Tidal Lagoons
What are tidal lagoons & where are they found around Victoria?
A tidal lagoon is a protected body of ocean water that is semi-enclosed by a barrier such as a rocky headland or a sand spit. Tidal lagoons may also be estuaries, when they contain a source of freshwater from a river or a stream. Because they are not fully exposed to the ocean, tidal lagoons experience limited flushing from the tides and waves. These conditions create a unique and protected environment that is rich in life, but can also lead to problems when pollution is introduced.
One of the most unique physical features in the marine areas of Victoria is the tidal lagoon called Portage Inlet. This lagoon is connected to Victoria Harbour through a narrow rocky channel called the Gorge Waterway. Two other well-known local tidal lagoons are Esquimalt Lagoon in Colwood and Witty’s Lagoon in Metchosin.
What lives in tidal lagoons?
As in estuaries in general, tidal lagoons are usually rich in marine life. The plants and animals that live here vary quite widely from one area to another, and depend largely on the local geology and environmental conditions.
- Where there is soft sediment, eelgrass often predominates, and creates underwater meadows that host a huge array of fish, shellfish, birds and mammals. Juvenile salmon and Cutthroat Trout often spend part of their life cycles in tidal lagoons, particularly among eelgrass, and herring deposit their eggs on eelgrass during spawning.
- Native Oysters also live among mud and sand; in Portage Inlet, these relatively rare shellfish have established quite a large population, one of the largest known on the west coast of British Columbia.
- Sea lettuce, mussels and barnacles, crabs and starfish are common in tidal lagoons with rocky shores.
- Many types of seabirds, shorebirds and waterfowl make use of tidal lagoons for feeding, resting and breeding. Victoria is located on the Pacific Flyway, a route traveled by millions of birds during seasonal migrations that can span tens of thousands of kilometres and two or more continents.
- The rich plant and animal life in tidal lagoons in turn supplies higher predators such as otters, eagles, ospreys, bears and raccoons.
Why are tidal lagoons important?
Tidal lagoons are unique because they are sheltered from the brunt of ocean forces that usually limit the types and numbers of organisms that can live on a shoreline. Their rich plant and animal life (see “What lives in tidal lagoons?”) therefore makes them an important component of the marine food web.
People also make use of tidal lagoons. For example, the Gorge Waterway was highly valued by the Songhees and Esquimalt people, long before Europeans colonized Victoria, for its abundance of food such as fish, shellfish and birds. Herring roe, which is deposited on eelgrass, kelp or other vegetation, was a prized treat; today, a valuable roe-on-kelp commercial aquaculture industry is practiced, mostly by First Nations, in some tidal lagoons (though not in the Gorge).
Local tidal lagoons, such as the Gorge, Portage Inlet and Esquimalt Lagoon, have long been popular recreation areas for boating and/or swimming. The shallow water in these areas is much warmer than in the surrounding deeper bays.
Tidal lagoons are particularly valuable as “nurseries” for commercially important fish such as salmon and Cutthroat Trout. These fish are anadromous, meaning they spend most of their lifecycles in salt water, but return to fresh water to reproduce. When the juvenile fish first reach the ocean from the streams where they were born, they require sheltered areas in which to adjust to the new environment. Tidal lagoons provide shelter, often in the form of eelgrass beds, plentiful food such as insects, copepods and other invertebrates, and relatively few predators. Since recreational, commercial and subsistence fisheries are important to the economy and culture of people in British Columbia, tidal lagoons are also important.
What threatens tidal lagoons?
Tidal lagoons are by nature partially cut off from the ocean; this attribute can also be a liability, since tidal currents may not flush out lagoons as adequately as in more open areas. Consequently, pollution and sediment can accumulate. Two local examples help to illustrate these concerns:
In Esquimalt Lagoon, nutrient pollution is a concern. Excessive nutrients (i.e. nitrates and phosphates) enter the lagoon in the form of fertilizers, which are washed off lawns and gardens, and in sewage from improperly functioning septic systems. Although these nutrients occur in nature, in the form of wildlife feces and decaying organic matter, in excessive quantities they cause large algae blooms that rob the water of oxygen. Fish and shellfish require oxygen in the water; this pollution therefore makes the lagoon less valuable for wildlife. In a more open marine area, these nutrients are more likely to be diluted when mixed by waves and currents with the open ocean. But in the lagoon, mixing is sluggish and the pollution accumulates.
In Portage Inlet, soft sediment has been slowly accumulating on the bottom for thousands of years. The sediment is naturally washed off the land and carried by the two creeks that flow into the Inlet (Colquitz and Craigflower Creeks). Since Portage Inlet is only connected to the ocean by the long, narrow channel of the Gorge, little sediment is flushed out by the tides. Consequently, most of the Inlet is no deeper than 2 metres. This is its natural condition. However, some people are concerned that human activities are accelerating the rate of sediment deposition. For example, construction and farming practices, if not carried out carefully, can wash sediment into the creeks. If too much sediment were to become deposited in the Inlet, it would eventually fill up and become a marsh, leaving less habitat for fish and other aquatic organisms.
The problem of excessive sediment is closely related to the problem of erosion. When natural vegetation is removed, particularly from streamsides and shorelines, more bare earth is exposed to the elements and washed into creeks and marine areas including tidal lagoons. Impervious surfaces, i.e. roads and pavement that prevent rainwater from soaking into the ground, also contribute to erosion and sedimentation, as well as pollution.
Rainwater flows across these surfaces very quickly, and causes the water flows in streams to suddenly increase. These sudden “flash floods” can wear away stream banks and carry the eroded soils into tidal lagoons. Moreover, as rainwater flows over roads, driveways and parking lots, it picks up an array of toxic chemicals.
How can I help protect tidal lagoons?
For information on protecting lagoons, please visit our How Can I Help section.
© Image courtesy of Tim Flickr (www.flickr.com)