Search for Eurasian water-milfoil on Black Oak Lake September 28 and 29, 2008
Dr. Susan Knight
On September 28, our team, including Joe Beers, Walt Bates, Dirk Meyer, a group of
Conserve Students and I started out on a search for Eurasian water-milfoil (EWM), curlyleaf pondweed and purple loosestrife. I started off showing the students pictures of
EWM and several plants with which it can easily be confused, including northern water
milfoil, common and purple bladderwort, water marigold and coontail. I also explained
that it is especially efficient to look for EWM in the late summer or fall because, if
present, it will be at its greatest height, perhaps even up at the surface and will be easiest
to spot this time of year. (Of course if it is found in the fall, it could have been spreading
all summer. It is vitally important for the Black Oak Lake vigilantes to watch for EWM
all summer and not rely on a fall lake-wide search.) Early on in the search we found
several EWM look-alikes, including purple bladderwort, water marigold and alternateflowered water-milfoil. We also found a few individuals of whorled water-milfoil. All of
these are native plants and their presence, along with all the other species in Black Oak
Lake indicates a vibrant plant community.
We followed our grid of points we used last spring to thoroughly cover the lake. Joe
Beers and I also paddled part of the west and north bays where the water is especially
shallow to be sure we did not miss any part of the lake. After a day and a half of
searching, we found plenty of the native northern water-milfoil but did not find any
EWM, curly-leaf pondweed or purple loosestrife. Last summer, someone brought me a
plant they suspected might be curly-leaf pondweed. Luckily, it was clasping-leaf
pondweed, the native plant that most closely resembles curly-leaf. We looked especially
hard in the area where the suspicious plant had been collected and found plenty of
clasping-leaf but no curly-leaf pondweed. There had also been a report of a plant that
might have been purple loosestrife on shore. Under closer inspection, I could only find a
few Canada thistles that are, like purple loosestrife, exotic, invasive and noxious weeds.
Also like purple loosestrife, Canada thistles are tall, prickly beasts with purple flowers
and should be dug up and removed wherever found. If anyone finds anything suspicious,
I hope they will bring it to me to identify. I would much rather look at a thousand “false
alarms” than have anyone hesitate to show me something that might be a problem.
Although EWM is one of the first plants visible in the spring, it is still much harder to
find in spring than in fall. It seems unlikely any new EWM will arrive over the winter
and so I believe a spring search is unnecessary. However it is extremely important for
the vigilantes to keep watch all summer long. Fall 2009 might be a good time for
another lake-wide search.
Below is the GPS tracker’s “bread crumb trail” of our two day search. Captain
Dirk Meyer accurately “mowed the lawn” maintaining the track prescribed by Dr.
Knight with lateral pass spacing of 27 meters.
Dr. Susan
Knight
prepares
Conserve
students for the
AIS search.
Here
she instructs (from left) Danny Figueroa from Philadelphia, Thomas
Crawford from Phelps, Miguel Mendez from Chicago, and Karina Kim
from Miami. Dr. Knight already enjoys a good reputation at Conserve
since her son, Eliot, attends there. You may recall that Eliot is the one
who built our Black Oak Lake website in 2005.
This may look
like a
suspicious
milfoil.
Actually, it is
not a milfoil at
all.
It is Water
Marigold and
has unique
multiple
branching of
its leaves.
It is very
common in
Black Oak
Lake.
Eight pairs
of trained
eyes scanned
Black Oak
Lake for a
two day total
of eleven
hours.
Numerous
things were
brought up and examined.
Dr. Knight
uses this
device for
closely
examining
complex
areas.
If anything
looks
suspicious
she can
then bring
it up with a
specially
designed
rake. One
of her
favorites is
Purple
Bladderwo
rt, a
carnivorou
s plant which has a small pouch with a one-way trap door.
When a microscopic zooplankton swims by it sucks in the prey, much
like a Venus Flytrap. The Purple Bladderwort is very common in the
west bay of Black Oak Lake.
We
carried a
canoe
along for
examinin
g the
large
areas that
were
too
shallow to
allow the
pontoon
boat in.
Here Joe
Beers
shoves
the canoe
through
nearly
impassabl
e weeds while Dr. Knight
searches for AIS. Though Eurasian Milfoil will not normally grow
in such shallow water, when protected by other heavy growth like
this it can thrive quite well. This is the case in the far west bay
and the equally shallow north bay in the area of the osprey nest.
The north side of the entire west bay, where this photo was taken,
is also far shallower than normal. Dr. Knight repeatedly noted the
barren state of the bottom in these shallow areas. She stated that
this is certainly due to high powered boat traffic. She explained
that this rototills the bottom making it more receptive to AIS
establishing itself and rooting in. A normal solid turf of natural plants
is a great defense against this happening. Secondly, stirring up bottom
sediments releases trapped phosphorous into the water which can then
fuel algae growth. Just one pound of phosphorous can grow 500 pounds
of algae! This is one more reason to use phosphorous-free fertilizers
when caring for our own landscaping and keeping the DNR recommended
35 foot natural buffer from the water’s edge. Dr. Knight urged us to pursue
and observe a No-Wake ordinance while our water level is so low.
We also reviewed our lake chemistry (see LAKE INFORMATION > WATER QUALITY
DATA). Dr. Knight said our high lake clarity (40 foot plus in June) cannot be explained
entirely by our low phosphorous level. She feels that a "food chain component" must be
involved. In other words, our zooplankton population is disproportionately high and they
are gobbling up the algae. There are many possible explanations for this but suffice to say
- this is a good thing!
She gave us several tips for differentiating between Northern and Eurasian Watermilfoils
in our own inspections. The whorls (the four splayed out leafs in a circle around the main
stem) of Northern are much closer together (like less than an inch) on the main stem than
in Eurasian which can have over four inch spacing. The main stem of Northern is green to
nearly white while in Eurasian it is tinged red. The tops of Northern come to an abrupt
tapered end while the tops of Eurasian are tasseled and frequently have a reddish flower
on it.
She noted the denuding of the bottom in many shallow areas and said it is due to boat
traffic. She voiced strong support for no-wake zones in such areas.