Lake Mead is a large impoundment of the Colorado River along the Arizona - Nevada border which is an important component of water management in the Southwestern United States and California. It is also the first aquatic system in the west of the Mississippi River to be infested with Dreissenid mussels. Quagga Mussels (Dressenia bugensis) were first identified in zooplankton samples in 2007 but are believed to have been introduced into the lake in 2004 or 2005. Lake Mead is routinely monitored for a wide range of ecological parameters including phytoplankton composition, chlorophyll a, zooplankton composition and biomass, inorganic nutrients and dissolved oxygen. Negative impacts of quagga mussels have been observed in many systems following introduction and colonization. Reservoir systems, like Lake Mead could be particularly vulnerable given the changes to the riverine environment and the introduction of infrastructure concerns to the system. This work investigates the impact of quagga mussels on the Lake Mead ecosystem during the first 10 years following invasion. Confounding this analysis is the addition of other major changes in the watershed over the same period: extended drought and nutrient loading management.
Results/Conclusions
During the period following introduction quagga mussels, as evaluated by the juvenile veliger stage, increased in abundance for several years exhibiting 2 periods of reproduction each year. These periods of reproduction were in the spring and fall of each year, when epilimnetic temperatures were not at their annual peak. Phytoplankton composition and chlorophyll concentrations showed large decreases over the quagga infestation period. Zooplankton abundance varied seasonally and annually but has not increased or decreased appreciably, except for the addition of quagga veligers to the overall abundance. Inorganic nitrogen concentrations have been stable through this period while phosphorus concentrations have decreased significantly near the confluence of Lake Mead and the Las Vegas Wash. The Las Vegas Wash is a highly-treated wastewater dominated stream draining the Las Vegas Valley. Oxygen concentrations have remained high in most regions of the lake throughout the water column. Overall the ecosystem impacts of the quagga mussels appears to have been quite small but has been difficult to evaluate in Lake Mead given the overall changes. It is clear that the large foodweb and ecosystem changes that have been observed in other systems have not been seen in Lake Mead. The limitations imposed by drought, nutrient reduction, and quagga mussels are unresolvable.