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Aquatic Plants

        Aquatic plants, or macrophytes, are often classified according to their growth form. “Emergent” species such as cattails and sweetflag (Acorus calamus) are common in near-shore areas in southeast Pennsylvania, and provide shading, surfaces for aquatic insect emergence and protection against bank erosion. Oxygen generated in photosynthesis by leaves and stems above the water is often translocated to the roots, which must exist in (usually) anoxic sediments via specialized passageways termed “aerenchyma”. Most are pollinated by either wind or insects.
        “Rooted, floating-leaved plants” (e.g., waterlilies) are usually found in quiet water over soft, flocculent sediments. Leaves are often waxy to shed water and are designed to resist damage due to currents and wave action.
        “Submersed” species often extend from the sediments into the upper portion of the water column (e.g., water milfoil, Elodea), where light levels sustain more rapid growth. Leaves are usually thin and filiform or highly dissected for more rapid gas exchange with the surrounding water.
        In contrast to most aquatic plants, which obtain the bulk of their nutrients from the sediments, “free-floating” plants have no attachment to the sediments and obtain their nutrients from the water column. Duckweed (Lemna, Spirodela) and watermeal (Wolffia) may become abundant in smaller, hyper-eutrophic ponds that are protected from wind action. Examples of these growth forms are shown in Figure 39a–c.
        Aquatic plants collectively provide surfaces for periphyton, help to control blooms of excess phytoplankton and metaphyton, provide shelter and feeding sites for invertebrates, and are used as cover and feeding sites by fish. As such, a diverse plant community is important to the overall productivity of a pond. Rooted plants are often light-limited in deeper ponds, relegated to near-shore areas and shaded out by the phytoplankton and metaphyton above them in deeper water. Extensive feeding by ducks or swans may also reduce aquatic plants to very low levels. Loss of plant cover may have a devastating impact on fish populations, and ponds without aquatic plants often experience obnoxious phytoplankton blooms.

Fig. 39a  Emergent plants typical of ponds in southeast Pennsylvania.


Fig. 39b  Rooted, floating-leaved plants, and free-floating plants common to ponds in Chester County.


Fig. 39c  Common submersed plants in ponds of southeast Pennsylvania.

        Two ponds in the study were strongly influenced by the presence of the invasive pondweed Potamogeton crispus (Fig. 39c). Unlike most other aquatic plants in southeast Pennsylvania, which grow all summer and die off in the fall, P. crispus dies and decomposes in late June (Nichols and Shaw, 1986), releasing its stored nutrients to the water column where they may stimulate phytoplankton growth. As shown in Figure 40, the two ponds with P. crispus actually had lower concentrations of both total nitrogen and total phosphorus in the water column during March and May when the plants were actively growing. Concentrations of both nutrients increased in the ponds with P. crispus relative to other ponds during July, however, likely resulting from its characteristic pattern of decomposing in early summer.

Fig. 40  Comparison of seasonal trends in total nitrogen (left) and total phosphorus (right) in 2 ponds with abundant Potamogeton crispus, compared to values for 11 other ponds in the study. The line through each box is the median; upper and lower bounds of the boxes indicate quartile, and the range of observations are shown as “whiskers”.