Anyone who has spent any time on the coast knows that the sea level periodically rises and falls; this regular motion is called the tide. The sea level rises with the flood tide and falls with the ebb tide. Technically, a tide is the deformation of earth and water materials at different places on earth in response to the combined gravitational effects of moon and sun. Although the sun produces tides, the effect of the moon is much greater, since it is much closer to the earth. Therefore, we will consider only lunar tides in this section.

What causes tides?

Basically, the gravitational pull of the moon causes a “bulge” on either side of the earth’s oceans, in line with the moon. This occurs due to differences in the gravitational force at different points on the earth, relative to a point at the centre of the earth. At the point closest to the moon, the moon’s pull is strongest and at the point furthest from the moon, the moon’s pull is weakest. Consequently, the overall shape of the earth, and the layer of oceans surrounding it, is slightly compressed.


 
Image: Bulge of water (greatly exaggerated) caused by the gravitational pull of the moon; arrows represent net forces caused by differences in gravitational pull over the entire earth.

The bulges cause high tides. Some points on the earth pass through both of these bulges during a 24 hour period. Therefore, many locations experience a semi-diurnal tide, or two high tides per day. Because the moon’s orbit is at an angle to the earth, other areas “miss” passing through one of the bulges, and therefore experience only one high and low tide per day; this situation is called a diurnal tide. Victoria experiences a mix of diurnal and semi-diurnal tides.

When the moon is aligned with the sun, at the new moon and full moon, the sun’s gravitational pull adds to that of the moon. This causes spring tides, which result in higher than usual high tides and extra low tides. The opposite are called neap tides, and occur when the sun is positioned at right angles to the moon, causing the sun’s gravitational pull to counteract the moon’s. The variation between high and low tide is much less extreme during neap tides. 


 

Image: Spring and neap tides, represented with a greatly exaggerated tidal bulge

Tides also vary depending on the latitude and local geography. At locations on the earth in line with the moon’s orbit, generally at higher latitudes, the tidal range is greater. Geographically, a shallow, narrow inlet will experience a larger tidal range than a straight, steep-sided coastline. Tidal currents result when the flow of water with the changing tide is constricted by a channel or headland.

How do tides affect marine life?

The vertical movement of water once or twice a day has tremendous importance for intertidal biotic communities, those plants and animals that colonize the shoreline between the high and low tides. They must be able to withstand a number of stresses such as the force of waves, exposure to the air and changes in salinity. With exposure to the air they are at risk of desiccation (drying out), freezing, overheating or starvation, since most animals can only feed when submerged.

Tides therefore create niches in the shoreline ecosystems that are available to different organisms, depending on their tolerance to these conditions. This leads to intertidal zonation, the tendency of plants and animals to form visible communities along the marine shoreline, between the high and low tide lines.

Tide pools are depressions in rocks that are flooded regularly with sea water, but isolated at low tide. They provide rare opportunities to observe marine life that may normally live below the low tide line in the subtidal zone. Many intertidal and subtidal areas are particularly rich in marine life due to their exposure to strong tidal currents, which carry abundant nutrients. Race Rocks, a Marine Protected Area west of Victoria, is a good example. Some areas in the harbours of the CRD that experience significant tidal currents include the Gorge Waterway and the entrance to Esquimalt Harbour.

Tides control salinity and the flushing of wastes and nutrients in tidal lagoons, estuaries and tidal rivers. The high ecological value of estuaries is due primarily to the influence of tides. Esquimalt Lagoon and Portage Inlet are two local examples of tidal lagoons.

Tides also affect erosion and coastal sediment processes, since they control to what height waves can influence the land. Storm surges (high energy waves) that occur during spring tides can be particularly damaging to the shoreline and/or human property.

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