Students of the history of the earth are resigned to the fact that geologic events like mountain-building and sea-floor spreading take millions of years to happen. During a lifetime, earth scientists rarely witness significant changes in mountains, seas or climates. Small wonder that they flock to any scientific arena where dramatic action is guaranteed. Where do some scientists go to study a rapidly-changing ecosystem? They go to the beach!
The beach is constantly in motion. Tides, surf and winds change its appearance daily. Erosive storm waves in winter and gentle waves in summer account for seasonal variations. In addition, longshore currents, negligible but persistent, can alter the beach over a period of several years.
On a summer beach, it is pleasant to speculate about the origin of the sand that squishes between one's toes. It is rather awesome to contemplate the same thought after a winter storm, for the entire summer beach may have disappeared. A stark, narrow ridge of sand and rocks has replaced the broad, flat beach. High on the beach, like stranded whales, lie rounded boulders too heavy to be lifted by surf and tides. Storm waves have carried away the sand of summer and deposited it on offshore sand bars. While breakers pound the coast, the summer beach rests a few hundred feet from shore.
In the spring, small constructive waves begin to rebuild the beach. They are strong enough to lift the sand grains onto the shore, but too weak to haul them back out to sea again.
If the seasonal movement of sand on and off shore were the only force acting on the coastline, beaches would stay the same from one year to the next. As anyone who owns beachfront property will tell you, this is not the case! Over the years, alongshore movement of sand removes sand from one area, typically where it is wanted, and deposits it in another, usually where it is not wanted. This sand shift is generated by storm waves that strike the beach at an angle, bounce back at an angle, and pick up sand grains as they rebound. The zigzag action of waves hitting the shore has the overall effect of moving large quantities of sand parallel to the beach just beyond the breakers.
Visiting the sandy shore in winter provides a new perspective on the beach. It vividly portrays how harsh this habitat is for the plants and animals that must withstand the drying wind, salt spray, storms, freezing temperatures and constantly shifting sands. These ever-changing conditions prohibit intertidal plants and animals from establishing permanent residence on the beach. Because they have no place to anchor themselves on the shifting sands, no seaweeds except microscopic algae live on the wave-washed portion of the beach. Crabs, worms and mollusks are noticeably less evident, as they must burrow into what little sand there is to escape waves, wind, cold temperatures and predators. Except for the lines of debris, seaweed and dead organisms stranded at the high tide mark, the winter beach appears desolate and lifeless.
If you look closely, though, you will find numerous examples of adaptation to winter conditions. As winter progresses, periwinkles migrate down the shore to reduce their air exposure time to only a few hours. You'll also find snails and crabs who have sought shelter under seaweeds or rocks to avoid the freezing winds. Intertidal organisms will also relocate to high salinity tide pools, as a higher percentage of salt results in a lower-temperature freezing point. Like cats, some dark-pigmented seaweeds and mobile animals can also expose themselves to the winter sun to absorb solar radiation, which will help raise body temperature and prevent freezing.
Many organisms also benefit from metabolic or biochemical changes in cold weather. Some animals produce glycerol, an alcohol compound that works as an antifreeze. Seaweeds, mussels and fish are known to increase the salt content of their body fluids by either losing water or taking in salt. Again, this higher salt content helps to prevent freezing.
As foreboding as the winter beach appears, it is teeming with life. Some would say that there is no greater pleasure than to walk a deserted beach on a cold, sunny day in winter.
The Winter Beach was written by Mary Cerullo, who has authored a number of articles, curricula
and special projects the Gulf of Maine Aquarium. Special thanks to Ray Gerber, professor at St.
Josephs College in Windham, for his writing on animal adaptations.