Don’t worry, it is not “red algae” and it is not an immediate threat to wildlife or water quality. It is Eurasian watermilfoil, an aquatic invasive species. If you have lived here for any length of time, you have likely heard people talk about milfoil. While Eurasian watermilfoil, referred to as EWM for short, has a stronghold throughout Lake Pend Oreille and the Pend Oreille River, this year there is a particularly large and obvious patch that you can see from the north end of the long bridge. There are numerous aquatic plants in the system, but EWM has a distinguishing red hue that can help identify it. Idaho Department of Agriculture (ISDA) is the agency that manages aquatic invasive species for the State, and they will be surveying the area to develop a plan of attack.
Over time, if not controlled, invasive plants become very dense and take over diverse and less dense aquatic habitats, creating a monoculture. When large amounts of aquatic biomass breakdown it depletes dissolved oxygen in the water, which in turn, is a threat to fish. Local, state and federal water managers worked diligently to eradicate Eurasian watermilfoil, for about 7 years. However, several years ago, it fell from the state’s “eradication” list to the “control” list, due to its tenacious nature. This means that instead of trying to treat the whole Pend Oreille Basin, they will focus on high use areas such as busy boat launches, public swim areas and small bays deemed especially problematic. Boat launches are targeted because boats are the main pathway for spreading EWM (and other invasives). Spreading by just a tiny plant fragment, boat props exacerbate the problem by chopping it up. Plants also attach to boat props and trailers, where they can be moved to other areas of the lake or waterbodies.
Eurasian watermilfoil is not the only aquatic invasive species (AIS) of concern in the Pend Oreille Basin. Flowering Rush is fast becoming the most challenging aquatic plant in the system. Please keep your eyes peeled for a follow-up post on Flowering Rush.