Joseph Stachelek

Desmarestia viridis, a brown algae, also known as sourweed

The rocky coast of Acadia National Park contains many different kinds of marine algae, or seaweeds. These organisms thrive in the dynamic area between the high and low tide marks where the land meets the sea. Seaweeds that grow in the rocky intertidal zone must cope with both aquatic and terrestrial environments to survive.

Most of these algae are adapted to live in a particular part of the shore that is uncovered at low tide, not the entire shore. For example, many seaweeds in the lowest part of the shore live there because they cannot survive the long periods of high temperatures, drying (or freezing), and high light that occur higher on the shore at low tide.

A combination of physical and biological factors determines where each different alga is found. Important physical factors that control distribution include levels of light (including UV), drying, temperature, wave action, and ice scour. Biological factors that control distribution include competition for space and resistance to predation; animals that eat algae (or other plants) are called herbivores. These factors all play a part in defining the distinctive zones on the shore, which consist of the splash, high, mid-, and lower intertidal zones.

In the splash zone and upper intertidal zone, spray from high tide moistens the algal mats (diatoms and Ulothrix laetevirins), which makes footing slippery. The most abundant seaweed is usually the rockweed Fucus spiralis. The mid-intertidal zone is typically covered by other rockweeds (Ascophyllum nodosum and Fucus vesiculosus), whereas the lower intertidal zone and lower intertidal pools contain many different kinds of seaweeds including sea lettuce (Ulva lactuca), kelps (Alaria esculenta), and Irish moss (Chondrus crispus). The algae field guide will help you identify many of the different algae on the shore.

Most of the algae on shores of Acadia National Park belong to one of four groups:

• Red algae (Rhodophyta)
• Green algae (Chlorophyta)
• Brown algae (Phaeophyceae)
• Diatoms (Bacillariophyceae)

These groupings reflect differences in anatomy, reproduction, life cycles, and type of light-harvesting pigments. Most algae are flat blades or filaments (branched or unbranched); filaments consist of a chain of cells and appear to the eye as hair-like or stringy seaweeds. As a group, most algae are not closely related to each other, but all algae contain chlorophyll a (a light-harvesting pigment) and need water at some stage in their life cycle.

Explore the field guide to algae in the waters surrounding Acadia National Park. The field guide (as well as this text) was developed by Joseph Stachelek and Sarah Hall, students at the University of Maine.