Science in the CEE Department:

Trina McMahon

Bacterial communities in Lake Mendota are extremely dynamic. Over seasonal time scales, their patterns of change appear to be linked to broad changes in temperature and water chemistry. From weeks to months they seem to respond to climate drivers that are manifest in water column thermal structure, circulation patterns, and phytoplankton regime. On a daily time scale, we hypothesize that internal waves, microstratification, and light are major drivers. We hope to use high-resolution measurements of temperature, dissolved oxygen, phytoplankton pigments, and meteorological variables captured using the Lake Mendota buoy to ask how these drivers structure bacterial communities and populations. We also use the buoy parameters to guide adaptive manual sampling of the water column. Eventually we hope to develop automated sampling devices to collect microbial community samples at high temporal resolution without the need for manual sampling.


Research Highlights

Cyanobacterial population structure in the Yahara Lakes

Water quality in many south-central Wisconsin lakes is severely impaired by noxious cyanobacterial blooms. The occurrence of these blooms has been attributed to changes in nutrient loadings associated with agricultural activity and other land-use changes. More recently, blooms have included cyanobacterial strains capable of producing toxins, creating an urgent need to forecast bloom formation in order to inform the public of potential toxin presence. In this project, we seek to relate environmental parameters to bloom formation, and to evaluate the extent of toxic bloom formation over ten years in Madison-area lakes. Since 2000 we have been building a long-term archive of bacterial community samples from these lakes. We propose to use these samples to track cyanobacterial community composition over time with a special emphasis on toxin-producing strains. The population structure of toxic Microcystis strains over time and across lakes will also be investigated. We are particularly interested in the seasonal dynamics of Microcystis and other genera capable of producing cyanotoxins, in Lake Mendota. We will use molecular biology tools to describe the cyanobacterial community composition and to detect the presence of genes involved in toxin biosynthesis, in time series collected weekly during the ice-free season. The key outcomes will contribute to a broader understanding of cyanobacterial bloom ecology in Madison-area lakes and should ultimately enable the construction of predictive models for toxic bloom formation.

Below are some photos from the field sampling undertaken by members of McMahon’s lab during the summer of 2008. Click on “i” in top right corner to show/hide picture description.



Internal loading of phosphorus to Lake Mendota: the role played by microbes

A large proportion of the phosphorus fueling the planktonic food web in Lake Mendota enters the water column via internal loading from phosphorus-rich sediments. During the stratified open-water period, the bottom thermal layer of the lake becomes anoxic and accumulates high concentrations of dissolved reactive phosphorus. Internal waves and entrainment of bottom waters following storm events can deliver this phosphorus to the mixed surface waters, where it becomes available to drive algal blooms. We are studying the role of bacteria and other microbes in this process. We hypothesize that sediment bacteria are releasing stored phosphorus via a process that is commonly exploited in wastewater treatment “activated sludge”. We are applying what we have learned by studying engineered activated sludge systems to understand how and why these sediment bacteria cycle phosphorus.

Lake Hydrodynamics Process


Circulations, Wind Wave Characteristics, and Ecosystem

Blooms of blue-green algae (i.e. cyanobacteria) are temporally and spatially variable in eutrophic lakes. Algal species that constitute the blooms are also highly variable and sub-species characteristics of algae differ widely (e.g., clumping vs. non-clumping or toxin-producing vs. non-toxin producing genotypes). Because blue-green algae are often buoyant, hydrodynamic processes result in large intra-lake spatial variability in algal abundance with the potential for high inter-lake spatial correlation in bloom patterns including noxious bloom pile-ups on downwind shorelines. As a result of all these factors, the ecological and public health consequences of blue-green algal blooms can be major.

The effects of hydrologic, hydrodynamic and wind wave characteristics on environmental impacts of Madison Lakes such as bloom formation, water quality and shoreline erosion have been concerns and interests of the local authorities and Wisconsin citizens. We are developing a three-dimensional non-hydrostatic and stratified flow model (3DNHYS) to examine general circulation pattern, surface and internal waves and their breaking over shoaling bathymetry. The model would take into account the effects of temperature stratification, steep bathymetry, and wave-current interactions. In addition, the 3DNYHS model is coupled with a water quality model and a cohesive sediment transport model to examine the environmental impacts of LTER Lakes. Currently we are developing a nowcasting WISBIN system for the North Temperate Lakes, one of the global ecological observatory network (GLEON).

Circulation Pattern in Lake Mendota

An interdisciplinary approach will be used to characterize spatial/temporal dynamics of bloom development. For remote sensing technologies, we are currently developing a remote controlled model aircraft (DigiDot2) with a high precision CCD camera to sample across the lake. In addition, we are developing an Internet real-time video imagery with aerial photogrammetry technique for monitoring water quality in eutrophic lakes under various biophysical environment. At the lake district scale, IKONOS, QuickBird, SPOT, and Landsat will be used to study blooms on lakes. For in-situ data, a state-of-the-art wireless buoy, vertical profiling buoy, and the BEDS will be used to measure nutrients, phytoplankton and zooplankton species densities, velocity and temperature profiles. Molecular characterization of cyanobacterial taxa will be used to detect community change in response to in-situ and remote chemical and physical measurements. The interdisciplinary approach will permit us to assess algal bloom as the synchronicity of bloom development among lakes and the spatial variability of such external drivers as weather or climate change.