Covering Benthic Mats

Covering Benthic Mats

The Lake Lowdown – Issue 2

Lake Management just released a new policy –permitting installation of barrier sheets on the lake bottom around docks to suppress growth of submerged aquatic vegetation (SAV). Buffer strip permit holders must obtain a permit from Lake Management Office (LMO) and follow a set of guidelines.

Access to information on this option is limited. There is no mention of mats on the DNR DCL web site: We have posted information on the Friends of Deep Creek Lake website, found here

The installation guidelines make one’s head spinFor example:

  • Movement of the mat is to be avoided… A weighted frame is necessary to hold the mats to the bottom and mats must be secured by a rope to the dock. This is a Catch 22. If the mat is secured to the dock, how does one move the dock out without moving the mat? Further, holes must be cut in the mat to prevent gaseous buildup which could lift and float the mat away
  • To ensure the mats will be effective during mid-season emergence of SAVs, they will need to be placed deep enough to accommodate lake draw down in July and August. A strong crew and possibly a scuba diver seems to be needed for installation of the mat and the weighted frame. Sediment accumulation over the season may make removal very difficult.

Is it worth the cost– to you?

Our estimate runs somewhere north of $500–  annual $70 permit, $300-600 for the mat, weighted frame materials, and labor for installation and removal. The costs fall on the private property owner, not the State which owns the lake bottom where SAVs grow or manages the lake. This is a repeated theme of DCL management.

Should we kill SAVs?

DNR has stated repeatedly SAVs are a valuable part of a healthy lake ecosystem and indicators of good water quality. Submerged vegetation absorbs and filters out nutrients and helps oxygenate water. In fact, DNR says that the absence of submerged vegetation is cause for concern (NRMA, Resource Guide, “Submerged Aquatic Vegetation”,

Why is there a new policy now?

  • Are SAVs a new problem? No. The DCL Recreation and Land Use Plan incorporates a 1998 report which found many sections of the lake, especially south of Glendale Bridge, were heavily impacted by SAVs, limiting recreational usage or even preventing it during some parts of the season. The remedy provided in the plan and in effect today allows property owners to obtain DNR permission to rake SAVs around their docks. ( Last year the policy was liberalized when the Lake Manager issued a blanket permit for raking. We do not know the duration of this blanket permission since it has not posted on the Lake website. We do know that SAV density and distribution is spreading in DCL, as predicted in the Recreation and Land Use Plan.
  • Are benthic mats the best approach? Though DNR has been studying SAVs in DCL, no recommendations have been produced, no determination of need for SAV controls nor delineation of best management practices
  • Is there a Eurasian Watermilfoil (EWM) Connection? Eurasian Watermilfoil is an invasive SAV species which had explosive growth all around the lake in 2011. DNR acknowledges that DCL is now infested with EWM. Is it just a coincidence that DNR announced the benthic mat option at the same Policy and Review Board meeting that the top DNR research scientist announced plans for an EWM survey in 2012?

To benthic mat or not...   

  • If your recreational enjoyment around your dock is seriously hindered by lake grassesand you have tried raking and want to try something new, and have over $500 to spare on an experimental approach, the benthic mat may be an option.
  • Are there known unintended consequences? Yes. Killing off SAVs in dock areas will allow for increase in turbidity since SAV roots retain and filter sediment.
  • Are benthic mats the preferred option? We have yet to receive recommendations from DNR scientists on this.
  • Will benthic mats solve the spread of EWM at DCL? Definitively not. EWM grows in depths between 6 to over 20 feet—well beyond our docks.

DNR initiative to allow the use of benthic mats represents a substantial policy departure from previous positions regarding SAVs in our lake. Such change should be followed by an assessment of the mats effectiveness, negative impacts, and discussion on who is financially responsible.

Lake Manager, Carolyn Mathews (
Local source of benthic mats, contact Tripco Property Management (

Upcoming Events:
DNR is conducting two relevant studies this season: 1) The 3rd year of the DNR SAV study of 6 small sample sites; and,  2) Survey of distribution and density of Eurasian Watermilfoil in DCL. Study reports are expected by early 2013. and will be posted on our web site as soon as they are released. Contact: Bruce Michael (

© Friends of Deep Creek Lake 2012, All Rights Reserved.
Produced by Friends of Deep Creek Lake, July 2012