Aquatic environments host an astonishing variety of plant life that has adapted to life in and around water. Aquatic plants, also known as hydrophytes, are uniquely specialized organisms that have evolved specific structures to survive prolonged submersion or saturation. These plants form the foundational structure of the aquatic food web and are essential to the health and function of the entire pond ecosystem. The diversity of aquatic flora is typically categorized by their location within the water column, with each group playing a distinct role in maintaining water quality and supporting wildlife. The presence of these specialized plants helps ensure a balanced environment where fish, amphibians, and insects can thrive.
Submerged Aquatic Plants
Submerged aquatic plants grow entirely beneath the water surface, with some species rooted in the bottom sediment and others floating freely in the water column. These plants possess specialized features to cope with their underwater existence, such as a lack of rigid stems because the surrounding water provides physical support. They often have finely dissected leaves, which increases the surface area for absorbing nutrients and dissolved gases directly from the water. Unlike terrestrial plants, they do not require a thick cuticle, which prevents water loss, since they are constantly immersed.
A primary function of these plants is their direct absorption of nutrients like nitrogen and phosphorus from the water column. This process actively competes with algae for food sources, helping to keep the water clear. Common examples of rooted submerged plants include Elodea and various pondweeds (Potamogeton species), while Coontail (Ceratophyllum demersum) is an example of an unrooted species. These plants are sometimes called “oxygenators” because they release oxygen into the water during photosynthesis, which is beneficial to aquatic animals.
Free-Floating Aquatic Plants
Free-floating aquatic plants are not anchored to the pond bottom; instead, they drift freely on the water’s surface with their roots hanging down into the water column. This unique positioning allows them to absorb nutrients directly from the water via their dangling, lace-like roots. They are characterized by rapid growth rates, often reproducing through runners or vegetative budding.
These plants can quickly cover large areas of the pond surface, which is a double-edged sword for the ecosystem. Common examples include the tiny Duckweed (Lemna spp.), Watermeal (Wolffia spp.), and the larger Water Hyacinth (Eichhornia crassipes) and Water Lettuce (Pistia stratiotes). Their ability to spread rapidly means that pond managers must monitor and control their populations to prevent them from completely covering the water.
Emergent and Marginal Plants
Emergent and marginal plants occupy the shallow edges of the pond, forming a transition zone between the aquatic environment and dry land. These plants are rooted in the saturated soil of the shoreline, but their stems, leaves, and flowers grow up and out of the water. Because their foliage is exposed to the air, they require more rigid stems than their submerged counterparts to support their weight and withstand wind.
This group includes easily recognized pond plants, such as the tall Cattails (Typha spp.), Pickerelweed (Pontederia cordata) with its spikes of blue flowers, and various species of Iris. Marginal plants depend on having their roots wet or submerged to access nutrients. The root systems of these shoreline plants play a role in securing the pond’s edge.
Ecological Functions in the Pond
The various groups of aquatic plants work together to perform several functions for a balanced pond ecosystem. Submerged plants are the primary producers of dissolved oxygen, which fish and other aquatic life need to breathe. This oxygenation is a direct result of photosynthesis occurring underwater.
All types of aquatic plants, particularly the submerged and free-floating varieties, are effective at nutrient removal, absorbing excess nitrogen and phosphorus from runoff. By absorbing these nutrients, the plants limit the food supply for nuisance algae, helping to maintain water clarity. Marginal and emergent plants, with their dense, fibrous root structures, stabilize the shoreline. These roots hold the soil in place, preventing erosion and keeping sediment from clouding the water. Additionally, the dense foliage provides shelter and habitat for juvenile fish, amphibians, and invertebrates, protecting them from predators.