edible or Bay Mussel which shelters a variety of small animals. The lowest layer is a wide zone of several species of brown, green and red marine algae such as Sugar Kelp, Crustose Coralline Algae and Sea lettuce.
When the tide comes in, the intertidal world changes. Not only is there fresh sea water for breathing, flushing away waste products, bringing in nutrients, and protection from the heat or cold, but it also brings in the predators or allows the ones who stayed to come out of hiding. For example the Purple Sea Star moves up to feed on the mussels and barnacles. Several species of marine fish such as the Tidepool Sculpins and Clingfish move among the seaweed feeding on small crustaceans. The limpets and periwinkles arise from their low tide slumber to begin feeding on algae. TIDE POOLS which served as a refuge at low tide now join to open water so the oxygen and nutrients can be refreshed. Found under rock shelves or large boulders are Rock Pricklebacks, Tube Sponges and the Northern Feather Duster Worm.
Subtidal rocky sites in protected as well as high current areas have many more species. Since they require diving to view they are not covered here. However many of the animals and plants that naturally occur on subtidal rocky sites also can be found under floats and on piling at local marinas.
Sandy Beach
Colonial Diatom, Green String Lettuce, Red String Seaweed, Rockweed, Turkish Washcloth, Wireweed, Pickleweed, Moonglow Anemone, Aggregating Anemone, Burrowing Sea Cucumber, Moon Snail, Bay Mussel, Spotted Aglaja Nudibranch, Dungenese Crab, Graceful Crab, Red Rock Crab, Smooth Bay Shrimp, Ostracod, Cumacean, Jointed-tube Worm, Spaghetti Worm, Sandworm, Yellow-bellied Ribbon Worm, Pacific Sand Lance, Surf Smelt, Glaucous-winged Gull, Ring-billed Gull, Greater Yellowlegs, Least Sandpiper, Northwestern Crow, Black Brant and Great Blue Heron.
Animals associated with subtidal (deeper than the intertidal zone) sandy habitat are: Sea Lemon nudibranch, Blackeyed Hermit Crab, Speckled Sanddab, Giant Sea Cucumber and Leather Sea Star.
As you move away from the protected back bays of Puget Sound to the mouths or open channels, high tidal currents and wind-driven wave action leave fewer fine sediments in the intertidal zone. The beaches become sand dominated rather than mud dominated. As a result the kinds and numbers of plants and animals change.
Tolmie State Park beach, west of the Nisqually River delta, is characteristic of sand dominated local beaches (see below photograph). The upper intertidal zone is composed of gravel and cobbles (and occasionally a short strip of sand) where wave action makes them act like millstones crushing any animals or plants. In addition, water easily moves through the course sediment leaving the upper few inches dry. Further down slope lies the more stable sand/gravel/boulder zone with a dramatic increase in life. The sand binds the gravel and cobbles resisting wave action and holding moisture. Here are Littleneck and Manila clams, green sea lettuce, a red seaweed called Nori, amphipods under the seaweed, juvenile shore crabs under larger boulders and juvenile fish (such as Staghorn Sculpins and various flatfish) in small tide pools. If there are hard surfaces consisting of untreated wood you may find Gribbles and Teredos burrowing into it.
The sand flat extends out to a minus four feet and beyond. The upper portion is home to small beds of a short, small-bladed eelgrass, sand dollar beds, a variety of burrowing polychaete worms, the white sand clam , cockle clam, horse clams and ghost shrimp. As the lower intertidal grades into the subtidal zone, beds of the large-bladed eelgrass may be present along with their rich diversity of associated animals and plants.
Sandy Mud
Colonial Diatom, Green String Lettuce, Sea Lettuce, Rockweed, Nori, Red String Seaweed, Pickleweed, Moonglow Anemone, Light-edged Ribbon Worm, Thread Sludgeworm, Polyclad Flatworm, Jointed-tube Worm, Spaghetti Worm, Sandworm, Pile Worm Bentnose Clam, Littleneck clam, Manila clam, Butter clam, Geoduck clam, Horse clam, Moon Snail, Bay Mussel, Pacific Oyster, Frilled Dogwinkle,, Marine Pill Bug, Cumacean, Ghost Shrimp, Graceful Crab, Red Rock Crab, Mud-flat Crab, Hairy Hermit Crab, Ostracod, Arrow Goby, Pacific Staghorn Sculpin, Plainfin midshipman, ring-billed Gull, Glaucous-winged Gull, Least Sandpiper, Northwestern Crow, American Wigeon and Great Blue Heron.
Inside the protected inlets of South Puget Sound and other locations, wind waves and tidal currents are much reduced. As a result sediments from streams or rivers along with cliff erosion will remain in the area resulting in wide beaches with a mixture of mud and sand. Depending on the ratio of mud to sand, as well as the percentage of gravel, the beach will be home to a different assemblage of animals and plants.
Frye Cove County Park in Totten Inlet is an example of a mud dominated beach (see photograph). The upper intertidal zone is characterized by cobbles and muddy sand mixed with gravel. However the expansive flat beyond is composed of sandy/mud that hosts a variety of worms and other organisms. Some of those organisms are only present if rocks lie on the surface. Most of the seaweeds need a hard surface to attach to while the High Cockscomb Prickleback lives along the underside of the rock where moisture occurs at low tide. If that surface is untreated wood you may find Gribbles and Teredos burrowing into it.
Sandy mud substrate is ideal for preserving the evidence of creatures from the uplands that will invade the beach when the tide recedes. See a discussion of those animals HERE.
Pilings & Floating Docks
Sugar Kelp, Green String Lettuce, Sea Lettuce, Tube Sponge, Fried Egg Jelly, Lion’s Mane Jelly, Nanomia Siphonophore, Hydroid, Giant Plumose Anemone, Sea Gooseberry, Red Sausage Jelly, Crystal Jelly, Bay Mussel, Rock Oyster, Teredos, Opalescent Nudibranch, Shaggy Mouse Nudibranch, White and Orange Nudibranch, Branched Dendronotus, Rainbow Nudibranch, Hedgpeth’s Sapsucker sea slug, Common Squid, Calcareous Tube Worm, Pile Worm, Feather Duster Worm, Slime Fanworm, Polyclad Flatworm, Mottled Sea Star, Purple Sea Star, Orange Sea Cucumber, Puget Dwarf Brittle Star, Pink Short-spine Sea Star, Sunflower Sea Star, Giant Acorn Barnacle, Gribbles, Dock Shrimp, Stout Shrimp, Thickclaw Porcelain Crab, Pigmy Rock Crab, Decorator Crab, Kelp Crab, Ostracod, Kelp Isopod, Marine Pill Bug, Cumacean, Skeleton Shrimp, Kelp Encrusting Bryozoan, Spiral bryozoan, Mushroom Sea Squirt, Transparent Sea Squirt, Northern Anchovy, Pacific Herring, Bay Pipefish, Shiner Perch, Striped Surfperch, Threespine Stickleback, Tubesnout, Slimy Snailfish, Belted Kingfisher, Great Blue Heron, Glaucous-winged Gull, River Otter and Harbor Seal.
Where is a great place to see marine animals and plants without getting your feet wet? At your local marina, of course. However, be sure you have permission to go on the floating docks and don’t disturb the animals or boaters.
The floating docks at a marina (or a private dock) offer an ideal habitat for lots of local flora and fauna. Piling that tie the floating docks to the bottom are also commonly used by a small variety of animals.
Floating docks provide a constantly submerged, hard surface that attracts many species who normally would be found on subtidal rocky reefs and sunken ships. The sides of the floating docks receive sunlight which allows many types of seaweed to thrive. There is generally good water movement around floating docks due to tidal action which brings food, nutrients, and oxygen to the attached animals and plants. However floating docks in low salinity water will have fewer species.
Piling, while a hard surface, is a more difficult place to live. Depending on water depth, some of the piling will be exposed at low tide offering suitable habitat only for species that can protect themselves from drying, heat, and cold like the Acorn Barnacle. In addition the mechanism that secures the floating dock to the piling will move up and down with the tide and scrape along sections of the piling removing attached animals or plants. See video of piling animals.
Like piling and floats, the hulls of marine vessels can become encrusted with many of the organisms mentioned below. The marine growth, called a “fouling community”, can slow down the boat by increasing the friction of the hull moving through the water. It therefore increases the amount of power needed to move the boat. As a result sailors have used various materials to prevent the growth of marine organisms. Copper containing paint is a common material. The leaching of the copper poisons the animals and for some types of paint the copper will slough off over time stripping the hull of organisms.
Open Water Habitats
Plankton, Bull Kelp, Fried Egg Jelly, Lion’s Mane Jelly, Moon Jelly, Nanomia Siphonophore, Sea Gooseberry, Red Sausage Jelly, Crystal Jelly, Hyperiid Amphipod, Sea Lice, Common Squid, Rainbow Nudibranch, Pink Short-spined Sea Star, Sunflower Sea Star, Northern Anchovy, Pacific Herring, Pacific Sand Lance, Surf Smelt, Sand Sole, Sailfin Sculpin, Starry Flounder, Caspian Tern, Common Goldeneye Duck, Common Loon, Common Merganser, Double-crested Cormorant, Horned Grebe, Pigeon Guillemot, Surf Scoter Orca and Harbor Seal.
Open water is the largest habitat type in Puget Sound. Most marine and estuarine animals and plants use open water either as adults or more often when they are young. The vast numbers of planktonic plants offers abundant food for both adults and young (who are also part of the plankton community).
Many of the open water animals can be seen from boats, piers or marinas. However a number live in deeper water or on the bottom and are seldom seen unless they are caught while fishing, seen by divers or are drawn to lights placed in the water at night to lure them.
Tides
Why are the tides? The answer is not obvious unless your observation of nature has led you to notice a relationship between the moon and high tides. The solution lies in the gravitational interaction between the moon, the sun, and the earth, along with rotation of the earth, and the orbits of those bodies. That combination results in two daily bulges in the ocean level that moves around the earth following the moon. One bulge is opposite the moon (due to the pull of gravity) and the other on the opposite side of the earth (due to centrifugal force of earth/moon system rotation). When the bulge arrives in Puget Sound it causes a high tide and when it leaves, the tide is low resulting in two series of tide changes per day. The height of the bulge varies daily depending on the relationship of the sun to moon. When the sun and the moon are aligned with the earth the gravitational pull is greater and the high and low tides have a much greater range (called a “spring ” tide, meaning “to jump”). When they are at 45 degree angles then the pull is less and the tide range is less, the so called neap tides. The change occurs about every seven days. The tidal range also varies with local bathymetry.
The tide can be predicted using mathematic equations which are generally reliable but can differ from reality due to weather conditions: for example a storm can result in a higher tide than predicted due to the low atmospheric pressure.
Let’s use the Boston Harbor tide chart to discover what the expected tide level will be. The two mountains represent high tides with the valleys representing low tides. The chart axis on the left is height of the water level. The bottom axis is time from midnight to midnight. The first high tide is predicted to be at around 4:29 am with a height of 11.69 feet above mean lower low water (the 0.0 tide line or tidal datum). Then 6 hours and 15 minutes later will be the low tide of 2.02 feet at 10:43 am and so on through out the day. The extra 15 minutes is due the lag in the moon’s orbit relative to the earth’s rotation making the next pass of the moon a total of 50 minutes later each day. If a low tide is below the 0.0 tidal datum it is referred to as a minus tide.
What does this mean to the animals and plants of the intertidal zone?
The tide chart indicates that over this 24 hour period an organism like a speckled limpet or rockweed living at the 11.00 foot tide level would have been dry for 17.25 hours out of 24 hours or 72% of the time. An organism like a clam or shore crab living at the 3.00 level would have been dry only 14% of the time.
Most of the low gradient beach, as shown on the below photograph, is covered with water more than 50 % of the time (below the mean sea level mark of a +8.2 ft). The extreme low tide line at a – 4.5 ft marks the boundary between the intertidal zone and the subtidal zone. Note that some of the boats are going aground as the water recedes. The low water line in the photo is around the – 3.0 ft mark. The green seaweed, Sea Lettuce, covers the beach from around the +5.0 ft line to extreme low tide. The Turkish Washcloth seaweed and Rockweed grow above this line. There are fewer clams and Mud-flat crabs above this line as well. (Note: the indicated tide levels are based on mathematical estimates rather than measurements)
Tidepools
A tide pool is a wonderful place. It allows the patient and watchful visitor a way of seeing animals and plants usually found only in the low tide or subtidal zones. Any depression in rock that traps water when the tide goes out will do, however the bigger the more to be seen. In south Puget Sound there are few such places, but more can be found in rocky areas further north in Puget Sound and in rocky coastal beaches. Tide pools can be found at any tide level and range in size from a few gallons, only inches deep to thousands of gallons, several feet deep.
A tide pool is nature’s aquarium. Like an aquarium in your home, a tide pool provides oxygen, proper water chemistry, waste product removal, tolerable water temperature and food. In your home this work is done by air pumps, water filters, careful monitoring and daily feeding. In nature the incoming tide does the work. However when the tide falls, things change. Shallow tide pools in the high tide zone are subjected to high temperatures in the summer and low temperatures in the winter. Few animals or plants can tolerate such conditions and so those tide pools house only a few species. At mid tide level and lower even small pools a few inches deep, such as shown in the picture, can be brimming with life. Shrimp, shore crabs, a variety of small seaweeds, sea anemones and small fish scuttle about. When the tide comes in the animals will roam the surrounding rock hunting for food returning to a pool as the tide ebbs. One fish, the tidepool sculpin (Oligocottus maculosus), knows the pools so well in its territory that it returns to the same one each day.
