Exploring Coastal Plant–Water Interactions

Imagine standing at the coast and looking out to the sea. It might look like a vast, empty space, but beneath the surface, something amazing is happening: under water and along the coast, plants like salt marsh grasses and seagrasses are quietly doing something incredible. These plants help protect people from sea-level rise. They also offer homes to animals like fish, crabs, and birds, and they even help reduce climate change. Unlike most plants you can find on land, coastal plants can survive salty seawater and being under water for part, or even all, of the day. If you watered your houseplants with seawater or put them underwater all the time, they would die. These coastal plants, however, are built for this salty world. When the tide comes in, they may be completely underwater. When the tide goes out, they can be exposed to the air again. While the plants are underwater, they are affected by how the water moves.

At the coast, when the wind is pushing against you, you might lean into the wind to stay standing still but flexible plants like grasses cannot do that. They are more like your hair, bending, fluttering, and moving with the wind. However, the grasses can also affect the way that the water moves because they can partially block the flow of water. But how does this interaction between the moving water and flexible plants, such as grasses, work exactly?

Hydrodynamics: How Water Moves

There are two main ways that water can move at the coast, and if you have spent a whole day at the beach, you have seen both in action. The first way is through waves. Wind blows across the surface of the sea, creating ripples that grow into waves. The stronger the wind, the bigger the waves can get. Although it looks like waves travel toward the beach, the water particles only move in circles and transfer the energy of the wave from one particle to the other (Figure 1A). The second way water can move is through the tide. The tide makes the water at the coast rise and fall, usually twice per day. When rising, the tide can flood the land with seawater, and when it falls, it pulls the water back to sea. This flow of water in and out is called the tidal currents. The water particles move along with these currents (Figure 1B).

Together, waves and tides are often referred to as hydrodynamics (meaning moving water). Hydrodynamics push and pull on everything in their way, including plants. Hydrodynamics affect how coastal ecosystems look. In areas with strong wave action or fast tidal currents, plants will struggle to survive. In sheltered areas, the plants have evolved to specifically deal with and take advantage of the saltwater the tides bring in or the waves that move them around. Some plant types even rely on gentle water movement to spread their seeds or pieces of their roots to new places, helping them colonize and expand.

Flexible Plants

There are many different types of coastal plants. Whether it is a salt marsh plant or a seagrass, all coastal plants have special traits that help them deal with waves and tides [1]. Some plants are stiff and stand firmly against hydrodynamics, like trees against the wind. Others are flexible and move with the water, like a field of grass does in the wind. Under the waves, they move back and forth with the circular movement of water particles (Figure 1A), while under tidal currents, they bend in one direction (Figure 1B).

Different types of plants respond to water forces and movement in different ways. Stiff plants are good at slowing down waves but may break more easily under high waves or strong currents, like the big trees snapping in a windy forest. Flexible plants, on the other hand, can move with waves and tides, like the grasses in a field, and they break less easily. In their research, scientists look at things like how strong and flexible a plant stem is, whether the stem is hollow or solid, or whether the stem is thick or thin. These details all affect how a plant survives in the moving water. Roots are important too. Coastal plants need strong roots to stay in place when waves and tides try to wash them away. Plants with deep or wide root systems can hold on better during a storm.

The overall number of plants, their shapes, how close together they grow, and the total area they cover are important too [1]. When many plants grow close together in a meadow, they form what looks like a green wide wall. Salt marshes are full of different plant types, some stiff, some flexible, some tall, and some short. This mix creates a strong team that can spread out over wide coastal areas. Seagrasses, which grow underwater all the time, are usually very flexible. They bend and sway with waves and tides, like grass in the wind. This flexibility helps them avoid breaking and allows them to thrive in constantly moving water. So, to understand how coastal ecosystems interact with waves and tides, we need to study more than just one type of plant—we need to study the teamwork between plants.

How Plants and Water Work Together

What happens when waves and tides flow through a patch of coastal plants? First, the plants slow the water down. Their stems and leaves partially block water flow, reducing its speed (Figure 2). They do this along their full height, which is why tall plants reduce water flow more than small plants. The amount of light, the temperature, the type of soil, and other conditions can influence how plants grow and respond to flowing water. For example, a plant that gets only a little sunlight might grow larger or thinner leaves, which changes how it affects the waves and currents. If multiple plants grow in a group and there is little space between the plants, it is difficult for the water particles to move between them—a bit like cars on a narrow street compared to a wide motorway. The more plants there are, and the more space they cover, the more they slow the water down (Figure 2).

At the same time, the water affects the plants. If the current is strong, it can bend the plants or even pull them out of the ground, depending on the plants’ flexibility and strength [2]. Many plants, like seagrasses, are flexible, so they bend with the flow instead of breaking. Bending makes plants appear smaller than they are because they do not stretch to their full length. The height of the plants in the water is called the canopy height [3], and plants can only slow down the water that flows below the canopy height. That is why the canopy height is more important for slowing down the flow than the actual height of the plant. So, how much the flow bends the plants also influences how much the plants reduce the water flow. In this way, the plants and the water are constantly affecting each other. Water changes the shape of the plants, and the plants change how the water flows [4]. It is like a circle: one affects the other, and the other affects the one, over and over again (Figure 3).

Coastal Plant–Water Interactions Matter

Along the coast, water and plants constantly influence each other. The moving water changes the shape and strength of the plants, while the plants slow the water flow. These natural processes shape the coastline and create important places for animals to live.

Observing these interactions is not only fascinating, but it also helps scientists and engineers understand how to protect our coasts better. By learning how nature works, we can find smarter ways to live with water. You can help by keeping these areas clean and by respecting local habitats. Small actions can make a big difference in keeping our coasts healthy. Even discussing or reading more about the benefits of plants can make a difference, because it helps people understand how important the plants at the coastline are. Next time you stand at the coast watching the waves roll in, think about the quiet power of the plants that help protect the land behind you.

Glossary

Climate Change: ↑ When Earth’s temperatures and weather patterns change. This can be natural but nowadays is mainly caused by things people do, like burning fuels and cutting down trees.

Hydrodynamics: ↑ The study of how water flows, for example by waves and tides.

Canopy Height: ↑ The height of a group of plants that changes when the plants bend. If plants are not bent, the canopy height will be high; and if they are bent, the canopy height will be lower.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

AI Tool Statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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