Tidepools and beaches
Tidepools and beaches are dynamic and ecologically rich environments located along coastlines, primarily shaped by natural forces such as waves, tides, and sediment deposition. Beaches consist of various sediment types, including mud, sand, and pebbles, and are categorized into three main types: sediment strips along rocky coasts, river borders, and regions associated with barrier islands. The structure of a beach includes a sloping foreshore and a more stable backshore, with the foreshore being significantly influenced by wave action and sediment movement. Tidepools, formed by the retreating tide, contain diverse marine life, including seaweeds, mussels, and various species of fish and invertebrates, with their inhabitants varying based on the pool's location and connection to the ocean.
These ecosystems serve critical ecological functions, providing feeding grounds for migratory birds and habitats for numerous marine organisms. However, they are vulnerable to environmental changes and human activities, such as pollution and habitat destruction, which can disrupt the delicate balance of these habitats. Conservation efforts are essential to protect the biodiversity and ecological health of beaches and tidepools, as their sustainability is crucial for the broader marine environment and the interconnected life forms that depend on them.
Tidepools and beaches
The term beach indicates the muddy, sandy, or pebbly shore of a body of water such as an ocean or lake. Most beaches are located around Earth’s seacoasts. They are composed of mud, sand, and pebbles, sediments that accumulate in arrangements and forms dependent on coastal actions, sediment sources, and rates of sediment deposition. There are three main beach types—sediment strips edging rocky coasts, borders of sediment accumulations along rivers, and sediment regions parallel to coasts and associated with barrier islands.
At most times, a beach has a seaward, sloping foreshore lacking vegetation and a landward, horizontal backshore that can hold some hardy plants. Near the land’s end, waves rise and break along the foreshore. Sediment is moved both along this shore and perpendicular to it. During storms, waves erode beaches, moving sediment back into oceans. This temporarily leaves only the foreshore. In the ensuing calm, waves move sediment landward again and rebuild the beaches.
Due to rapid changes in wave sizes, the appearance and sediment types of a foreshore often change significantly from day to day. Backshores, normally unaffected by waves, are altered most by winds. There are also variations in beach form along and perpendicular to the beach shoreline. The most common are undulating foreshores that vary from beach to beach and time to time. Beach sands in areas having mild climates are mostly quartz and feldspar. In tropical regions, beaches are often made of calcium-containing remains of marine organisms.
Natural Forces and Beaches
Beaches develop along coastlines due to natural processes, including waves, longshore currents, tides, climate, and gravity. Waves, in their continual beachward motion, vary in size at different coastal areas and at different times. They interact with the ocean bottom as they move into shallows, suspending sediment and moving it landward. Large waves suspend sediment from deeper water and move larger sediment particles than small waves. For example, small waves only move sand, while large waves can even move boulders. During storms, large waves return beach sediment to deep water. Waves also erode coastal bedrock by abrasion from suspended pebbles and larger rock fragments.
The important effects of climate on beach development begin with rainfall, which creates runoff streams, transports sediment to seacoasts, and causes differences in the volume and types of sediment sent to coasts in different world areas. Temperature is also quite important because it causes weathering of coastal sediment and rock. This is most extensive in cold areas, where water freezes in rock cracks, causing their fragmentation. Wind action is also important because of its relation to wave size. Coasts with fairly constant, strong winds have high-energy waves. Onshore winds around the Earth’s coasts produce sand dunes wherever sediment is available and can accumulate. Gravity acts indirectly in wind and wave production and directly in the down-slope movement of sediment.
Most beaches cannot support large plants, though some hardy plants are found on their backshores. Near beach waterlines, organic matter such as decaying seaweed can be found. However, a bit further seaward, mud, and sand flats (intertidal flats) hold burrowing animals, including worms, clams, mussels, and burrowing shrimp. The worms ingest sediment and eat the organic matter it holds. Other burrowing organisms, such as clams, use tubes to reach into the water above them to filter out food when they are covered by the tide. Crustaceans and starfish also use intertidal flats to seek prey.
Intertidal flats are important feeding grounds for wading birds, such as sandpipers, terns, and plovers. In temperate climates, these birds remain all year. Hundreds of thousands of them also make seasonal migrations to specific beaches located between their summer and winter habitats. During such migrations, the birds rely on intertidal flats for food along the way. For example, hundreds of thousands of sandpipers stop each July and August on the intertidal flats of Canada’s Bay of Fundy, each eating tens of thousands of burrowing shrimps before they migrate to South America.
Tidepools
Tidepools are pools of various types and sizes originally or permanently composed of water left behind by the ocean when the tides go out. Their presence is due to the topography of beaches and coasts. Some tidepools occupy beach crevices or fissures with seaward ends open to the sea and landward ends abutting cliffs, caves, or boulders. Others occur in rocky basins with high rims on the seaward side. The rims hold back water when the tide ebbs. Seaweed lines a tidepool’s walls or bottoms. Sponges, hydroids, anemones, sea slugs, insects, mussels, fishes, jellyfish, and starfish live in many pools, either temporarily or permanently.
Fish and jellyfish may become temporary tidepool residents when swept into them by tides. Almost as suddenly, these visitors enter the pools to seek prey and leave with one of the next few tidal cycles. Permanent tidepool inhabitants include seaweeds. The seaweed species depend on the position on the beach of a given tidepool, the acidity or alkalinity of the pool, and its salt content. Tidepools high on a beach contain only plants. This is because they are almost entirely isolated from the sea, and so their temperatures are too high to allow animal survival. Even plants may have some trouble living in such pools.
Lower on the beach, tidepools provide far more stable conditions because they are connected to the ocean, filling and only partly emptying during tidal cycles, so plants and animals live in them. Yet they, too, are affected by the duration of the ocean’s presence or absence, and the inhabitants of a pool in the middle of the foreshore are very different from those of low-foreshore pools, which are separated from the sea only very briefly.
Other Types of Beach Sites
Beaches form along depositional coasts wherever sediment accumulates due to longshore currents, waves, and tides. Locations other than long, open coastal areas are associated with river deltas, estuaries, lagoons, and barrier islands. Deltas are accumulations of sediment at river mouths. They vary in size and shape. However, all require that more sediment is deposited at a river mouth than is carried into the ocean and that shallow sites are available for sediment accumulation. Delta size is proportional to the size of the river involved, and delta shape is dictated by the strength of the river, its tides, and the waves. Deltas can be river-dominatedsuch as the Mississippi River Deltawhen waves and tides do not affect water discharge or accretions of sediment. Such deltas are irregularly shaped. A wave-dominated delta experiences sediment erosion that simplifies and smoothes its edges. Tide-dominated deltas are funnel-shaped.
Irregularly shaped depositional coasts are rich in stream-fed bays or estuaries. They receive a lot of sediment due to coastal runoff. Seaward estuaries are sandy barrier islands that parallel the shoreline, formed by waves and longshore currents. The islands are separated from the mainland by lagoons, which are long, narrow bodies of water. Barrier islands contain well-developed beaches, dunes, and tidal flats on their landward sides.
Beaches are very important land features. They serve to protect coastlines from storms and erosion. In addition, they are ecologically important, serving as homes in intertidal flats and tidepools for useful and sometimes rare flora and fauna. However, when humans remove important species, like muscles, anemones, urchins, sea stars, and indigenous plants like surfgrass from these ecosystems, the balance of marine life is altered, which can disrupt the health of all organisms in the tidepool. For example, if seagrass is removed from a tidepool, the water temperature may increase drastically, and light may increase to an unlivable degree for some species. As key links in the food chain fail to adapt to this change, more species are impacted as their food sources and hiding spots are eliminated. Sometimes, this can even occur by accident. During low tide, moon snails lay their eggs on the sand with a mucus that forms a dark casing around the eggs. Humans may mistake these for trash and throw them away.
Climate change is also changing the Earth’s coasts and tidepools. Some animals have adapted to thrive in warmer water, but in species that need cold water to live, populations continue to decline in the twenty-first century. In some areas, tidal pools and beaches are protected by various governmental and conservation agencies, but the harmful impact of human activity is long-lasting, and the repercussions of things like chemical groundwater runoff and oil spills may take decades to fully reveal their harm.
Principal Terms
Backshore: the horizontal beach part farthest from the ocean
Barrier Islands: islands seaward of estuaries, parallel to the shore and separated from it by a lagoon
Delta: an accumulation of sediment at a river mouth
Estuary: a stream-fed bay
Foreshore: the sloping beach part closest to the ocean, which is continually altered by waves, tides, and longshore currents
Intertidal Flat: a beach part located in shallow water just seaward of the foreshore, made of mud and/or sand, holding burrowing animals
Longshore Current: a current parallel to a seashore, from shoreline through breakers, created by waves approaching the coast at a small angle to the shore
Tide: the regular rise and fall of sea level due to gravitational fields of the moon and sun
Tidepool: a pool of water originally or permanently composed of water left behind at ebb tide
Bibliography
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Braun, Ernest. Tideline. Viking Press, 1975.
Carson, Rachel, and Bob Hines. The Edge of the Sea. Canongate, 2021.
Denny, Mark W., and Steven D. Gaines. Encyclopedia of Tidepools and Rocky Shores. University of California Press, 2007.
Kelly, Ryan P., et al. Between the Tides in California: Exploring Beaches and Tidepools. University of Washington Press, 2024.
Lencek, Lena, and Gideon Bosker. The Beach: The History of Paradise on Earth. Viking, 1998.
Levinton, Jeffrey S. Marine Biology: Function, Biodiversity, Ecology. 6th ed. Oxford University Press, 2022.
United Parks and Resorts. "Human Impact and Conservation." SeaWorld Parks & Entertainment, seaworld.org/animals/ecosystems/tide-pools/human-impact-and-conservation. Accessed 30 Sept. 2024.
"What is a Tide Pool?" National Oceanic and Atmospheric Administration, 20 Jan. 2023, oceanservice.noaa.gov/facts/tide-pool.html. Accessed 30 Sept. 2024.