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 1.10, 1.11).
Glacial lakes with low nutrient conditions, exceptionally clear water, and sparse growth of aquatic plants are termed oligotrophic. Some of these lakes drop off quickly to depths up to 30 m; as a result the littoral zone is narrow and sediment accumulation is low. Well developed bogs are not present. These lakes may be isolated kettle lakes, or water bodies with both inlets and outlets. Some appear to have been formed due to damming of streams by glacial deposits; but all are characterized by low within-lake biomass production. Many of the aquatic plants classified as endangered and threatened in Pennsylvania are found in lakes at the oligotrophic end of the spectrum (see below).
Other natural lakes in Pennsylvania would probably be classified as mesotrophic. These are lakes with moderate nutrient availability that support the growth of a variety of plants. Mesotrophic conditions may reflect natural inputs from the underlying geology or moderate enrichment due to watershed conditions. Glacial lakes in northwestern Pennsylvania are in this category due to the limestone content of the underlying glacial deposits.
Figure 1.9. Distribution of lakes in Pennsylvania with respect to the southern boundary of the Wisconsinan (most recent) glaciation; lake location data provided by the Pennsylvania Department of Environmental Protection Clean Lakes Program.
Figure 1.10. Succession from lake to bog and eventually dry land in a glacial kettle lake.
Figure 1.11. A glacial kettle lake surrounded by a floating bog mat in Sullivan County, PA.
Eutrophic lakes, at the other end of the spectrum, are systems that have undergone nutrient enrichment with nitrogen and phosphorus derived from external sources such as agriculture, sewage systems, or fertilized lawns in the immediate watershed.
In addition to natural lakes, Pennsylvania has many lakes that were formed by humans damming a stream or wetland. These lakes are usually shallow (except perhaps in the former channel) and support abundant aquatic growth throughout. When coupled with nutrient enrichment, they become eutrophic. Abundant plant growth results in rapid build-up of sediments and accelerated successional development toward marsh and terrestrial conditions.
Rivers and Streams
Rivers and streams provide a variety of habitats for aquatic plants, from rapids, where riverweed (Podostemum ceratophyllum) and aquatic mosses cling to the rocks, to shoreline shallows and backwaters where a diversity of emergent and submergent species may be found. Factors such as water quality and rate of flow affect the occurrence of aquatic vegetation. Lack of light may limit the growth of some aquatic species in small streams in forested areas. In urban and suburban landscapes scouring due to heavy flow during and after rainstorms may limit the growth of rooted submergent species.
Delaware Estuary
The tidal influence of the Atlantic Ocean extends up the Delaware River to the fall line, where elevation of the river exceeds the elevation of the high tide line. The Pennsylvania portion of the estuary extends from the Pennsylvania/Delaware state line to Morrisville in Bucks County. Twice a day the water level in the estuary rises and falls as much as 2 m, creating tidal marshes along the shoreline of the river and its tributaries (Figure 1.12). The tidal influence also extends up tributary streams; however, in some cases, such as the Schuylkill River and Neshaminy Creek, dams have truncated its extent.
Figure 1.12. Freshwater tidal marsh on the bank of the Delaware River in Bucks County, PA.
Classified as freshwater tidal marshes, despite the fact that the water in the estuary is slightly brackish, the marshes support a group of