EXECUTIVE SUMMARY

Submersed vascular plants are native to many aquatic ecosystems where they influence a number of ecosystem processes and provide food and shelter for fish and wildlife. This report examines factors that effect the light environment of submersed macrophytes and evaluates the responses of submersed plant communities to changing light conditions. Other requirements and stresses important to submersed macrophyte communities are discussed also.

The amount of light available for photosynthesis and growth of submersed plants depends on the combination of the turbidity of the water and the depth at which they grow. Turbidity due to suspended sediments may vary greatly depending on the energy of currents and waves that keep them in suspension or on human activities that may create new sources of particulate matter. Deposition of sediments directly on leaf surfaces may reduce light available to plants. Increases in plankton density, matting filamentous algae, and dense epiphytic growth, all common consequences of eutrophication, can reduce the amount of light available to submersed macrophytes. Other factors affecting the survival and growth of aquatic macrophytes are grazing and feeding activities of fish and waterfowl, fluctuating water levels, hydrostatic pressure, and sediment type. All of these factors are reviewed briefly and examples given from pertinent studies.

Data were assembled on depth distribution records of submersed angiosperms and the Secchi transparencies of water for mostly North American lakes. These were lakes in which the maximum depth of submersed macrophyte establishment was limited by light availability rather than shallow depth. Although the data base lacks rigorous precision, several patterns reveal that species respond quite differently to reduced light levels with increasing depth. Laboratory and field studies generally show that most shade tolerant species have a rapid photosynthetic response to increasing light in the low range of intensities. In clear shallow waters, the competitive advantage is shifted toward species in which photosynthesis is saturated only by extremely high light intensities.

Further analysis of depth distribution patterns of species and water transparency allowed identification of turbidity tolerant and non-tolerant species. This was expressed as a turbidity tolerance index, which in turbid waters (Secchi transparency <= 2.5 m), is the ratio of the depth maxima of species to the Secchi transparency depth. The index ranked ten species according to their resistance to adverse effects of turbid systems.

Submersed macrophyte communities may respond differently to short term and long term perturbations. Short term changes may vary from a few weeks to a few years and usually have no lasting detrimental effects on the aquatic macrophytes. Long term changes associated with factors such as eutrophication result in elimination of species sensitive to light reduction and often in the appearance of exotic species.

Based on long term studies of northern lakes, a survival index for submersed species was developed. This index and the turbidity tolerance index were used, along with other information, to identify five groups of species that have varying degrees of resistance to ecosystem alteration.

Finally the possible effects of human activities on alterations in aquatic ecosystems and an array of impacts on submersed plant communities are considered. Since a wide range of ecosystem changes can be associated with a single type of perturbation and since responses of submersed macrophytes to system changes cannot be predicted with confidence, these impact evaluations are tentative.