EVALUATION OF INVASIONS AND DECLINES OF SUBMERSED AQUATIC MACROPHYTES

During the past 60 yr, sightings of aquatic macrophyte species in geog raphic regions where they had previously not been found have occurred with increasing frequency, apparently due to both greater dispersal of the plants as a result of human activities as well as better document ation of plant distribution. Intercontinental invasions, such as Myrio phyllum spicatum and Hydrilla into North America, Elodea canadensis in to Europe and Elodea nuttallii, Egeria densa and Cabomba caroliniana i nto Japan, have generally been well documented. However, the spread of an exotic species across a continent after its initial introduction ( e.g., Potamogeton crispus in North America) or the expansion of a spec ies native to a continent into hitherto unexploited territory (e.g.,th e expansion of the North American native Myriophyllum heterophyllum in to New England) have received little attention. Natural declines in aq uatic macrophyte communities have also received little scientific stud y although there are many accounts of macrophyte declines. The best-do cumented example comes from the marine literature where extensive decl ines of eelgrass (Zostera) occurred in the 1930s along the Atlantic co ast due to a pathogenic marine slime mold (''wasting disease''). The a im of this workshop was to identify examples of invasions or natural d eclines of aquatic macrophyte species throughout the world and assess the importance of environmental factors in their control. Forty-five s cientists and aquatic plant managers from ten countries participated i n the workshop. Eleven of the participants contributed written evaluat ions of species invasions and declines in their geo-graphic region. Th ese were distributed to registered participants prior to the meeting a nd served as the starting-point of workshop discussions. To address th e topics raised in the working papers, the participants divided into f our working groups to evaluate: 1. Environmental controls of species i nvasions. 2. Biotic controls of species declines. 3. Abiotic controls of species declines. 4. Impact of management practices on macrophyte i nvasions or declines. Each working group was asked to identify existin g evidence, the need for additional evidence and management implicatio ns of their topics and then requested to discuss their findings with t he entire workshop at the conclusion of discussions.