5. EFFECTS OF ENVIRONMENTAL ALTERATIONS ON SUBMERSED AQUATIC PLANTS

This section is intended to provide a better understanding of why reductions in submersed macrophytes occur and how specific environmental alterations may affect submersed plants. It should be emphasized at the outset that the capacity to predict changes in macrophyte communities due to natural or human induced environmental alterations is quite low. However, the examples of long term and short term changes in macrophyte communities just described show that some generalizations can be made that may allow some predictive capacity. Where natural episodes are responsible for changes, efforts toward conservation and management of submersed plant communities are not usually applicable. Thus management efforts must be directed toward human induced environmental changes. The condition of 'overabundance' of aquatic macrophytes that may result from environmental manipulation or from invasion of adventive species will not be considered.

Since reduction in abundance of submersed plants can result from an array of factors, it would be instructive to examine how the forcing functions that cause these changes affect macrophyte communities. The diagram in Figure 9 is a conceptual model that separates these forces into three categories: light attenuation, toxicity, and biomass removal. Some of these may result in long term and others in short term changes of submersed plant comnunities. Light attenuation due to suspended sediments and eutrophication acts by reducing the energy available for photosynthesis. For suspended sediments, this may be either short term, as in the case of storms or floods, or long term, where the source of suspended sediments persists. Eutrophication is generally a long term effect since nutrient levels in aquatic ecosystems can persist long after the source of these nutrients are eliminated. Sediments of aquatic ecosystems often have a high capacity for storing elements critical to plankton growth and may continue to supply these nutrients to the water column after inputs to the system cease.

Figure 9. Conceptual model illustrating the effects of environmental forces on submersed aquatic macrophyte communities. These forces (circles) are separated into three categories of stress: Light attenuation, toxicity, and biomass removal. Lines represent pathways of energy flow and bold arrows may either reduce flow (indicated by "-" sign) or accelerate flow. Thus all bold arrows represent stress on the macrophyte community, except for the one indicating a positive feedback of macrophyte reproduction. Symbols after Odum (1971).

Toxicity due to herbicides, heavy metals, and other toxic substances acts by altering the metabolism of plants (Figure 9). The affinity of herbicides for small particles may result in the accumulation of these substances in the sediments as previously discussed, and effects may persist for long periods depending on the stability of the compounds in the environment and whether degradation products are also toxic.

Environmental factors resulting in biomass removal (Figure 9) are generally short term so long as the reproduction of aquatic plants is not impaired. Of these, only burial by sedimentation would normally be induced by human activities, such as dredging and instream mining, while the others are not amenable to control. Damage to macrophyte communities by the simultaneous occurrence of more than one stress should also be considered as a possibility.

With this model as a basis for understanding stresses on submersed plants, the separation of factors due to human and natural forces is facilitated. The management of submersed plants in relation to anthropogenic influences is confounded by a spectrum of problems associated with many variables such as the soil types which are disturbed (including aquasoils), the nature of the aquatic system (lotic vs. lentic, etc.) and the nature, time, and duration of the activity generating the pollution. The resiliency of the submersed macrophyte system under stress will be related to the ecological tolerance of the species present. For example, northern potamogetons are very sensitive to stresses associated with increased suspended sediments. As discussed in other sections, long term stresses may result in a greatly altered communities where exotic species often dominate. Under extreme stress by one factor or a multiplicity of several stresses, macrophyte communities may cease to exist.

Examples of human activities which may adversely affect natural systems and their possible effects on submersed plant populations are given in Table 4 along with possible plant community responses. These perturbations are a function of the stress factors discussed for Figure 9. Literature reviews relating to the effects of suspended sediments and sedimentation on aquatic organisms include Cordone and Kelley (1961), Hynes (1970), Baxter (1977), Norton (1977), Sorensen et al. (1977), and Stern and Stickle (1978). Estimates of the magnitude of pollution of aquatic ecosystems given in Table 4 were compiled mainly from information in these reviews. However, little information on the impact of changes in aquatic systems affecting submersed plants was presented. This paucity of research relating to clearcut examples of the effects of human activities on submersed plant communities is evident in this review as in others dealing with submersed plants (i.e., Spence, 1967; Westlake, 1968; Westlake, 1973; Wetzel and Hough, 1973). Evidence that long term environmental degradation associated with agricultural and urban pollution contributed to loss of many submersed species was given in a previous section. Some other examples of community response to stress are cited in Table 4.

(a)Butcher (1933)
(b)Haslam (1978)
(c)Ozimek (1978)
(d)O'Rear (1975)
(e)Odum (1963)
(f)Hynes (1970)
(g)Davis and Brinson (1976)