Todd Tietjen

Nutrient dynamics in the lower Mississippi River floodplain: Comparing present and historic hydrologic conditions

Alterations to the lower Mississippi River-floodplain ecosystem to facilitate commercial navigation and to reduce flooding of agricultural lands and communities in the historic floodplain have changed the hydrologic regime. As a result, the flood pulse usually has a lower water level, is of shorter duration, has colder water temperatures, and a smaller area of floodplain is inundated. Using average hydrologic conditions and water temperatures, we used established nitrogen and phosphorus processes in soils, an aquatic ecosystem model, and fish bioenergetic models to provide approximations of nitrogen and phosphorus flux in Mississippi River flood waters for the present conditions of a 2-month (mid-March to mid-May) flood pulse and for a 3-month (mid-March to mid-June), historic flood pulse. We estimated that the soils and aquatic biota can remove or sequester 542 and 976 kg nitrogen ha−1 during the present and historic hydrologic conditions, respectively. Phosphorus, on the other hand, will be added to the water largely as a result of anaerobic soil conditions but moderated by biological uptake by aquatic biota during both present and historic hydrologic conditions. The floodplain and associated water bodies may provide an important management opportunity for reducing downstream transport of nitrogen in Mississippi River waters.

Potential ecological consequences of invasion of the quagga mussel (Dreissena bugensis) into Lake Mead, Nevada–Arizona

Abstract The recent invasion of the quagga mussel (Dreissena bugensis) in Lake Mead, Nevada–Arizona, USA has the potential to alter biological relationships in this western reservoir. We evaluated the potential impacts by examining several measurements in the Boulder Basin of Lake Mead after the introduction of quagga mussel (2007–2008). Analysis of variance did not reveal any basin-wide changes in chlorophyll a concentrations or water clarity (Secchi disk depth). Although significantly lower chlorophyll a concentrations were found in the outer basin, this reduction was likely related to the decline of dissolved phosphorus concentrations. The abundance of cladocerans, copepods or rotifers has not changed since 2007. Overall, the results suggest that there are no statistically significant changes to many of the standard water quality indices routinely measured in the Boulder Basin of Lake Mead; however, given the complexity of biological, chemical and physical processes driving this ecosystem, the long-term impacts of quagga mussels remain uncertain. This manuscript identifies impacts known to be altered by quagga and zebra mussels in other systems and aims to help lake managers develop experimental and monitoring programs that will accurately address the impacts of quagga mussels.

Persistence of Daphnia in the epilimnion of Lake Mead, Arizona–Nevada, during extreme drought and expansion of invasive quagga mussels (2000–2009)

Abstract Lake Mead has experienced drought conditions since 2000 and the introduction of invasive quagga mussels since about 2004. Empirical evidence from the Great Lakes region suggests a potential for energy transfer to the benthos and away from planktivorous fish through competition between quagga mussels and filter-feeding, herbivorous zooplankton. We describe Daphnia dynamics from 2043 analyses performed on the 3 basins of subtropical Lake Mead during 2000–2009. Daphnia were generally most numerous in shallower areas strongly influenced by nutrient inputs and with decreased clarity and higher chlorophyll a (chl-a) concentrations. Daphnia populations increased in mid- to late winter and early spring prior to the spawning of gamefish populations and rapidly decreased with increasing predation pressure, water temperature and cyanobacteria populations. Concurrent with the reduction in Daphnia populations in late spring, quagga veliger abundance rapidly increased, declined in summer, and peaked again in the fall. Quagga veligers in Lake Mead were generally most numerous in open water areas with maximum depth, greater clarity and low chl-a. Although mean Daphnia abundance since the beginning of drought conditions and the establishment of quagga mussels is lower in Boulder Basin, the temporal and spatial dynamics of the zooplankton community remain comparable to previously described historic patterns in all 3 basins of Lake Mead.

Three-dimensional management model for Lake Mead, Nevada, Part 1: Model calibration and validation

Abstract Lake Mead, the largest-volume man-made reservoir in the United States, faces a variety of challenges, including increasing demands for municipal water, 10 years of drought in the Colorado River system, lower water surface elevations, discharges of highly treated wastewater effluent, invasive mussels, and climate change. Lake Mead is an important source of water for 25 million people in the southwest U.S. and is also a National Recreation Area. Thus, it is imperative that the lake be adequately protected and managed to meet the often competing needs of the multiple users. A well-calibrated and validated three-dimensional hydrodynamic and water quality model of Lake Mead has been a key component of this management strategy, enabling hydrodynamics and water quality within the reservoir to be predicted and assessed for a wide range of anticipated conditions. The model was developed using the ELCOM and CAEDYM simulation codes, and has been calibrated and validated for the 2000–2008 period using measured field data for temperature, conductivity, perchlorate, bromide, chlorophyll a, nutrients (phosphorus and nitrogen), total organic carbon, pH, and dissolved oxygen. The model captured the hydrodynamics and water quality of this complex system well, and the standard errors of the model results for selected parameters were found to be larger than, but of the same order of magnitude, as the accuracy of the measured field data.

Three-dimensional management model for Lake Mead, Nevada, Part 2: Findings and applications

Abstract A 3-dimensional numerical hydrodynamic and water quality model of Lake Mead was used to improve understanding of the general limnology and aquatic ecology of Lake Mead and to assess the benefits and refine the designs of 2 large-scale capital improvement projects, with total project costs estimated to exceed

Development of a phosphorus budget for Lake Mead

1.4 billion. The model, which was developed and validated as described in a companion paper (Preston et al. 2014), was utilized to illustrate the rapid advection of algae on the lake surface by wind, the variable insertion level and fate of inflows, the velocity fields within the lake, and the replenishment of dissolved oxygen in the hypolimnion through 2 different mechanisms. Water quality information from the model was critical in the selection of the final location for a new municipal drinking water intake, allowing designers to optimize the intake elevation and to identify the benefits of maintaining the existing intakes (such as enabling selective withdrawal). Alternative discharge scenarios were modeled and ultimately demonstrated that chlorophyll a concentrations could be reduced to lower levels than the water quality standards by discharging treated wastewater effluent to the hypolimnion of Lake Mead via a proposed diffuser, rather than discharging to the surface via the Las Vegas Wash. Model results were used to facilitate port design and operation planning for the deep-water effluent diffuser to increase dilution and manage effluent concentrations at key locations.

Effects of clay mineral turbidity on dissolved organic carbon and bacterial production

Abstract Phosphorus is the growth-limiting nutrient for phytoplankton in Lake Mead. Multiple agencies have developed an extensive phosphorus dataset with low detection limit since 2007 by obtaining monthly or biweekly samples at 19 monitoring stations throughout the lake and at all the major inflows. Approximately 2000 phosphorus grab samples were collected and analyzed as part of this effort. We developed a phosphorus budget for Lake Mead during 2007–2008 using these measured phosphorus data and evaluated its accuracy using the water and bromide budget developed in this study. Based on the phosphorus budget, total phosphorus (TP) loading from the Colorado River dominated external TP loading to Lake Mead, accounting for 98% of total external loading. The orthophosphate (Ortho-P) portion that is soluble and bioavailable accounted for <2% of the TP loading from the Colorado River, indicating that a significant portion of the Colorado River TP loading was in particulate form. Our analysis showed that 88% of this TP loading settled within 8 km of the river confluence. Calculated annual retention coefficients for Ortho-P indicated that Lake Mead retained about three-fourths of the influent Ortho-P; thus, Lake Mead is a significant phosphorus sink for the Colorado River, limiting the transport of phosphorus to downstream reaches or attached systems. In Boulder Basin, the most downstream basin where major drinking water intakes are located, Las Vegas Wash was the largest Ortho-P contributor during 2007–2008, validating the recent management efforts to reduce phosphorus loading in the Las Vegas Wash.