Caring for Our Shores – Chapter 1: Water, Shores, and Intertidal Ties
…some different shore types and how to identify them
…an introduction to our tides, and marine food web
On the B.C coast, there are two unequal high tides and two unequal low tides each twenty-four hours. Intertidal exploring in the Strait of Georgia is best during those summer days when extremely low tides expose the shore. In the winter, night owls can take advantage of nighttime low tides. The greatest tidal ranges occur in June and again in December. In the Cowichan region ranges vary from 16 feet on the northeast side of Valdes Island, to 12 feet in Saanich Inlet. For approximate tides in your area, purchase a tide guide at your local marina, sporting goods store, or book store.
The kinds of animals and seaweeds living on the seashore are determined by a variety of local conditions: salinity, or the concentration of saltwater, water movement, and the surface characteristics of the shore. While these are some of the more obvious conditions, it is important to remember that other factors like nutrients have big effects, but leave no immediate “footprint”.
To an oceanographer, nearly all the Strait of Georgia is an estuary, a place where saltwater and freshwater mix. During times of high freshwater runoff, the eastern shores of Valdes, and Galiano Islands are bathed in mainly freshwater while the western shores are more than 67% saltwater. This huge freshwater input is from the Fraser River.
Most marine animals cannot tolerate drops in salinity caused by such significant dilutions of saltwater with freshwater. If you live on one of the eastern Gulf Islands, you may be aware that the western shores of your island are much richer in marine wildlife than eastern shores. Similar differences also occur on a smaller scale in the estuaries of the Cowichan, Chemainus, and Nanaimo Rivers.
The Strait of Georgia does not generate the same continuous swells and oceanic waves as the open Pacific Ocean. In the open strait, waves occur during local storms or from passing boats, and are rarely higher than six feet. The areas most protected from waves such as Ladysmith Harbour experience warm surface water during the summer. Oysters require such warm water to spawn, and up until recently, Ladysmith Harbour was known as Oyster Bay.
Where land masses constrict water passage, the rise and fall of the tides translates into currents, rapids, and overfalls. Whereas some marine wildlife live in quiet conditions, many sponges, bryozoans, and sea squirts thrive on surfaces in contact with currents. These attached creatures rely on the drifting food carried to them by moving water. Bull kelp also lives in near shore waters with tidal currents. In the Cowichan region, these kinds of bottlenecks occur at several inter-island passes including Gabriola Pass, and Sansum Narrows. The richness of life associated with high currents in Gabriola Pass led to its recommendation for Marine Protected Area (MPA) status.
The characteristics of intertidal surfaces are important for marine animals and seaweeds. Is a surface hard or soft? Is it smooth, or does it have cracks and crannies? Is a shore made of stable rocks, or is it unstable like a muddy sand beach? Whether a shore is horizontal, sloped, or vertical is also an important factor in determining which marine wildlife will use it for a home. For examples of some of the dominant shore types found in the southern Strait of Georgia, visit our shore types page.
Have you ever been surprised by a ripple of sparkles in the moonlit sea? Waves hitting the shore or wading feet stir up the water and cause some resident phytoplankton to chemically produce light internally or bioluminesce. The best time to enjoy a sparkling swim or paddle is during the hot summer months when the plankton are growing well and the evening waters are not too cold. The type of phytoplankton responsible for this magical display are called dinoflagellates.
To order a copy of Caring for Our Shores: A Handbook for Coastal Residents in the Strait of Georgia, contact:
The Cowichan Community Land Trust Society
#6-55 Station Street, Duncan, B.C, V9L 1M2
P: 250-746-0227, F: 250-746-9607
The Marine Ecology Station
Images used with special permission from Kerry L. Werry. To view more pictures of ocean creatures found in B.C waters, visit the B.C Diving and Marine Life I.D Page.