1. INTRODUCTION

There are many examples of waters that at one time supported luxuriant growths of macrophytes but are now largely devoid of these plants. The causes for the reduced abundance vary, but most am associated with increases in water turbidity. However, the absence of submersed macrophytes does not always imply that adequate light is lacking. Submersed macrophytes generally colonize high energy and rather unstable zones of lakes, reservoirs, rivers, and estuaries and are subjected to a number of stresses that may exclude them. Some of these stresses are discussed to put in proper perspective the importance of light availability. However, the main subject area of this review is the effects that reduced light availability and increased turbidity have on submersed plants in inland waters.

The probable effects that increased water turbidity might have on submersed angiosperms will be evaluated. A number of human activities such as dredging, waste disposal, boat traffic, road construction, land use, etc. may add suspended solids to lakes and rivers at levels above background. However, background or 'natural' turbidity can vary greatly in rivers depending on current velocity following heavy rains or snow melt and in lakes depending on mixing by wind, bank erosion, and inflows from turbid rivers and streams. The changes in turbidity and thus the light environment of submersed macrophytes corresponding with these events need to be evaluated relative to other factors that limit macrophyte distribution. The importance of these 'residuals' is often very difficult to quantify.

The importance of submersed macrophytes in aquatic ecosystems is widely recognized by biologists, but public perception of their value focuses primarily on their utilization by migratory waterfowl as food. Many aquatic vertebrates and invertebrates utilize aquatic macrophytes for food and cover. In fact, the metabolism of many flowing and limnetic ecosystems is dominated by photosynthesis and respiration of aquatic macrophytes which are at the base of the detritus food web. Exchange of nutrients among the compartments of the sediment-plant-water system and calcium carbonate precipitation in plant beds can have striking effects on the water chemistry of aquatic ecosystems.

Most of the data and examples reviewed are from North America. Examples from north-central and northeastem United States and contiguous Canadian areas dominate the review. This is due partly to the abundance of lakes in those districts and partly to the large number of limnological studies conducted there. Data are presented for areas other than North America, but a serious review of the world literature was not made. Some of the most relevant studies on turbidity-macrophyte relationships have been conducted in shallow estuaries rather than lakes.

While the studies cited date back to the early decades of this century, many of the relevant data are of recent origin. Problems associated with overabundance of certain macrophytes (e.g., Myriophyllum spicatum, Hydrilla verticillata) have provided the impetus for some of these studies. few investigators in the field have recently focused much of their effort toward resolving the relationship between distribution of aquatic macrophytes and water transparency. We hope that this review will provide additional insight, identify some critical gaps in our knowledge, and suggest important areas of research.

North American species mentioned in this report are listed alphabetically in the table in the Appendices with additional information on the family, common name, and distribution. Distribution is given according to the regions developed in Shelter and Skog (1978). [The appendices are not included in this version]