Starry stonewort (Nitellopsis obtusa) is an invasive macroalga, a type of large algae that resembles an aquatic plant. Native to northern Eurasia, it was first introduced to North America in the St. Lawrence River in 1974, likely via ballast water from cargo ships. This species has since spread across the Great Lakes basin and into numerous inland lakes, posing a significant threat to freshwater ecosystems.
Identifying Starry Stonewort
Distinguishing starry stonewort from native aquatic plants and other algae involves observing its unique physical characteristics. This macroalga can grow tall, reaching lengths of up to 2 meters (7 feet) or even 10 meters (33 feet) in the water column. Its appearance is bushy, with thin, bright green branchlets arranged in whorls of four to six around the main stem.
The branchlets extend at acute angles from the stem nodes and may have irregular forks or divisions at their tips. A distinguishing feature is the presence of small, white, star-shaped bulbils, typically 3-6 mm wide, which form on threads at the base of the plant or at nodes along the stems. These bulbils are firm to the touch and are not found on native macroalgae in North America.
Starry stonewort forms dense “pillows” or mats of vegetation, sometimes up to 3 meters (9.8 feet) thick, in shallow areas. While it resembles true plants with root-like filaments called rhizoids that anchor it to the sediment, it lacks a vascular system. Unlike some native charoid algae, starry stonewort does not have a strong musky or garlic odor.
Ecological Impacts
Starry stonewort has negative effects on aquatic environments. It forms dense mats that can cover the lake bottom and reach the water’s surface. These aggregations block sunlight from reaching native submerged vegetation, outcompeting them for resources and lowering biodiversity.
The mats impede fish movement and disrupt spawning activities, as native fish species may not spawn in starry stonewort mats. Beyond direct competition, starry stonewort can alter water chemistry by releasing phosphorus previously bound in the sediment. This release can trigger harmful algal blooms and the growth of cyanobacteria.
The dense mats hinder recreational activities such as swimming, boating, and fishing. The alga’s rough texture can irritate skin, and its dense growth can damage boat equipment. Economic impacts to shorelines and marinas have also been reported due to these infestations.
How it Spreads and Prevention
Starry stonewort spreads through vegetative means in North America, as only male individuals have been observed in introduced populations. The spread involves small fragments of the alga or its star-shaped bulbils. Even tiny fragments can establish new populations.
Human activities are a factor in its dispersal. Fragments and bulbils can be carried on boats, trailers, anchors, fishing gear, waders, and other water-related equipment. Infestations are often found near docks and marinas. Within a waterbody, currents, waves, and recreational activities can also contribute to its spread.
To prevent the spread, individuals should follow “Clean, Drain, Dry” practices for watercraft and equipment. This involves inspecting and removing all plants, mud, and debris from boats, trailers, and gear before leaving a waterbody. All water-holding compartments, such as live wells and bilges, should be drained. Drying boats and equipment for at least five to seven days in warm, dry conditions, or disinfecting with hot water (140°F or 60°C) for at least 30 seconds, are recommended.
Management and Control
Managing existing starry stonewort infestations is challenging and requires ongoing effort. Mechanical removal methods include hand-pulling, which can be effective for small, early-stage infestations, but it is difficult to remove all bulbils and plant fragments.
Diver-assisted suction harvesting (DASH) is another mechanical method that uses divers to remove whole plants, including those below the sediment, by feeding them into a suction line. Mechanical harvesting, using aquatic vegetation harvesters, can address large and dense infestations, providing immediate relief for recreational use.
Chemical treatments involve applying algaecides, such as copper-based products, endothall, or diquat. These contact herbicides damage or kill only the parts of the plant they touch. While some chemical treatments have shown effectiveness, especially when applied before large mats form, they may be less effective on tall stands where the chemical is quickly absorbed by the upper parts, leaving lower portions unharmed. Professional application and permits are required for chemical treatments.
There are no known biological control options for starry stonewort. Control efforts are costly and time-consuming. Research continues to explore more effective management practices for this invasive species.