Plants are primary producers that provide food and nutrients that support herbivores and ultimately predators as well. Without plant life, nothing else would survive. In aquatic systems, plant life consists of single celled microscopic plankton, as well as vascular plants. Both play a key role in freshwater ecology.
Healthy Freshwater System
Nutrients are essential for both plant and animal growth. Herbivores get their nutrients from plants, and carnivores get their nutrients from herbivores. But as primary producers, plants and algae get their nutrients from the environment in the form of nitrates, phosphates and minerals. By transforming these nutrients into carbon by harnessing energy from the sun in the process of photosynthesis, plants are able to sustain other life on earth.
In a healthy, balanced freshwater ecosystem, photosynthesizing aquatic plants absorb nutrients and carbon dioxide from the water column in the presence of sunlight, transforming them into carbon and releasing oxygen in the process. This oxygen allows aquatic animals, such as fish and freshwater invertebrates to survive underwater. Should oxygen be depleted, these aquatic organisms would die. The animals in turn release carbon dioxide which is absorbed by the plants. The ecosystem is healthy and balanced.
Unhealthy Freshwater System
Nutrients occur naturally in the environment as a result of decomposition and other processes, but are also added to the environment by man made activities. These include runoff of fertilizer and animal waste products from agricultural practices, from sewage runoff and discharge from wastewater treatment plants, which can result in nutrient loading. When excessive nutrients flow into a waterway, both unicellular algae and aquatic plants take advantage of the available nutrients and will flourish, removing these 'contaminants' from the water in the process.
However, when conditions are favorable, aquatic plants can quickly multiply to form dense algal blooms or mats of weed that cover the surface of a pond or lake. The system rapidly becomes unbalanced, often with dire consequences.
In the case of unicellular algae, the algal cells rapidly grow and reproduce while nutrients and light are abundant, but once the nutrients in the surface water are depleted they have to move deeper to find nutrients to sustain them. Eventually there is insufficient nutrients in the surface layers, and as light cannot penetrate the deeper water layers, there is insufficient light in the deeper, nutrient rich layers. When this point is reached, the algae cells die off and sink to the bottom of the lake, where they decompose. Oxygen is stripped from the water during the decomposition process, resulting in water that is low in oxygen, which can result in mass mortality of fish and other aquatic organisms that require oxygen to survive.
Aquatic plants, especially fast growing non-native species that herbivores tend to avoid, can also spread rapidly, clogging waterways and making it difficult for fish and other organisms to move about with ease. Aquatic organisms may die or move away. A once thriving ecosystem, home to a rich biodiversity and abundant wildlife is reduced to a monoculture of impenetrable aquatic weed.
Balance is Key
Aquatic plants play a vital role in maintaining ecosystem health and supporting biodiversity. However, it is essential that the ecosystem remains in balance for it to function effectively.
Algal blooms and the spread of invasive aquatic plants are both likely to occur more frequently in the future as a result of climate change, more intensive agricultural activities, ecological imbalances, and/or as a result of invader species changing ecosystem dynamics. We therefore need to find creative ecologically sound solutions to maintain balanced freshwater ecosystems and to protect biodiversity.