Dreissena rostriformis bugensis has a high environmental impact in the Great Lakes.
Realized:
The quagga mussel is a filter-feeder and at high abundances removes substantial amounts of phytoplankton and suspended particulates from the water. As such, the mechanism of its impacts is similar to that of the zebra mussel. However, the magnitude is greater than for zebra mussels in deep lakes by virtue of more extensive distributions in deeper depth zones and higher abundances (Karateyev et al. 2015).
Extensive multi-seasonal changes have been documented in the southern basin of Lake Michigan following the proliferation of quagga mussels. The fraction of water column cleared (FC) was measured experimentally for quagga mussels in 2007–2008 and determined to exceed the phytoplankton growth rate at depths of 30–50 m, likely by a factor of five (Vanderploeg et al. 2010). This excessive filtration is hypothesized to cause a mid-depth sink of carbon and phosphorous; this is similar to the nearshore phosphorus shunt caused by zebra mussels, except that it occurs at mid-depth levels where quagga mussel densities are high (Vanderploeg et al. 2010). In support of this theory, total phosphorus (TP) and mean chlorophyll a concentrations both markedly fell in spring seasons after the expansion of quagga mussels, and TP levels remained low into summer (Mida et al. 2010). Meanwhile, silica and nitrate concentrations have significantly increased during spring and summer seasons post-expansion (Mida et al. 2010). Increased silica is likely a result of decreased uptake by diatoms, which are being filtered out of the water by the mussels. Evidence for this phenomenon was documented, particularly in the spring isothermal period when mixing occurred throughout the water column (typically before July). From 1983–2008, average spring silica concentrations in open water increased in Lake Michigan and increased slightly in Lake Huron, while summer silica concentrations experienced significant increases in both lakes. Dramatic increases in summer silica were initiated in the early 2000s in Lake Huron and in 2004 in Lake Michigan and seem to be associated with the expansion of quagga mussel populations in the lakes at those times (Evans et al. 2011). Spring phytoplankton biomass and primary production, which can be primarily attributed to diatoms, decreased 87% and 70%, respectively, in Lake Michigan from 1995–98 to 2007–08 (Fahnenstiel et al. 2010a). While diatoms previously accounted for >50% of phytoplankton composition at the deep chlorophyll layer, they composed less than 5% of it in 2007–08 (Fahnenstiel et al. 2010a). Further, the phyoplankton community in southern Lake Michigan has shifted towards being dominated by pico- and nanoplankton (Carrick et al. 2015).
There is also evidence that quagga mussels can and have had an impact during the late winter season. Parts of Lake Michigan, Lake Huron, and Lake Erie are known to have a late winter phytoplankton bloom that is likely important for spring fish recruitment and over-wintering zooplankton species. In the Southern basin of Lake Michigan from 2001 to 2008, chlorophyll a concentrations fell from 1.1–2.6 μg/L to 0.4–1.5 μg/L in the late winter, and percent light transmission, which ranged from 74–85% at deepwater sites in 2001, increased to 94–96% in 2008 (Kerfoot et al. 2010). Likely correlated to these changes, there was also a reported decline in cyclopoid and omnivorous calanoid copepods over this period (Kerfoot et al. 2010).
Conditions in Lake Michigan, especially in the critical late winter to spring season, indicate that the southern basin is transforming into a more oligotrophic condition, similar to that of Lake Superior in terms of levels of nutrients, chlorophyll, and primary production (Mida et al. 2010). These changes are very likely attributable to quagga mussels, especially since the most significant changes happened post-2005 (consistent with large-scale quagga mussel establishment) and impacts are most pronounced during the isothermal spring and winter seasons—the seasons in which the water is vertically well-mixed, and bottom-dwelling quagga mussels have access to particulate matter throughout most of the water column (Rowe et al. 2015b; Pilcher et al. 2017).
Quagga mussels likely decrease food availability for zooplankton through their filtration of phytoplankton, thereby altering the food web. Since the early 2000s, crustacean zooplankton densities have declined in Lake Michigan (Vanderploeg et al. 2012), Lake Ontario (Bowen et al. 2018), and Lake Huron (Barbiero et al. 2009). Concurrently, dreissena veligers in Lake Ontario have increased, and are now a substantial component of the zooplankton community (Bowen et al. 2018). Very little research has been done, however, on the contribution of dreissena veligers to the food web.
Populations of Diporeia spp., a native amphipod that once dominated benthic habitats, have decreased dramatically since dreissenids were first introduced in the 1990s. Diporeia is an important prey item linking the benthos to higher trophic levels, and it has been suggested that the shift from Diporeia to Dreissena has transformed the benthic community into an energy sink which may diminish support of the upper food web (Nalepa et al. 2009). Diporeia populations at 30–90 m depths declined by 99% in Lake Michigan between 1994/1995 and 2010, by 97% in Lake Huron between 2000 and 2012, and by >99% in Lake Ontario between 1994 and 2008 (Nalepa et al. 2007, 2009, 2014, and 2018, Watkins et al. 2007, Birkett et al. 2015). In agreement with these findings was a 13 yr annual benthic survey (1997–2009) initiated by the EPA, which also found significantly reduced populations of Diporeia in Lakes Huron, Michigan, and Ontario (Barbiero et al. 2011). These declines were coincident with the expansion of D. polymorpha and perhaps even more so with the following expansion of D. bugensis, and have progressed from shallow to deeper waters over time (Barbiero et al. 2011, Nalepa et al. 2007, 2009, 2014, and 2018, Watkins et al. 2007). While exact mechanisms for the negative response of Diporeia to zebra and quagga mussels are uncertain, declines may be a result of a reduction in diatom abundance, a food source of Diporeia spp., or to potential toxins and pathogens associated with dreissenids and their waste products (Cave and Strychar 2015; Fahnenstiel et al. 2010a, McKenna et al. 2017, Nalepa et al. 2006, Watkins et al. 2007).
The metabolic activity of dreissenid mussels has complex impacts on water chemistry and nutrient flows (Burks et al. 2002, Bykova et al. 2006, Conroy et al. 2005, Makarewicz et al. 2000). High water filtration rates and high dreissenid abundances have lead to the accumulation of biodeposits such as feces and pseudofeces (Claxton et al. 1998). Through nitrogen (N) and phosphorus (P) remineralization, the production of biodeposits may increase and redirect nutrient supply and turnover in colonized areas (Conroy et al. 2005, Hecky et al. 2004). Some detritivorous benthic organisms may benefit from the initial redirection of nutrients to the benthos via biodeposition; however, nuisance benthic algae (e.g., Cladophora) have also historically increased with these changes (Auer et al. 2010, Hecky et al. 2004). Furthermore, when high-density dreissenid colonies form, nitrate (NO3-) concentrations may significantly increase in the interstitial water at the colony base while dissolved oxygen concentrations drop, creating potentially detrimental conditions for some benthic organisms (Burks et al. 2002). Burks et al. (2002) found that nitrate concentrations in interstitial water drawn from mussel colonies in Lake Michigan were 162% greater than Lake Michigan water column above the colonies. Concurrently, dreissenid metabolic activity may lower the N:P ratio in the water column, which (along with selective feeding behavior of dreissenids) appears to favor the growth of toxic cyanobacteria (Microcystis spp.) (Bykova et al. 2006). Additionally, quagga mussels accumulate organic pollutants within their tissues to levels more than 300,000 times greater than concentrations in the environment, and these pollutants are also found in their pseudofeces (Snyder et al. 1997). Pollutants can thus be passed up the food chain, increasing wildlife exposure to organic pollutants, such as PCBs and hexachlorobenzine, and potentially mercury (Mueting and Gerstenberger 2010, Richman and Somers 2010, Snyder et al. 1997).
Potential:
Increasing amounts of biodeposits could also have an impact on multiple trophic levels via changes to the physical environment. A study in Brocton Shoal, Lake Erie, suggests that colonization of lakebed areas by dreissenid mussels and the consequent filling of remaining interstitial spaces with pseudofeces and fine-grained sediments may significantly eliminate valuable habitat for native fish (Gorman et al. 2011). Brocton Shoal, once thought to be an important area for lake trout spawning, appears to have diminished suitability as a spawning ground, potentially due to such impacts (Gorman et al. 2011).
Many of the long-term impacts of Dreissena are unclear due to the limited time scale of North American colonization. Nonetheless, it is clear that the genus Dreissena is highly polymorphic and has a high potential for rapid adaptation to extreme environmental conditions by the evolution of allelic frequencies and combinations, possibly leading to significant long-term impacts on North American waters (Mills et al. 1996). In Eurasia, it was documented that D. bugensis often replaced D. polymorpha as the dominant mussel species over time, possibly due to greater adaptability to different temperatures, levels of water mineralization, or habitat types (Zhulidov 2010). This evidence, along with recent reports of dreissenid populations in the Great Lakes, indicates that quagga mussels may dominate U.S. waters in the long-term as well. Dreissena rostriformis bugensis also inhabits a wider range of water depths, and has been found at depths up to 175 m in the Great Lakes (Nalepa et al. 2014).
Much of the work on quagga mussel impacts has focused on Lake Michigan, which has a thorough record of conditions pre- and post-quagga mussel establishment (Fahnenstiel et al. 2010b). It is quite likely that the quagga mussel has had and may continue to have effects similar to zebra mussels in other regions of the Great Lakes, as well as in the western U.S., where it has already established in Lake Mead and the Colorado River Basin (Nalepa 2010).
Dreissena rostriformis bugensis has a high socio-economic impact in the Great Lakes.
Realized:
Although D. r. bugensis lacks the keeled shape that allows D. polymorpha to attach so tenaciously to hard substrata, it is able to colonize both hard and soft benthic habitats (Mills et al. 1996). Dreissena species’ ability to rapidly colonize hard surfaces causes serious economic problems. These major biofouling organisms can clog water intake structures, such as pipes and screens, thereby reducing pumping capabilities for power and water treatment plants and financially impacting industries, companies, and communities (Connelly et al. 2007). Recreation-based industries and activities have also been impacted; docks, breakwalls, buoys, and boats have all been heavily colonized and beaches have been incidentally littered with dead shells. The full extent of negative impacts on industries and recreation due to quagga mussels is as of yet unclear.
Potential:
The quagga mussel has the potential to cause major costs for dams and the hydropower industry, particularly if its westward expansion continues. Colonization has already occurred at the Hoover, Imperial, Davis, and Parker Dams on the Lower Colorado River, causing various degrees of clogging and subsequent expense (Claudi and Prescott 2007a, 2007b).
The reemergence of nuisance algal species Cladophora in Lake Ontario, Lake Erie, and Lake Michigan has been largely attributed to the resulting changes in nutrient cycling and water clarity due to zebra mussels (Auer et al. 2010, Hecky et al. 2004). Similar observed effects between zebra and quagga mussel filtration suggest that quagga mussels could also contribute to this impact.
There is little or no evidence to support that Dreissena rostriformis bugensis has significant beneficial effects in the Great Lakes.
Potential:
Increased water clarity following dreissenid introduction is perceived as a benefit by some, especially business owners and residents on invaded water bodies (Limburg et al. 2010).
Quagga mussels have been proposed and tested for use as bio-indicators (both in the Great Lakes and Western U.S.) due to their ability to accumulate toxins and metals at much higher levels than those found in the environment, especially when small environmental levels are difficult, and yet important, to measure (Mueting and Gerstenberger 2010, Richman and Somers 2010). For instance, the National Oceanic and Atmospheric Administration’s Mussel Watch program has been monitoring contaminants in Great Lakes dreissenids since the early 1990s.