Montana Field Guide

An aquatic, filamentous algal that can be 2 meters long (tall) with a main stem that can be 80 cm long.

When growing plants are light-green in color (Kipp et al. 2018). Stems have long branches that are variable in length and arranged in whorls. Branches grow at acute angles from the stem. Bulbils are creamy-white in color and occur at nodes and where the main stem and substrate interface. Rhizoids (root-like structures) are star-shaped.

Between nodes, cells are often a few centimeters long (Kipp et al. 2018). Most stem and branch cells are about 1mm wide.

Species of Chara (Musk-grass), Nitella (Brittlewort), and other Nitellopsis can look similar.

Nitellopsis obtusa lacks a musky-odor and has irregular branching, star-shaped rhizoids, and orange-color oocytes.

Starry stonewort occurs at depths ranging from 1 to 5 meters in relatively protected areas with slow flow (3-11 centimeters per second) (Kipp et al. 2018). It can grow in freshwater or brackish water with salinity up to 5%; however, it can tolerate salinity fluctuations of up to 17% for about one week (Kipp et al. 2018). It has occasionally been found in deep, slow moving water where other plants are scarce, such as around docks and marinas (Kipp et al. 2018). It colonizes soft substrates (silt, sand, and fine detritus) where light transmittance ranges from 1% to 50% (Kipp et al. 2018). It has been found in water temperatures ranging from 0 to 24 degrees Celsius, and experiences suppressed growth at 30 degrees Celsius (Kipp et al. 2018).

In its native habitat, it occurs in deeper habitats (3-8 meters) with low light transmittance, and water with relatively high calcium and phosphorus content, and where other stoneworts occur less frequently (Kipp et al. 2018).

Starry stonewort becomes more prevalent at a time when most other algae and aquatic plants are declining, which makes it compete well in North America (Kipp et al. 2018). In the St. Clair-Detroit River system in Michigan, it appears in July, reaches its highest biomass levels by September, and gradually declines to decomposition by March (Kipp et al. 2018). It develops dense mats (called pillows) on the substrate of the waterbody. This dense mat becomes a benthic barrier that accumulates phytotoxins that can cause redox conditions. The mat also reduces vegetative biomass and may displace fish, clams, and/or other invertebrates (Kipp et al. 2018). In the Midwest, bass and sunfish which spawn in dense mats of native Chara algae did not spawn in dense mats of Nitellopsis obtusa (Kipp et al. 2018).

The oocytes easily attach to the fur of mammals and the feathers of birds, helping the algae to disperse (Kipp et al. 2018). The algae also spreads by fragmentation (Kipp et al. 2018).

Nitellopsis obtusa has also been associated with occurrences of zebra mussels (Dreissena polymorpha).

Male and female plants are separate (dioecious) (Kipp et al. 2018). In the upper nodes of branchlets, female oogonia develop between bracts. Oocytes are orange to red. Plants can develop calcified, spiral-shaped fructifications (gyrogonites).

It is thought that starry stonewort was introduced into the Great Lakes in ballast water (Kipp et al. 2018). This alga is a relatively new invasion, and its ecology, biology, and impacts to the environment and economy are only beginning to be studied. A large investment is being made to manage and control invasions of starry stonewort, in order to maintain recreational fishing and swimming areas (Kipp et al. 2018). The filamentous algae can entangle swimmers, are potentially damaging to boats, and are aesthetically displeasing when they wash up on the lake shores and beaches ("scum").

PHYSICAL
It is difficult to mechanically remove because it produces lots of biomass (Kipp et al. 2018). It is a strong competitor that disperses and re-establishes easily.

CHEMICAL
It is very sensitive to algaecides that contain copper and endothall based compounds (Kipp et al. 2018).

BIOLOGICAL
No known biological control methods have been developed (Kipp et al. 2018).

Contact information for Aquatic Invasive Species personnel:
Montana Fish, Wildlife, and Parks Aquatic Invasive Species staff
Montana Department of Natural Resources and Conservation's Aquatic Invasive Species Grant Program
Montana Invasive Species Council (MISC)
Upper Columbia Conservation Commission (UC3)