Benthic marine algal flora of the north shore of Prince Edward Island (Gulf of St. Lawrence), Canada

A seaweed community along 60 km of shoreline in the southern Gulf of St. Lawrence has been studied for 5 years. Because of severe ice action along the shores, the algal flora is largely restricted to subtidal levels, which are accessible only during the ice-free months, April to early December. A total of 52 Rhodophyta, 50 Phaeophyta, 19 Chlorophyta and 5 Cyanophyta was identified, mostly occurring between mean low water and 10 m depth. The normally intertidal dominants Focus vesiculosus L. and Chondrus crispus Stackh. flourish at depths of 0–2.5 m and 2.5–5 m, respectively. Chondrus and Furcellaria lumbricalis (Huds.) Lamour. co-dominate the algal benthos at 5–7.5 m, and F. lumbricalis becomes prevalent at 7.5–10 m. Phyllophora truncata (Pallas) A. Zin. and Phyllophora pseudoceranoides (Gmel.) Newr. & A. Tayl. are the main perennials at 10–15 m, and often at 15–20 m as well. Below 15 m, the flora diminishes markedly, and at 20–25 m consists chiefly of Agarum cribrosum (Mert.) Bory, crustose corallines and Polysiphonia urceolata (Lightf. ex Dillw.) Grev. Laminarians other than Chorda occur only as scattered plants or small patches and do not exceed 1 m length. The algal community is relatively undiversified and is especially deficient in green algae. Despite higher surface-water temperatures during summer, the flora is essentially a cold-water one as is indicated by both composition and low R/P and (R+C)/P ratios of 1.06 and 1.45, respectively.