October 6, 2004
Lake Mishnock Preservation Association
c/o Ken Fallon
17 Ragnell Road
West Greenwich, RI 02817
Re: Lake Mishnock Aquatic Plant Survey Results and Recommended Management Program
Aquatic Control Technology, Inc. conducted an Aquatic Plant Survey of Lake Mishnock in West Greenwich, RI on September 2, 2004. You accompanied Marc Bellaud, Aquatic Control’s Senior Biologist for the survey
in which the entire littoral zone of the lake was toured from a row boat. Findings
from our survey along with nuisance aquatic plant management recommendations are presented in the following report.
Lake Mishnock has an irregularly shaped shoreline, but it is effectively divided into
two primary basins connected by a narrow channel. The smaller West Basin is approximately 10 acres and the larger East Basin is approximately 38 acres, resulting in a total lake surface area of
48 acres. These acreages were calculated using available hydrology coverage that
was edited to match the lake shoreline shown in the orthophoto coverage of the lake available through RIGIS.
The West Basin is shallower with
a maximum water depth encountered of just 8 feet. The entire cove supports moderate
to dense aquatic plant growth. The East Basin is deeper with 12 foot water depths encountered towards the middle. Aquatic plant growth is found in varying densities throughout the shoreline littoral zone. Open water is found though the center of the East Basin with only low, growth bottom weed growth present. The
immediate shorelines around both basins support light to moderate residential development.
There appear to be several intermittent inlet streams and storm water drainage culverts that flow
into Lake Mishnock. The outlet is located on the north shore of the East Basin. A culvert directs outflow under Mishnock Road. There does not appear to be any flow control
structure to regulate water levels. Outlet waters flow north through Mishnock Swamp and eventually reach the East Pawtuxet River.
couple field tests of water quality were taken during the survey. Water clarity
was excellent with the Secchi Disk being clearly visible to a depth of 12 feet where it became obscured by weed growth along
the lake bottom. Water temperature and dissolved oxygen concentrations were also
measured with a YSI meter.
The lake is too shallow
to thermally stratify. The upper surface waters were nearly saturated with dissolved
oxygen. Oxygen concentrations did drop off near the bottom, which is common in
productive lakes with decomposing organic matter on the lake bottom. Temperature
and oxygen readings were more than adequate to support warmwater fish species and most other aquatic organisms common to the
The dominant aquatic plant assemblages
seen during the survey are depicted in Figure 1. The most notable plant in Lake
Mishnock is the dense growth of variable watermilfoil (Myriophyllum heterophyllum)
found throughout the West Basin and along
several stretches of shoreline in the East Basin. This is considered to be a non-native and highly invasive species in the northeast. It is a canopy forming plant that forms dense monocultures. Its primary mode of reproduction is through vegetative fragmentation, so it can quickly spread once it
becomes established in a waterbody. Variable watermilfoil is capable of reaching
the surface in 12 or more feet of water, but it typically most problematic in water depths between 4 and 8 feet. Nearly all of the West Basin supported moderate to dense milfoil growth.
Patchy milfoil growth was found in several shoreline locations in the East Basin.
Most of the dense patches were found within 100 feet of shore in water depths less than 8 feet, while more widely scattered
milfoil plants were observed in deeper waters.
Floating leafed waterlilies
are widely distributed in the lake. The two dominant species are white waterlily
(Nymphaea odorata) and watershield (Brasenia schreberi). Dense waterlily
growth rings the entire shoreline of the West Basin, while moderate density lily growth is more commonly encountered in the
East Basin. While both are considered to be native species, they are capable
of invasive growth.
Other secondary species
noted during the survey included submersed bladderwort (Utricularia sp.) and ribbonleaf pondweed (Potamogeton epihydrus) and
emergent water willow (Decadon verticillatus), pickerelweed (Pontederia cordata) and rushes (Scirpus and/or Juncus).
MANAGEMENT OPTIONS AND RECOMMENDATIONS
The principal aquatic plant
management objective at Lake Mishnock must be containment and control of the variable watermilfoil growth. This nuisance weed has the potential to spread and render the majority of the East Basin as unusable as
the West Basin, where it is presently challenging to even row a boat.
Mechanical Controls (Not
Because vegetative fragmentation
is its primary mode of propagation, mechanical harvesting and hydro-raking are generally not recommended management strategies
for milfoil control. These techniques will generally only provide several weeks
to a few months of effective control, before regrowth reaches nuisance densities. Escaping
milfoil fragments that are unavoidable with these operations are also capable re-rooting and hastening its spread. Mechanical harvesting and hydro-raking are usually only considered for milfoil control when there are no
other suitable large scale management techniques available.
Drawdown (Not Presently
Winter drawdown or water
level lowering can provide effective milfoil control, but there is not a suitable outlet structure at Lake Mishnock. A comprehensive Diagnostic/Feasibility Study would be needed to evaluate the engineering
necessary to modify the outlet structure and to determine whether or not there is enough of a drop in gradient downstream
for the lake to be effectively lowered.
Biological Controls (None
There are no known biological
controls that specifically target variable watermilfoil. The herbaceous weevil
(E. lecontei) used to control Eurasian watermilfoil (M. spicatum) does not feed on the variable watermilfoil found in Lake
Mishnock. Sterile (triploid) grass carp are presently illegal in Rhode Island. Furthermore, these fish are non-selective feeders that would likely consume desirable
native plants as much as milfoil.
Harvesting and Bottom Barriers (Not Appropriate)
New or pioneer infestations
of milfoil can sometimes be effectively controlled through diver hand-pulling, suction-harvesting and the use of bottom weed
barriers. Sometimes these techniques are effectively used as follow-up to chemical
treatment or to control widely scattered growth as part of an integrated management program.
Effectiveness is typically limited to very small (<1 acre) areas with very low density milfoil growth. Presently, the milfoil growth found throughout Lake Mishnock is too extensive for these approaches to be
used cost effectively. They should be considered as follow-up strategies once
the milfoil infestation is reduced to more manageable levels.
Chemical Treatment (Recommended)
Whole lake or large scale milfoil infestations are often most cost effectively managed through the
application of aquatic herbicides. Proper application of EPA/State registered
products can provide area and species selective weed control with negligible risk to humans, fish, wildlife and other non-target
organisms. Aquatic herbicides are registered by the EPA and the State of Rhode
Island and they must be applied by Commercially Certified Applicators and pursuant to a site specific permit that is issued
Fortunately milfoil is susceptible to a few different herbicides.
Generally, it is preferable to use systemic-acting herbicides that kill the entire plant including the root structures. The most effective systemic herbicide for control of variable watermilfoil is Aqua-Kleen
or Navigate (2,4-D granular). It usually provides 2-3 years of effective milfoil
control following treatment. 2,4-D can also be used to effectively control waterlilies,
but would not adversely impact the other non-target native species in Lake Mishnock.
From an effectiveness standpoint we would recommend the use of 2,4-D at Lake Mishnock, but it is unlikely that DEM
would issue a permit for its use. Figure 2 shows that Lake Mishnock lies within
Groundwater Reservoir and Groundwater Recharge Areas mapped by the State. There
are also two Public Wells for Kent County Water located directly north and downstream from the lake. Regulators in several different states have raised concern over the potential for groundwater contamination
from 2,4-D applications to lakes and ponds. As a result its use is often restricted
in water supply watershed areas. DEM has raised these concerns over previous
treatments in Rhode Island. We would explore the use of 2,4-D at Lake Mishnock
prior to filing permits with DEM, but are doubtful that it would be approved based on our previous experience.
The two other systemic herbicides that could be applied to control variable watermilfoil are Sonar
(active ingredient fluridone) and Renovate (active ingredient Triclopyr). We
have treated dozens of lakes and ponds in New England with Sonar where variable watermilfoil was predominant or a secondary
nuisance species and response has been mixed at best. High concentrations that
reduce plant selectivity and early season applications that are subject to dilution from spring run-off are necessary for
treatments to be effective. Whole lake applications are also more effective because
of the high solubility of Sonar. These all result in higher treatment costs without
Renovate just received EPA registration in 2003. It is
very effective on Eurasian watermilfoil, but has not been used extensively on variable watermilfoil, especially in the northeast. It is reported to provide similar levels of control as 2,4-D, or 2-3 years of
effective milfoil control. However, the per acre cost of chemical is 3-4 times
that of comparable 2,4-D or Reward treatments. Renovate can be used to effectively
control waterlilies along with milfoil, so it may be worth considering at Lake Mishnock.
Aside from 2,4-D or possibly Renovate, the herbicide of choice for milfoil control at Lake Mishnock
is Reward (active ingredient diquat). Reward is a contact-acting herbicide that
effectively kills the stems and foliage, but does not penetrate into the roots. It
usually provides one year of effective milfoil control, with occasionally some carryover control into a second year. Its fast action makes it effective for partial lake or shoreline treatments. Reward does not control waterlilies, so they would need to be selectively targeted
with a topical application of Rodeo (active ingredient glyphosate). A treatment
program utilizing Reward and Rodeo would involve a late May or early June application of Reward for milfoil control, followed
by Rodeo applications in July and August for waterlily control.
None of the herbicides discussed above have any swimming restriction following treatment, but we believe
that it is prudent practice to close the lake to swimming on the day of each treatment.
Restrictions on using treated lake water for irrigation purposes (i.e. watering lawns or gardens) or for drinking and
cooking purposes would also be imposed for some of the products. Fact sheets
and information on the various herbicides are provided for your review. Optional
treatment programs and associated costs are provided on the following page.
Lake Mishnock is infested with the non-native and highly invasive variable watermilfoil weed. It has already rendered the smaller West Basin unusable during the summer months and
is capable of spreading to similar densities throughout the majority of the larger East Basin.
Applications of EPA/State registered aquatic herbicides appear to be the most selective and cost-effective means of
controlling milfoil and thinning-out the floating-leaf waterlily growth in Lake Mishnock.
The duration of nuisance plant control will vary from 1-3 years following treatment depending on which herbicide is
used, but regardless of which product is used there will be to maintain an ongoing weed management program at Lake Mishnock.
We trust that this report will assist the Association with its lake management planning efforts. Please feel free to contact us if you have any questions or would like to discuss
these options in greater detail. We look forward to working with you in the coming
Aquatic Control Technology, Inc.
Gerald N. Smith
AQUATIC PLANT MANAGEMENT PROGRAM AT LAKE MISHNOCK
§ Prepare and file Pesticide Permit Application with RI DEM.................................................... $200
Chemical Treatment Programs
§ Option A – West Basin Only (10-acres) – Reward herbicide
treatment for milfoil control and Rodeo herbicide treatment for waterlily control. 2005 season only.................................. $4,950
§ Option B – West Basin Only (10-acres) – Reward herbicide
treatment for milfoil control and Rodeo herbicide treatment for waterlily control. Three-Year Contract – 2005-2007 seasons.....................................................................................................................
§ Option E – West Basin Only (10-acres) – Renovate herbicide
treatment for milfoil and waterlily control – 2005 season only............................................................................... $8,275
§ Option C – Entire Lake (48-acres) – Reward herbicide
treatment for milfoil control and Rodeo herbicide treatment for waterlily control targeting >80% milfoil control and area
selective waterlily control. 2005 season only...................................................................
§ Option D – Entire Lake (48-acres) – Reward herbicide
treatment for milfoil control and Rodeo herbicide treatment for waterlily control targeting >80% milfoil control and area
selective waterlily control. Three-Year Contract – 2005-2007 seasons........................... $21,975