(a) Invasions and declines of submersed macrophyte
communities have been reported worldwide. In general,
factors contributing to invasions are most poorly
understood. Factors potentially contributing to
macrophte declines are numerous, and include reduced
irradiance, nutrient depletion, parasites and
pathogens, toxin accumulation, animal damage, climatic
fluctuations, and other factors. Attention to these
dynamics in submersed macrophyte communities is of
more than academic interest, since coordination of
macrophyte management policies and procedures with
natural controls could improve the efficiency of
macrophyte management efforts. Indeed, there is some
evidence that intense management may prolong the
dominance of invasive exotic submersed macrophyte
species. As part of the International Symposium on
the Biology and Management of Aquatic Plants, held in
July 1992 in Daytona Beach, FL, a workshop was
conducted to better understand invasion and decline
phenomena, from a regional perspective, within the
context of aquatic plant management.
(b) During the past 60 years, aquatic macrophyte
species have been discovered with increasing frequency
in geographic regions where they had not previously
been found, apparently due to greater dispersal
resulting from human activities and better
documentation of plant distribution. Intercontinental
invasions have generally been well documented.
However, the spread of exotic or native species across
continents has received little attention. Given this
introductory information, the aim of the workshop was
to identify examples of invasions or natural declines
of aquatic macrophyte species throughout the world and
assess the importance of environmental factors in
their control. While chance was acknowledged as an
important factor determining species invasions,
factors determining successful establishment following
invasion were recognized to vary with the spatial
scale of consideration (e.g., continent vs. lake
district). Few natural declines of aquatic
macrophytes have been studied quantitatively, although
personal accounts suggest that these declines may be
common. Presently, there is limited evidence of
biotic controls of declines due to a lack of "before
and after" data. In contrast, abiotic factors have
been documented as causing declines in aquatic
macrophyte communities. Management practices can
potentially influence both invasions and declines.
