Article and Photos by Scott Brown
ML&SA Executive Director
Starry stonewort (scientific name: Nitellopsis obtusa), a member of the Characeae family, considered a highly beneficial, though increasingly rare “connoisseur of clean waters” (Lambert, 2009; Stewart, 1996) within its native range of Europe and Asia, was first observed as an aquatic invasive species within the North American waters of the St. Lawrence Seaway in 1978, and was later discovered in the St. Clair-Detroit River system during the summer of 1983 (Geis, 1980; Geis et al., 1981; Schloesser et al., 1986; Nichols et al., 1988). First detected in Michigan inland lakes in February of 2006 (Pullman and Crawford, 2010), successful colonization of over two hundred inland lakes had been confirmed by the Michigan Department of Environmental Quality by the spring of 2014. Due to the repeatedly observed ability of invasive Starry stonewort to degrade ecologically sensitive areas of critical shallow water habitat within colonized inland lakes, federal and state government agencies, including the United States Aquatic Nuisance Species Task Force and the Michigan Department of Environmental Quality, have classified Starry stonewort as a highly invasive aquatic species. Michigan Lake and Stream Associations frequently receives questions from lakefront property owners about the dense meadows of invasive Starry stonewort that may have suddenly appeared near their docks or shorelines. This article was written to address the commonly asked questions we have received regarding the biology and ecology of this unique and interesting, though highly invasive ancient plant that has inhabited the freshwaters of Europe and Asia for well over fifty million years.
Why is invasive Starry stonewort so harmful to our inland lake ecosystems?
Invasive Starry stonewort is capable of rapidly producing dense aquatic meadows that have been observed in Michigan inland lakes extending from near shore areas or canals in water depths of less than one foot to the outer most edges of the littoral zone in depths of up to twenty nine feet, completely engulfing the most ecologically sensitive of areas within colonized inland lakes. Meadows of invasive Starry stonewort often form dense benthic barriers of up to eight foot thick that prevent the growth of an important array of native submerged aquatic plants. In colonized inland lakes, it is not uncommon to observe littoral areas that once supported diverse native aquatic plant communities now entirely dominated by vast Starry stonewort meadows and completely devoid of native submerged aquatic plants. Dense aquatic meadows possess the ability to significantly alter and/or destroy submerged native aquatic plant communities (Schultz and Dibble, 2012). Native submerged aquatic plant communities play a vital role in inland lake ecosystems by contributing to sediment stability, water transparency, moderate biological productivity levels and the promotion and sustainability of plant and animal biodiversity (O’Neal & Soulliere, 2006). Investigators Pullman and Crawford (2010) also observed that successful Starry stonewort colonies often form dense aquatic meadows that exact their greatest physical, biological and chemical influences on the most ecologically sensitive areas of inland lake littoral zones. In addition to their negative impact on native aquatic plant communities, dense aquatic meadows of Starry stonewort often prevent fish from accessing spawning beds and areas hosting coarse woody habitat that provide optimal growth conditions and refuge for the hatchlings and juveniles of a myriad of important fish species (Schultz and Dibble, 2012). Severe degradation and/or loss of native submerged aquatic plant communities and areas critical to fish reproduction represent a significant threat to the immense ecological, recreational and economic value of Michigan’s inland lakes.
Why has invasive Starry stonewort been so successful in colonizing Michigan inland lakes?
The history of freshwater biological invasions reveals that the exotic plant and animals most likely to succeed are those that possess physiological requirements that closely align with the ecological characteristics of recipient aquatic ecosystems (Gherardi, 2007; Ren and Zhang, 2009). Many of Michigan’s inland lakes feature high quality aquatic ecosystems that are capable of supporting the basic physiological needs of Starry stonewort which are identified in existing scientific literature as follows:
minimum Secchi disk (water) transparency of ≥ 3 feet;
water temperatures ranging from 39° – 75° F;
moderate levels of inorganic phosphorus;
aquatic plant dominated, stable state freshwater ecosystems;
hard water lakes possessing pH levels of ≥ 8;
the presence of marl formations (which provide a supply of calcium carbonate);
≥ 25 mg/l calcium carbonate levels
Moreover, the results of a study conducted by this author revealed that inland lakes in Michigan most vulnerable to successful colonization by invasive Starry stonewort were largely oligo-mesotrophic to mesotrophic, with the likelihood of successful colonization by Starry stonewort rapidly declining when trophic state conditions were significantly above or below this range. Oligo-mesotrophic and mesotrophic conditions closely align with the majority of the inland lakes sampled in Michigan (Fuller and Minnerick, 2008; Fuller and Taricska, 2011).
Based on the potentially large number and surface area of inland lakes known to possess trophic state conditions, calcium carbonate producing marl formations and basin characteristics capable of supporting expansive Starry stonewort meadows, Michigan may currently be hosting colonies that are comparable in area to the largest known colonies of the ancient plant that are located in the marl lakes of southern Scandinavia, northwestern Russia and northeastern Poland (Soulie-Marsche et al., 2002).
Why does Starry stonewort grow to such great lengths and high densities in Michigan inland lakes?
Investigators Kufel & Kufel (2002) have identified calcium carbonate usually associated with the presence of marl formations within the lake’s basin as the primary factor in determining the growth rates and density of Starry stonewort. Their research results also determined that minimum calcium carbonate concentrations of 25 mg/l are required for the establishment of Starry stonewort. During periods of intense photosynthesis (the warm summer months) Starry stonewort is often observed precipitating calcium carbonate (Kufel & Kufel, 2002). Calcium carbonate levels in many Michigan inland lakes often exceed the minimum concentrations established for growth of the species by seven times (Fuller and Minnerick, 2008; Pullman and Crawford, 2010) – high concentrations that are capable of fueling explosive growth rates that often allow the plant to grow to eight feet in length and to form dense aquatic meadows over relatively large areas.
How does Starry stonewort reproduce?
Although Starry stonewort is capable of reproducing sexually through the production and fertilization of oospores, colonies of invasive Starry stonewort now widely distributed throughout many of the inland lakes of Michigan and upstate New York consist of all male plants (Soulie-Marsche et al, 2002), thus making the entire population of the invasive species in North America dependent upon vegetative bulbil reproduction and fragmentation. In the late fall, winter and early spring, the translucent lower inter-node of Starry stonewort are frequently observed to host one or more distinctive star shaped vegetative bulbils, a unique characteristic that inspired the origin of the common nomenclature for the species, Starry stonewort (Bharathan, 1983; Soulie-Marsche et al., 2002). Ranging in size from 2 to 6 mm in diameter, the jewel-like star shaped bulbils of Starry stonewort are cream colored and possess five or six distinctive points (Bharathan, 1983; Naz et al, 2010). The star shaped vegetative bulbils of Starry stonewort are capable of remaining viable within the upper substrate layer of inland lakes for periods of up to several months, thus enabling an effective over-wintering reproductive strategy within northern temperate and boreal inland lakes.
Is Starry stonewort capable of affecting the trophic state conditions of colonized lakes?
Rapidly growing meadows of Starry stonewort may significantly influence the trophic state of colonized inland lakes by acting to increase water clarity by drastically reducing the re-suspension of coarse and fine particulate matter, by releasing substances that suppress phytoplankton production as well as the growth of undesirable algal species and by significantly reducing levels of available total phosphorus in the water column by acting as an effective nutrient sink (Gross, 2003; Kufel & Kufel, 2002; Van Donk & Van de Bund, 2002). Primary biological production levels in Starry stonewort dominated inland lake aquatic ecosystems may thus be constrained, directly resulting in enhanced water clarity due to significantly reduced chlorophyll-a concentrations (Kufel & Kufel, 2002; Van den Berg et al., 1998).
Are Starry stonewort growth patterns affected by the seasons?
In the uniformly cold water temperature and ambient light profiles that occur during northern temperate spring and autumn seasons, Starry stonewort frequently produces dense meadows that have been observed extending from near shore areas in depths of less than one foot to the outer most edges of the littoral zone in depths of up to 29 feet, thus engulfing the entire littoral area of colonized inland lakes. However, during the air temperature peak that usually occurs in Michigan during July or early August and water temperatures near the surface and in shallow areas of the lakes basin reach levels approaching 85 F, dense Starry stonewort meadows may completely collapse. Pullman and Crawford (2010) have hypothesized that bio-accumulation of volatile fatty acids and other toxins produced by dense meadows of Starry stonewort in shallow waters may interact with high water temperatures in inducing an abrupt and complete collapse. Following the collapse of dense Starry stonewort in shallow waters, less dense, though still actively growing meadows of Starry stonewort often continue to inhabit preferred cooler waters in a temperature and light defined zonation pattern that ends abruptly at the deeper outer edges of the littoral slope (Chambers & Kalff, 1985; Schwarz et al. 2002). Starry stonewort meadows occupying deeper portions of the basin are capable of successfully overwintering, producing relatively abundant new growth via starry bulbils in near complete darkness induced by seasonal ice cover.
What factors determine how much of an inland lakes basin may be colonized by Starry stonewort?
In inland lakes possessing naturally shallow basins, highly irregular shorelines, shallow bays, gradually sloping basins and/or complex bottom contours, the total area of the basin capable of hosting Starry stonewort and other submerged aquatic plants may approach 100% (O’Neal & Soulliere, 2006). The degree to which Starry stonewort is capable of forming meadows in deeper areas of the lakes basin is dependent upon water transparency – that is, greater water transparency equates to the ability of the species to form meadows at increased depths. Starry stonewort also possesses the unique ability to grow in depths of up to three times the Secchi disk transparency (Duarte & Kalff, 1986) and is often found at substantially greater depths and lower light conditions than are tolerated by most native submerged aquatic plant species (Chambers & Kalff, 1985).
Does invasive Starry stonewort have a competitor in Michigan inland lakes?
Eurasian water milfoil, a perennial submerged aquatic invasive plant first detected in Michigan in the late 1940s, and present in thousands of Michigan’s inland lakes, was observed in 88% of the 120 inland lakes initially reported by the Michigan Department of Environmental Quality as hosting colonies of Starry stonewort as of 2012. The frequent co-occurrence of invasive Eurasian water milfoil and Starry stonewort in Michigan’s inland lakes is primarily due to the fact that both of these highly invasive aquatic plant species evolved together in their native distribution range which constitutes most of the Eurasian continent and possess comparable ecological prerequisites that are found in oligo-mesotrophic and mesotrophic inland lakes – that is, within inland lakes hosting moderately productive, stable state aquatic plant dominated ecosystems with moderate phosphorus levels and good water transparency. In the spring of the year in particular, Eurasian water milfoil and Starry stonewort compete for dominance in the shallow productive areas of infected lakes. However, rapidly growing meadows of Starry stonewort ultimately overcome areas inhabited by Eurasian water milfoil by late spring or summer.
How can you help prevent the spread of Starry stonewort within Michigan inland lakes?
The evidence that Starry stonewort has now successfully colonized several hundred Michigan inland lakes continues to mount. It is important that lakefront property owners, recreational boaters, the fishing community and inland lake users in general learn to identify Starry stonewort. Early detection and rapid response in managing the rapidly growing invasive plant is critical to sparing your inland lake or favorite fishing spot from the ecological ravages of this unprecedented biological invasion.
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