Aquatic plants, scientifically known as macrophytes, are vegetation found in freshwater systems like lakes, rivers, and wetlands. This group includes flowering plants, ferns, mosses, and certain large algae adapted to live either fully or partially submerged. Their presence establishes the foundation of the aquatic food web and influences the physical and chemical properties of the water.
How Aquatic Plants Are Grouped
Freshwater flora is generally categorized into three main groups based on their relationship to the water surface and the substrate. Emergent Plants are rooted in the sediment underwater but extend the majority of their stem, leaves, and flowers above the water line. These plants, such as cattails (Typha) and bulrushes, are often found along the shallow margins and shorelines. Their stiff stems allow them to withstand wave action and thrive in waterlogged soils where many terrestrial plants cannot survive.
Submerged Plants live entirely beneath the water surface, occasionally reaching the top layer but rarely breaking the plane of the water. Species like coontail and American pondweed possess flexible stems and finely divided leaves, adaptations that minimize resistance to water currents. These plants absorb nutrients directly from the water column rather than solely through their roots. Submerged plants rely on water clarity for photosynthesis, limiting their depth to where sufficient light can penetrate.
The final category includes Floating Plants, which are divided into two distinct types. Rooted-Floating Plants maintain an anchor in the sediment while their leaves float flat on the surface, exemplified by water lilies and water-shield (Brasenia schreberi). These large, flat leaves shade the water below, which helps regulate water temperature and prevent the growth of submerged species in deeper areas.
Free-Floating Plants exist completely unattached to the bottom substrate. These species, which include tiny duckweed (Lemna) and the invasive water hyacinth (Eichhornia crassipes), drift freely on the surface, absorbing nutrients directly from the water. They often reproduce rapidly, forming dense, multi-layered mats that cover the surface of slow-moving water bodies.
The Functions of Freshwater Flora
Through photosynthesis, submerged plants release dissolved oxygen directly into the water column, sustaining fish and aquatic invertebrates. Emergent species utilize specialized tissues called aerenchyma to transport atmospheric oxygen down to their roots. This process improves the chemical conditions in the surrounding sediment.
Aquatic plants function as natural water purification systems by absorbing excess nutrients like nitrates and phosphates. They act as a biological sink, sequestering these compounds and preventing them from fueling excessive algal growth. Additionally, the root systems of these macrophytes stabilize the bottom sediment. This stabilization prevents fine particles from being resuspended into the water column, thereby increasing water clarity.
The physical structure created by the plants offers habitat and shelter. Dense stands of vegetation provide cover from predators for juvenile fish and serve as nesting sites for various bird species. The plants themselves are a direct food source for many herbivores. The surfaces of their leaves also host a community of smaller organisms.
When Aquatic Plants Become a Problem
Rapid, uncontrolled growth often signals an environmental imbalance or the introduction of non-native species. Invasive species possess a competitive advantage, allowing them to proliferate aggressively. Eurasian watermilfoil (Myriophyllum spicatum) is a common example, forming thick, tangled mats just beneath the water surface in North American and European waterways.
These dense surface canopies block sunlight from reaching the native plants below, reducing the diversity of local flora. When the biomass of these invasive plants dies off, the subsequent decomposition process consumes large amounts of dissolved oxygen. This biological oxygen demand can lead to anoxic conditions, causing stress or mortality for fish and other aquatic life. Furthermore, the physical presence of these dense mats restricts recreational activities, impeding the movement of boats and swimmers.