2006 Point / Intercept Survey

July, 2006, Plant Survey on Black Oak Lake

By Dr. Susan Knight
WI DNR and University of Wisconsin-Madison Center for Limnology

August 2006

We had spectacular weather for performing our plant survey on Black Oak Lake last month. Drs. Dean Premo, Dave Tiller, and Susan Knight collected data in one boat while Walt Bates, John Annin, and Bob Pierce assisted from a second boat.

A Little Background

In the last few years, the WI DNR experienced a “sea change” (or maybe that is a “lake change”) in how they deal with many aquatic plant (lake weed) issues. First, they realized that many citizens and lake groups requested permits every year to harvest or chemically treat nuisance plants, with no accounting of how successful they were. Second, the DNR realized that they could get more information from their own routine plant sampling surveys by taking advantage of new technologies such as the Global Positioning System (GPS). In order to address both issues, the WI DNR, with help from the Minnesota DNR, developed a new plant sampling survey system designed to systematically examine all parts of the lake. We are using a rake sampler to determine what species are present, where they occur, at what depths they are found, and in what kind of sediment. The spatially-explicit information can be used to create all kinds of maps such as the location of an invasive plant, where the plants grow most densely, or how a species of special concern is distributed. The survey will serve as a baseline for the future, and will be especially important should there be any changes in the lake such as a change in water level, or an invasion of an exotic species.

The new plant survey protocol was launched in 2005, adjusted, and resumed in 2006. Most lake groups creating an aquatic plant management plan, requesting a permit to do a large scale chemical treatment or getting assistance from the DNR are asked to perform a plant survey according to this new protocol.

The Protocol

This survey protocol is called a point-intercept sampling scheme because we collect data all over the lake, instead of sampling along a few straight lines laid out perpendicular to the shore (as we did in our routine sampling prior to 2005). First we lay a grid, like a sheet of graph paper, over a map of the lake – all done electronically. Dr. Jen Hauxwell, an aquatic biologist with the DNR’s Integrated Science Services, receives requests and determines the number of sampling points (how close the grid lines are to each other), depending on the acreage of the lake, the depth contours of the lake, and how convoluted the shoreline is. There is a latitude and longitude associated with each intersection point on the grid. These coordinates are loaded into a computer file and then loaded into a GPS unit.

The Black Oak Lake Survey

Armed with our GPS unit (loaded with GPS points spread over the lake), a couple of sampling rakes, and empty data sheets, we headed out to the lake. In launching the boat we encountered the team of Dirk Meyer and Fritz Hill inspecting boats for invasive plants! Wonderful! Our sampling rake is a double-headed metal rake secured to a pole expandable to 15’. For deeper points, we use another double-headed rake weighted and attached to a rope. The boat driver (Dr. Premo) used the GPS unit to navigate to each point, sometimes having to skip points too shallow or too choked with plants. Dean called out the depth from his electronic depth finder, and the raker (Dr. Knight) used the rake-on-a-pole (if the depth was less than 15 feet) or the rake-on-a-rope to scrape the lake bottom and haul up the catch. The raker called out the depth and sediment type (muck, sand or rock). She then identified each plant caught on the rake, and gave each species an abundance rating of 1 (few plants), 2, or 3 (overflowing the rake). The recorder (Dr. Tiller) recorded all the data and kept track of what had been and still needed to be sampled.


We started in the west bay and immediately found a wide diversity of plants. One of the first species we encountered was purple bladderwort, a very interesting carnivorous plant that eats mostly zooplankton (the way a pitcher plant eats mostly flying insects). We had to skip a number of sites in this bay as it was heavily choked with water-lilies and the protocol demands that if we cannot reach a point by motoring or fairly easy paddling, we skip it in the interest of time.

We gradually moved east, coming into some deeper areas of the lake, with a bit less diversity. In deep lakes, such as Black Oak, plants will not grow below a certain depth. In each lake, this depth will be different, mostly due to differences in water clarity. We must sample many deep points before we can be sure we have identified the maximum rooting depth. In Black Oak Lake, we sampled everything less than 25’ in depth. Once this maximum plant depth was established, we watched the depth finder and did not sample deeper points. In Black Oak Lake, that meant that we did not sample over half of our assigned points. This was taken into consideration when the points were assigned so we still had good coverage of the lake.

We found the following species in the lake, but have not worked up all the information we will be able to glean from the spatial aspect of the data.

Black Oak Lake Aquatic Plant Species List from Survey conducted on July 10 and 11, 2006 by Drs. Susan Knight, David Tiller, and Dean Premo

This represents an excellent diversity of plants and these species are typical of a healthy, low to moderately fertile lake. We found no invasive plant species.


Black Oak Lake riparian owners must be alert to our finding Purple Bladderwort in the west bay. Though native, this plant has become a nuisance in many lakes, especially in recent years, though no one really knows why. It is actually quite rare in WI, but for some reason is often abundant when it occurs.

In performing a plant survey, we must compromise between spending a reasonable amount of time on data collection and adequately sampling the lake. Dave Tiller, Dean Premo and I all know that there were a few species we saw traveling from point to point, but did not collect or see near a point. Also, because of the steep pitch of the lake bottom, we did not have very many sampling points along much of the north and south shores of the central body of the lake. Because the lake is so big, our sampling points were roughly spaced one per acre. At this spacing, the grid did not always situate a point very close to shore. As shoreline and shallow points are often more diverse than deeper sites, we may have missed a few species growing close to shore, or missed some occurrences of some species seen elsewhere in the lake. Those of us involved with developing the protocol are aware of and continue to think how best to address this limitation.

Purple Bladderwort

Closeup of flowers. (Purple Bladderwort Utricularia purpurea )

Tangled mass of Purple Bladderwort in a shallow bog pond.

Closeup of Purple Bladderwort traps.