they fix carbon, producing energy-rich sugars and starches. Other members of the aquatic community that eat plants are termed herbivores. They include fish, turtles, snails, insects, and some birds. Carnivores in turn depend on the herbivores for food. Predatory birds such as osprey, eagles, herons, and egrets are at the top of the food chain in these systems (Figure 1.5). Directly or indirectly all are dependent on plants.
Sugars and starches are not the only product of photosynthesis; the process also releases oxygen. In aquatic systems the oxygen produced by green plants is important in meeting the respiratory needs of plants and animals.
Limiting factors—The need for light to drive photosynthesis limits the depth to which plants can grow in a lake; this is especially true for rooted submergent species. Light does not travel as far through water as through air. In addition, plankton and suspended sediments create turbid conditions that limit light penetration. The clearer the water, the greater the depths at which rooted submergent plants will be found. See also discussion of stem length and branching patterns in the section on growth habit above.
Emergent, floating-leaf, and free-floating plants have unimpeded access to sunlight. However, they can form a canopy that reduces the amount of light reaching lower layers of aquatic vegetation, just as in a forest.
Figure 1.4. An aquatic food chain: powered by the sun, plants and algae in and around the lake (primary producers) photosynthesize, producing sugar and starch. Plants are food for many small fish, insects, and other invertebrates such as snails (herbivores), which are in turn consumed by carnivores. Larger fish and birds of prey are often the top carnivores in freshwater systems.
Figure 1.5. The Great Blue Heron and other fish-eating birds are top predators in aquatic ecosystems.
Carbon dioxide is the source of carbon for photosynthesis in terrestrial plants; in aquatic systems the carbon source can be either dissolved carbon dioxide (CO2), bicarbonate
Dissolved CO2 is used by most aquatic macrophytes and algae. Some algae and submersed macrophytes, including mosses and quillworts, can use only CO2. Submersed, rosette-type plants, including water lobelia (Lobelia dortmanna) and quillworts (Isoetes spp.), obtain up to 90 percent of their carbon dioxide directly from lakebed sediments through their roots. Interestingly, these plants have stiff leaves with a cuticle, which prevents loss of CO2 through diffusion into the surrounding water. Other plants, such as waterweed (Elodea spp.), can switch to bicarbonate when free CO2 is in low supply; however, photosynthesis under those conditions is less efficient.
Another modification seen in hydrilla, Brazilian waterweed, and common waterweed is similar to the C4 photosynthesis seen in warm season grasses and other terrestrial plants in high light/high temperature conditions. This allows the plants to capture carbon dioxide in the dark, later converting it to typical C3 sugars (Casati et al. 2000).
Decomposers
Detritus feeders and decomposers that feed on decaying plants and other organic matter form another important link in the system. These organisms release minerals to be utilized in a new cycle of growth. Decomposition may be slow in aquatic systems because of a lack of oxygen at the lake or stream bottom; very acidic conditions also inhibit decomposition, leading to a buildup of peat.
An excessive amount of organic matter, such as occurs when high nutrient levels stimulate an algae bloom, may create anoxic conditions as decomposers deplete the oxygen level, depriving other organisms of an adequate source.
Habitat Structure
The zone in which aquatic plants can grow is called the littoral zone (Figure 1.6). Greater water depth, where light cannot reach the bottom to support photosynthesis, is referred to as the benthic zone. The littoral zone often shows strong zonation with emergent plants such as pickerel-weed (Pontederia cordata), arrowhead (Sagittaria spp.), bur-reeds (Sparganium spp.), and spike-rushes (Eleocharis spp.) closest to the shore (Figure 1.7). Beyond them are the water-lilies and other rooted floating-leaf plants. Rooted submergents can be found at greater depths and throughout the littoral zone, although a dense stand of water-lilies (Nymphaea odorata and Nuphar spp.) or watershield (Brasenia schreberi) may limit the light too much to allow other plants to grow beneath them.
Plants provide cover for young fish, tadpoles, and salamander larvae. Many algae grow attached to the surfaces of larger plants as epiphytes. Insects and other invertebrates such as fresh water sponges also live on underwater plant surfaces. Emergent plants provide sites for dragonflies, and other insects with aquatic larval stages, to crawl up out of the water in order to emerge as aerial adults (Figure 1.8).
Figure 1.6. Diagram of the littoral zone showing bands of emergent, floating-leaf, and submergent plants.
Figure 1.7. Zonation in littoral zone vegetation at a lake in Wayne County, PA; note emergent plants closest to the shoreline with floating-leaf species farther out and open water beyond.
Figure 1.8. Shed skin (exuvia) left clinging to emergent vegetation following metamorphosis of a dragonfly from its aquatic larval stage.
TYPES OF AQUATIC ECOSYSTEMS
Lakes
All lakes, whether they are natural or created by damming, are on a trajectory that will result in filling and eventual conversion to swamp or marsh and then dry land. This process is accelerated by the production of abundant organic matter in the lake itself, but is slower in less productive systems.
Most of the natural lakes in Pennsylvania are found in the glaciated regions of the northeastern and northwestern corners of the state (Figure 1.9). Some are isolated kettle lakes that lack an outlet; depending on the depth and slope of the depression, they may be surrounded by a bog mat.
Bogs develop in shallow basins where acidic, low-nutrient conditions favor the growth of sphagnum mosses and associated bog vegetation around the lake margins. These lakes are termed dystrophic. Their water is typically stained brown by organic acids that leach out of the peat and other decaying vegetation. They will eventually become bogs, bog forests, and finally terrestrial forests as they fill with peat and dry out over time (Figures
