Field Guide to Algae - Brown Algae

Agarum clathratum - brown algae

Agarum clathratum
Agarum clathratum (“sea colander”) is a subtidal kelp, but can be found washed up after storms. This kelp is easily distinguished from Alaria esculenta because Agarum’s blade is naturally full of holes in addition to having a thick, central ridge (midrib).

Photo by Sarah Hall.

Alaria esculenta growing in water

Alaria esculenta
Alaria esculenta is found in the lower intertidal and shallow subtidal zones. It has a claw-like holdfast and a long, undivided blade with a thick ridge (midrib) in the center of the blade. This sea vegetable is economically important as human food. Present year round.

Photos by Sarah Hall.

Ascophyllum nodosum in its habitat

Ascophyllum nodosum
Ascophyllum nodosum (“rockweed”) is most abundant on sheltered rocky shores in the mid-intertidal zone of the North Atlantic. This alga makes one air bladder a year, which can be used to determine the age of the alga. Orange receptacles (males) and green receptacles (females) release sperm and eggs on 1-2 spring tides in late May and early June, sometimes causing the water to turn orange. Present year round.

Photos by Susan Brawley (left) and Sarah Hall (pop-up).

Desmarestia viridis

Desmarestia viridis
Sometimes called “sourweed,” D. viridis stores sulfuric acid within its cells, which may keep this alga from being eaten. However, sea urchins feed on it during calm seas. Desmarestia viridis is a subtidal alga that is also found in the lowest part of the intertidal zone. If exposed too long during a hot, dry low tide, its cells leak sulfuric acid and turn this brown seaweed into a limp, green mess (see photo). Present winter–late spring.

Photos by Susan Brawley (left) and Sarah Hall (pop-up).

Elachista fuciola - close-up

Elachista fuciola
This brown alga is often found growing in the mid-intertidal zone on other macroalgae, such as Fucus distichus ssp. edentatus. This alga grows up to 2 cm high from a basal cushion and consists of unbranched filaments. Present year round.

Photos by Joseph Stachelek (left) and Sarah Hall (pop-up).

Rockweed growing in rock cracks

Fucus distichus ssp. distichus
Fucus distichus ssp. distichus is a small rockweed that is found in tide pools in the upper intertidal zone. This alga survives under the ice when tide pools freeze in winter. It reproduces in winter and spring. Present year round.

Photos by Susan Brawley (left) and Joseph Stachelek (pop-up).

Fucus distichus ssp. edentatus growing on shore

Fucus distichus ssp. edentatus
Fucus distichus ssp. edentatus is found in the lower intertidal zone of rocky shores. The elongated tips (receptacles) are reproductive tissue containing eggs and sperm. All rockweeds contain high concentrations of tannins, which makes them taste badly to herbivores. Present year round.

Photos by Joseph Stachelek (left) and Susan Brawley (pop-up).

Fucus spiralis - habitat

Fucus spiralis
This species of Fucus develops a ridge of tissue around the inflated branched tips (reproductive tissue, containing eggs and sperm). This rockweed lacks the paired air bladders found in F. vesiculosus. Fucus spiralis is found in the high intertidal zone of rocky shores in the North Atlantic and is reproductive during summer. Present year round.

Photos by Susan Brawley (left) and Sarah Hall (pop-up).

Fucus vesiculosus with waves splashing over

Fucus vesiculosus
This rockweed dominates the mid-intertidal zone of moderately wave-exposed, rocky shores. Fucus vesiculosus often has paired air bladders, and individuals are dichotomously branched. However, individuals in the most exposed habitats sometimes lack paired air bladders. Rockweeds release gametes under calm, sunny conditions at high tide. Present year round.

Photos by Susan Brawley (left) and Sarah Hall (pop-up).

Laminaria digitata floating in water

Laminaria digitata
This kelp is found in subtidal beds, but can be observed in tide pools in the lower intertidal zone. Dark stripes on Laminaria digitata are patches of sori, which contain specialized reproductive cells called zoospores. The bladed form is only half of the kelp life history, and the other half is filamentous and microscopic and produces eggs and sperm.

Photos by Sarah Hall.

Leathesia difformis

Leathesia difformis
Leathesia difformis (“rat brains”) is a hollow ball that commonly grows in groups of individuals in the lower intertidal zone. Present late spring–summer.

Photos by Joseph Stachelek (left) and Sarah Hall (pop-up).

Saccharina latissima - long blade lying on bed of algae

Saccharina latissima
Saccharina latissima (“sugar kelp”) has a leathery, undivided blade that is about 2 to 5 meters long. This kelp forms a substantial belt below the intertidal zone and is an important habitat and food source for animals such as sea urchins. Saccharina can be found in lower intertidal pools and on extremely low tides. Kelps are economically valuable as food and sources of alginate (a thickener in many foods, toothpaste etc.). Present year round.

Photos by Sarah Hall.

Scytosiphon lomentaria

Scytosiphon lomentaria
Scytosiphon lomentaria are tubular algae that are constricted at intervals. This alga is found in the lower intertidal zone. Present winter–early summer.

Photo by Sarah Hall.

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