What are tides?
Tides are the periodic rise and fall of sea level due mainly to the gravitational and rotational forces of the Earth, moon, and sun. Most coasts experience semidiurnal tides – two highs and two lows each day. Some regions, like the Gulf of Mexico, experience diurnal tides: only one high and low per day.
You can't talk about Gulf of Maine water movement without mentioning tides. After all, the great tidal range of the Gulf of Maine is one of its most famous features. Whereas tidal range just south of Cape Cod is rarely over 4 feet, it increases dramatically as you move north and east. Passamaquoddy Bay sees tidal ranges of 28 feet, and the Bay of Fundy experiences the highest recorded tides in the world at 50 feet! (See image from Minas Basin, Nova Scotia, at right.)
Tides have significant impact on the structure of the coastline. Their power can bring mud to mud flats or salt marshes, contribute to the formation of sand bars, and increase erosion. Tides also affect the current flow and mixing of the water column.
Tides also provide a unique habitat: many organisms live between the high and low tide marks along the coast, in the tide pools created by the higher water receding, and in the "spray zone" above the high tide mark.
Beyond the Gulf: What causes ocean tides?
Tides are caused by: 1) the gravitational attraction of the sun and moon on the earth; and 2) Earth's rotation. The earth revolves around the sun, and the moon revolves around Earth; all three planetary bodies exert gravitational forces on ocean waters. Despite the sun's great mass, the moon exerts a greater force on ocean tides because it is much closer to Earth.
In addition to gravitational forces, the earth and moon generate a centrifugal force as they rotate. When the sun, earth, and moon are aligned, their gravitational forces and the centrifugal force complement each other and we get the highest tidal range. This called a "spring tide" and occurs at the full moon and the new moon (see Fig. 1). When the sun-earth-moon system form a 90° angle, we get the smallest tidal range. This occurs at the first and third quarter moons and is called a "neap tide" (see Fig. 2).
To complicate matters further, ocean tides are significantly modified by coastlines and sea bottom topography. In addition, the tilt of Earth on its axis contributes to unequal high and low tides. To learn more about tides visit the NOAA / National Ocean Service's website "Our Restless Tides: A Brief Explanation of the Basic Astronomical Factors Which Produce Tides and Tidal Currents."
- Apollonio, S. (1979). The Gulf of Maine. Courier of Maine Books.
- Conkling, P.W. (ed.). (1995). From Cape Cod to the Bay of Fundy: An Environmental Atlas of the Gulf of Maine. MIT Press.
- deCharon, A. (1999). Virtual Vacationland. URL: http://www.bigelow.org/virtual.
- deCharon, A., and M. Sieracki. (1999). Ship Mates: Explore the Gulf of Maine as Oceanographers Do. URL: http://www.bigelow.org/shipmates.
- Lalli, C. M. and Parsons, T. R. (1993). Biological Oceanography - An Introduction. Butterworth-Heinemann.
- NOAA / National Ocean Service. (1998). Our Restless Tides: A Brief Explanation of the Basic Astronomical Factors Which Produce Tides and Tidal Currents. URL: http://co-ops.nos.noaa.gov/restles1.html
