The Effect of Wave Action on Shore Organisms

 

 

Waves can be nice and gentle, Or.............not so!!

In relation to rocky shores, the type of waves which on average assail it give rise to discussion about its degree of exposure (to wave action).

Waves on any body of water are caused by wind, and the size of the waves is largely dependent on the distance that wind can blow across the water without interruption, this is called the 'fetch'. The other factor is the wind direction. Thus, a headland facing West into the Atlantic may be expected to suffer from much greater wave action than an inlet facing East.

Attempts have been made to both measure and estimate wave action. Estimates come from taking into account factors such as wind direction and fetch, but other factors like local topography complicate matters so much, this has not been a very successful approach.

Measuring wave action can be done with a piece of equipment called a dynamometer, here is an account of some recent work:

 

One factor that may affect the abundance of marine organisms – on a small and large scale – is the constant pounding of the surf against the shore.

"It's obvious that when waves crash, they apply really huge forces to things that live out there," Denny said, noting that a 6-foot wave applies the same kind of force to barnacles and other tiny creatures that a 1,000 mph wind would apply to a person standing on land.

"There aren't many things on land that would survive that kind of super-duper hurricane, but that's what these things put up with every 10 seconds when waves break on the shore," he noted.

To measure the force of individual waves, Denny and his colleagues developed a decidedly low-tech device that consists of a plastic golf ball attached to a spring scale.

"We call it a dynamometer, to give it a fancy name, but it's cheap to make, and we make them by the hundreds," he said. To measure wave forces along Monterey Bay, Denny and his coworkers drill holes into the rocky shoreline, then insert a dynamometer into each hole. When a wave hits the plastic golf ball, it stretches the spring, giving researchers an accurate measurement of the amount of force that struck the shore.

"The dynamometers see a lot of force, but it varies from place to place," Denny noted. "For instance, we have one transect with about 100 dynamometers placed about 18 inches apart. What we found is that the most exposed place will see about 13 times the force as the least exposed place just a few feet away."

Denny and his students are in the process of designing a high-tech version of the dynamometer that will replace the metal spring with an electronic sensor that continuously transmits wave force data back to the lab. He and his colleagues also are using nickel-size computers called I-Buttons that record the body temperature of shoreline organisms every 10 minutes over a two-week period.

"The take-home message is that the technology is available to measure these kind of small-scale variabilities," he concluded. "Doing big science on a big scale is important, but if you're really interested in opening the black box, you also have to look at smaller scales to be able to get the whole picture." From here

Probably, still the best way of deciding just how exposed a shore is involves using the organisms themselves, a kind of BIOASSAY. A similar type of thing is done when freshwater biologists use organisms too help them decide how polluted (or not) rivers and lakes are, a well known index being the Trent Biotic Index.

The most well known and widely used of these biological exposure scales is that of W.J.Ballantine. Here is the story of the research that led to his paper:


Dale Fort is remembered by many people as place where they stayed away from home for the first time, learned a lot about ecology, developed a passion for barnacles and a deep respect for the velvet swimming crab. It is also known by people all over the world as the place where the research was done for what may be the most famous paper in the the history of marine biology.

In the mid 1950s a sixth former called Bill Ballantine worked in his summer holidays at Dale Fort. He helped in the kitchens, he helped in the grounds, he helped maintain buildings, he was knocked out half-way up a cliff by a rock dislodged by the over-enthusiastic boot of the then warden John Barrett. On top of all this he developed a deep interest in marine ecology. This resulted in the publication in 1961 of the famous paper: A Biologically Defined Exposure Scale for the Comparative Description of Rocky Shores

It had been long established that exposure to wave action was a vital factor in determining the patterns of organisms that are so characteristic of rocky shores. Attempts to measure wave action in physical terms were (and still are) fraught with technical difficulties. How then could different shores be compared with regard to this important physical factor? Bill Ballantine noticed that the shores around Dale were inhabited by different organisms depending on how much wave action they received. He determined to visit every shore he could get to on the Dale Peninsula and survey the species there.

He then devised a scale which uses the assemblage of organisms on a shore as indicators of the degree of wave action (exposure) that the shore endures. The shore is then allocated a number to indicate its position on the scale. Ballantine's Scale runs from 1 (= extremely exposed) to 8 (= extremely sheltered).

It has generated much controversy during the last 40 odd years of its existence for instance:

It only works up to 100 miles or so from Dale. People have created modified versions for different latitudes, Ballantine suggested that there may be a trend involving exposure and latitude.

The whole thing is based on a circular argument. You are saying: "a shore is exposed (say) because it has these species on it. It's got these species on it because its's exposed".

Despite the above problems (identified in the original paper) the scale has proved extremely useful over the years. It has fulfilled its original aim to provide a means for ecologists to describe rocky shores to one another. It probably appears in more reference lists than any similar paper.

The author left the UK many years ago to take up a post at The University of Auckland, New Zealand. He became heavily involved in marine conservation and monitoring and was instrumental in the setting up of the worlds first marine nature reserve at Goat Island. He has been returning to Dale at regular intervals ever since to keep an eye on our limpet populations. He may have demonstrated an ocean warming trend reflected in the distributions of various limpet species around the UK.

He was awarded an MBE for services to marine conservation in 1999. (From here)

Ballantine's paper is a classic and well worth tracking down, the reference is given on the References page.

In order to get a full flavour of the obvious differences between these two extremes of shore type you should visit, say, a shore in Western Island and one in a sea loch in Scotland. In Ballantines paper shores of varying degrees of exposure in and around Dale are shown, this would be an excellent area to visit for anyone with a deep interest in exposure and its effects. Alternatively go here and here for virtual tours of shores at the extremes of the exposure scale.

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