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 highly specialized aquatic plants. Many freshwater tidal marsh plants are classified as endangered, threatened, or rare by the Pennsylvania Natural Heritage Program, including water-hemp ragweed (Amaranthus cannabinus), swamp beggar-ticks (Bidens bidentoides), showy bur-marigold (Bidens laevis), American waterwort (Elatine americana), dwarf spike-rush (Eleocharis parvula), mud-plantain (Heteranthera multiflora), long-lobed arrowhead (Sagittaria calycina), subulate arrowhead (Sagittaria subulata), river bulrush (Schoenoplectus fluviatilis), Smith’s bulrush (Schoenoplectus smithii), Walter’s barnyard-grass (Echinochloa walteri), and wild-rice (Zizania aquatica).
Freshwater tidal marshes are an endangered habitat in Pennsylvania. They are limited to the narrow strip of Atlantic Coastal Plain in the state, and development of Philadelphia and the adjacent riverfront areas of Delaware and Bucks Counties has taken a heavy toll. Several plants that once grew along the tidal shores of the Delaware are believed to be extirpated. Parker’s pipewort (Eriocaulon parkeri), awl-shaped mudwort (Limosella australis), and Nuttall’s mud-flower (Micranthemum micranthemoides) have not been seen in Pennsylvania in over 50 years. Nuttall’s mud-flower is believed to be extinct throughout its range.
Threats to freshwater tidal marshes include riverbank erosion, leading to loss of fine sediments (Figure 1.13) and colonization by non-native, invasive plants. Common reed (Phragmites australis) is the most troublesome of the invaders. In addition, Chinese lobelia (Lobelia chinensis), a low-growing plant from the Asia-Pacific region, has also become common along the tidal shores of the Delaware River.
Figure 1.13. Tidal riverbank showing absence of fine sediments and tidal marsh vegetation except in the area protected by a discarded section of dredge pipe.
Sea level rise, which is already occurring, is another threat. To survive, tidal marshes will have to accrete (grow upward) due to the gradual increase in elevation brought on by sediment deposition, or migrate inland. However, rates of sediment deposition are too slow to keep up with the current increase in sea level, which is expected to be approximately one meter by the year 2100. Unfortunately, a recent study has revealed that in very few cases is land available for freshwater tidal marshes in Pennsylvania to migrate inland (Titus et al. 2009).
MANAGEMENT OF AQUATIC ECOSYSTEMS
Endangered, Threatened, and Rare Species
About 60 species of aquatic plants are classified as endangered, threatened, rare, undetermined (candidate), or watch list by the Pennsylvania Natural Heritage Program at this time (PNHP 2010). One species, northeastern bulrush (Scirpus ancistrochaetus), is also listed as threatened under the Federal Endangered Species Act (Figure 1.14; Table 1.2).
Because of their rarity, these plants make an important contribution to the overall biological diversity of aquatic ecosystems. Many, such as water lobelia (Lobelia dortmanna), bayonet rush (Juncus militaris), floating-heart (Nymphoides cordata), slender water-milfoil (Myriophyllum tenellum), and horned bladderwort (Utricularia cornuta) are associated with oligotrophic glacial lakes; Pennsylvania populations represent the southern limit of range of these species.
An
