Environmental Engineering Reference
In-Depth Information
h e primary factor infl uencing the abundance and composition of aquatic plant
assemblages is nutrient availability, particularly nitrogen and phosphorus as these
are usually the most limiting nutrients in aquatic ecosystems aff ecting the growth
of aquatic plants. When these nutrients are readily available, growth of invasive
aquatic species can increase unchecked to the detriment of native aquatic fl ora. For
example, infestations of the fl oating aquatic macrophyte water hyacinth, arguably
the world's worst aquatic weed, are usually the symptom of eutrophication. Under
high nitrate and phosphate conditions, water hyacinth doubles its biomass every
11-18 days (Edwards and Musil 1975), and under ideal conditions, red water fern
(
Azolla fi liculoides
) can double its biomass every 5-7 days (Lumpkin and Plunckett
1982). Floating macrophytes absorb nutrients directly from the water through
their root systems, but because of the normally greater abundance of nutrients in
sediments compared to the water of most aquatic systems, sediments provide a
potentially large source of nutrient supply to rooted aquatic macrophytes (Barko
and Smart 1981). Submerged invasive species such as hydrilla (
Hydrilla verticil-
lata
), spiked water milfoil, and lagarosiphon (
Lagarosiphon major
), have all been
shown to increase in biomass under elevated sediment nutrient conditions, form-
ing dense canopies that shade out native fl ora (Agami and Waisel 1985; Barko
et al
. 1988; Rattray 1995; Van
et al
. 1999).
h e majority of invasive aquatic plants rely predominantly on vegetative modes
of reproduction, while sexual reproduction through seed production plays less of
a role in their spread. Most submerged plants are able to increase in popula-
tion size through stem fragmentation, which is particularly problematic in water
bodies utilized by recreational boaters and fi shermen because the spread of sub-
merged invasive weeds is enhanced by, and directly related to, recreational boat-
ing activities (Johnstone
et al
. 1985; Buchan and Padilla 1999; Johnson
et al
.
2001; Muirhead and MacIsaac 2005; Leung
et al
. 2006). h ese invasive species
are largely introduced to new waters from reproductive fragments attached to
boats, their motors, and trailers. New infestations develop from stem fragments
that root in the substrate, commonly beginning near boat ramps. Once estab-
lished, boat tra
c continues to break up the infestations, spreading the plants
throughout the water body (Langeland 1996). Furthermore, mechanical removal
of these plants is di cult because any stem fragments that remain after clearing
are capable of regenerating.
Similarly, fl oating aquatic macrophytes such as water hyacinth, water lettuce
(
Pistia stratiotes
)
and salvinia (
Salvinia molesta
) are capable of reproducing via vege-
tative budding and fragmentation. h ese species produce daughter plants from
buds that break off , resulting in new plants capable of regeneration. h ese plants
are spread by boats, animals, and wind and water currents. Red water fern repro-
duces both sexually via spores, which are both cold tolerant and drought resistant,
and vegetatively via detached plant fragments that are easily transported from one
water body to another on the feet or feathers of waterfowl (Hill 1999). An example
of an invasive aquatic plant that relies solely on seed production for its spread is
the fl oating, attached water chestnut (
Trapa natans
), indigenous to Eurasia and
