King County's Beach Assessment

Alki Beach, Seattle

Site Description

Alki Beach was sampled west of the Alki sewage treatment facility, south of the lighthouse and adjacent to Schmitz Park, a Seattle City Park. This beach was one of the five sampled in 1974 by Thom and Armstrong.

The beach is relatively flat, but is comprised of outcroppings of layers of clay and of Vashon till. Since the layers have different hardness, they have eroded at different rates. Consequently, there is a series of ridges relatively parallel to the sea wall that protects the upper beach from erosion. The spaces between the ridges are filled with gravel. The beach is exposed to the surf but it is highly resistant to erosion.

Much of the site consists of exposed bedrock, which consists of alternating layers of clay and sandstone. In places, the clay layers have eroded leaving the sandstone exposed as ridges.

Intertidal Inhabitants

Animal diversity here is extremely high because of the variety of habitats available. Various clams, crabs and shrimp live in the gravel in the tide pools between the ridges. Chitons, limpets, worms, snails and sea stars were commonly found on the hard rock. Piddocks burrow in the soft clay layers. The rocks have heavy growths of seaweed although they seem to be diminishing compared to a photograph taken more than 20 years ago. In 1994, both orange and white sea cucumbers were common, but they were found to be scarce two years later.

Invertebrates

Forty-nine (49) species of invertebrates were found on this beach, which is the second highest of all 13 beaches surveyed. The diversity of life on this beach is quite high, but it is diminishing because of little protection from harvesters.

Limpets
Plate Limpet	Acmaea scutum
Variegated Limpet	Acmaea persona
Finger Limpet	Acmaea digitalis
Snails
Sitka Periwinkle	Litorina sitchana
Checkered Periwinkle	Litorina scutulata
Moon Snail	Polinices lewisi
Hairy Triton	Fusitriton oregonense
Fringed Dogwinkle	Thais lamellosa
Channeled Top	Calistoma canaliculatum
Bivalves
Native Littleneck	Protothaca staminea
Manila Clam	Tapes japonica
Butter Clam	Saxidomus gigantea
Cockle	Clinocardium nutalli
Softshell	Mya arenaria
Inconspicuous Macoma	Macoma inconspicua
Pilsbry's Piddock	Zirfaea pilsbryi
Common Piddock	Penitella penita
Bay Mussel	Mytilus edulis
Jingle Shell	Posodesmus cepio
Native oyster	Ostrea luridea
Sea Stars
Mottled Star	Evasterias troschelli
Purple Star	Pisaster ochraceus
Sea Cucumbers
White	Eupentacta quinquesemita
Orange	Cucumaria miniata
Barnacles
Acorn Barnacle	Balanus glandula
Small Acorn Barnacle	Chthanalus dalli
Crabs
Green Shore Crab	Hemigrapsus oregonensis
Black Clawed Crab	Lophopanopeus bellus
Graceful kelp crab	Pugettia gracilis
Red rock crab	Cancer productus
Shrimp
Barred shrimp	Heptacarpus pugettensis
Anemones
Colonial anemone	Cribrina elegantisima
Pebbled Anemone	Telia coriacea
Green Anemone	Anthopleura xanthogrammica
Hydroids
Segmented worms
Corrugated worm	Glyseria
Shellbinder worm	Thelepus crispus
Scale worm	Halosydna
Irridescent worm	Hemipodus borealis
Sand worm	Nereis virens
Tube worms
Calcareous tubeworm	Serpula vermicularius
Flatworms
Large speckled	Kaburakia excelsia
Small speckled	Freemania
Nemerteans
Purple ribbon worm	Emplectonema sp.
Isopods
Brown Isopod	Idotea urotoma
Amphipod
Skeleton Shrimp	Caprella kennerlyi

Clams

Thirty-four (34) holes were dug during each sample date; nearly all contained at least one clam. The largest yield from a single hole was 1.5 lbs.

By both count and weight, the manila, littleneck, and butter clams were the most common. Mya arenaria (soft-shell) and Macoma clams and cockles are present. The clam population densities ranged from one to 65 per square foot but the average of all sites was about 14 clams per square foot.

The manila clams were larger than the average for all beaches but the rest of the species were a little smaller. About one third of the manila clams and about one fifth of the littleneck and butter clams were legal. That average was the second highest of all of the beaches.

Littleneck clams were found wherever a few inches of gravel covered the sandstone. Those gravel patches were scattered, but nearly all of them were inhabited.

The clams appeared to be most common above MLLW to about the +3' level; they were scarce below MLLW.

The clam band, which occupies about two acres, is about 700 feet long and 80 to 100 feet wide beginning at the jetty at the south end of the beach and continuing a northerly direction (see topographic map). Beyond that point, the gravel patches are very discontinuous. The sampling grid that we used was established by the Seattle Aquarium staff several years ago. The grid is 300 feet long x 80 to 120 feet wide and begins at about the +3 foot level at the south end of the park near the jetty. It extends north from there for 300 feet. The grid is 50 feet by 40 feet on this beach rather than the 100 foot x 40 foot format used on most other beaches.

Check out the following graphs for more information about the clam population at Alki:
Numeric composition of clam population
Weight composition of clam population
Clam distribution
Clam biomass distribution

Seaweed

Twenty-six (26) species of marine algae were found, making this site the second highest among the beaches surveyed. The brown algae (13 species) were the most varied overall, but there were more red species (10) at the lower tide levels on the beach. Only three species of green algae were noted, but they were the dominant species at the higher tide levels. Rockweed and Gigartina cristata (grapestone) were very common on the rock outcrops at the higher levels. Sargassum appears to be invading the area.

Green (Chlorophyceae)

Cladophora sp.

Enteromorpha intestinalis

Ulva lactuca

Brown (Phaeophyceae)

Costaria costata

Desmarestia intermedia

Desmarestia munda

Fucus sp.

Hedophyllum sessile

Leathesia difforimis

Petalonia debilis

Punctaria hesperia

Pylaiella littoralis

Ralfsia sp.

Sargassum muticum

Scytosiphon bullosus

Scytosiphon lomentaria

Red (Rhodophyceae)

Ceramium pacificum

Cryptosiphonia woodii

Gigartina cristata

Gigartina exasperata

Gigartina latissima

Iridea cordata

Porphyra perforara

Prionitis lanceolata

Rhodymenia palmata

Smithora naiadum

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Updated: 11/02/98