A Network of Lake Metabolism Sites

Understanding the factors influencing the carbon balance of lakes is a longstanding goal in aquatic ecology (Cole et al. 1994, Kling et al. 1991) because the carbon balance reflects both a lake’s biological activity and its links with its landscape and climatic settings. Scientists at the North Temperate Lakes Long-Term Ecological Research site have contributed uniquely to lake metabolism research, in part through ecosystem-scale measurements of important fluxes of carbon to and from lakes (Riera et al. 1999, Hope et al., 1996, Kratz et al. 1997) and biological processes influencing in-lake processing of carbon (Hanson et al. 2003). Here, we request supplemental funding to enable collaboration with colleagues in Taiwan for the purpose of studying the metabolism of Yuan Yang Lake, a sub-tropical lake that is part of the Taiwan Long-Term Ecological Research network. Through automated sampling of Yuan Yang Lake and data transmission over the Internet, we propose to compare its metabolism with lakes at the North Temperate Lakes LTER site.

Lake metabolism can be described as the balance between the complementary processes of gross primary production (GPP) and respiration (R) (Hanson et al. 2003). It is a fundamental lake characteristic that helps describe the source of carbon incorporated into all trophic levels of the ecosystem. If GPP is greater than R, then the lake is autotrophic and internally produces reduced carbon sufficient to fuel higher trophic levels. Alternatively, if GPP is less than R, then the lake is heterotrophic and must receive an external source of reduced carbon to fuel higher trophic levels. Recent studies have shown that two important drivers of net ecosystem production (NEP=GPP-R) are total phosphorus (TP) and dissolved organic carbon (DOC) concentrations of lakes (del Giorgio and Peters 1994, Hanson et al. 2003, Prairie et al., 2002). At moderate to low DOC concentrations, both GPP and R are directly related to TP. However, when DOC concentrations are moderate to high, GPP and R are uncoupled and NEP is negative, indicating heterotrophic conditions. These studies share a common methodological characteristic in that they were comparative surveys of north temperate lakes relying on short sampling durations on individual lakes. They describe neither the seasonal dynamics of lake metabolism for any given lake nor the drivers of seasonal variability.

Lake metabolism can be measured using high frequency (~0.002 Hz) observations of dissolved oxygen or carbon dioxide concentrations in the surface waters of lakes (Cole et al. 2000, Hanson et al. 2003). Over a diel period, for example, decrease in O₂ at night can be used to calculate R. Oxygen increase during the day reflects the difference between GPP and R. Thus, GPP, R, and NEP can be inferred from high frequency diel O₂ measurements.

At the North Temperate Lakes LTER site we currently use instrumented buoys to make continuous measurements of lake metabolism over long periods. These data will allow us to assess important seasonal characteristics of the metabolism signal, including: temporal variation in metabolism and the factors that may cause this variation; whether neighboring lakes vary synchronously in their day-to-day fluctuation in metabolism; and how physical processes such as gas flux with the atmosphere and mixing events that bring deeper waters with differing gas concentrations to the surface influence our ability to measure lake metabolism. These studies are being conducted on one to three northern Wisconsin lakes with the expectation that six lakes will be instrumented by the spring of 2004. One important feature of the Wisconsin buoys is that they use wireless digital spread spectrum radios to enable two-way communication to a base station. Therefore, the status of the lake and of the sensors can be determined from a remote location such as a field station or a University office. Information management specialists at the NTL LTER site have developed protocols that publish buoy data in near real-time to publicly accessible web sites. This wireless connectivity makes it possible, in theory, to quickly compare data from similarly equipped lakes anywhere in the world. One big advantage of making the data available over the web is that signal analysis and interpretation require fairly sophisticated algorithms and expert interpretation. This collaborative environment will allow scientists from both countries of our proposed activity to share their expertise in environmental science, as well as technology development.

A next step to understanding factors influencing lake metabolism is to increase the diversity of lake types on which measurements are made. Here we propose to work with our colleagues in Taiwan, particularly Dr. Chih-Yu Chiu of Academia Sinica and Dr. Fang-Pang Lin of the Taiwanese National Center for High Performance Computing, to instrument Yuan-Yang Lake in north-central Taiwan to make measurements of lake metabolism similar to the measurements made on Wisconsin lakes. Yuan-Yang lake is a study lake in the Taiwan Long-Term Ecological Research Program. It has a surface area of 3.5 ha, a maximum depth of four meters, and has no defined inlets and one outlet. It appears to be relatively nutrient poor and has a slight yellow-brown color, probably due to dissolved organic carbon leaching from the surrounding forest soils. Perhaps most interesting for this study is that the lake is subject to one or two typhoons each year when up to 5 meters of precipitation can fall on the lake. Five meters of precipitation falling on a 4 meter deep lake will cause rapid flushing of the lake. There is almost nothing known about the metabolic response of a lake to this kind of rapid flushing. Since it is not possible to sample during a typhoon by conventional means, remotely deployed sensors will be critical in collecting data during and immediately following a typhoon event. This will be the first true sub-tropical/temperate lake comparison by way of distance linkage.

The unique ecological characteristics of this lake evoke a series of specific questions to be addressed:

· How variable is lake metabolism on a day-to-day basis?

· What are the limnological, landscape or meteorological characteristics that influence this variability?

· Do lakes located in different parts of the world, but with similar physical and chemical characteristics, produce similar metabolic signals?

· In what ways do episodic events such as wind storms or typhoons alter metabolism processes, and does the lake return to base-line values or does it remain in an altered state?

A typical metabolism buoy developed for the Wisconsin lakes is instrumented with a Greenspan dissolved oxygen sensor to measure dissolved oxygen concentration of the surface water; a thermistor chain, with thermistors positioned every 0.5-1 meter depth to measure water temperature; an anemometer; and a downwelling photosynthetically active radiation (PAR) sensor. A battery, solar panel, datalogger, and spread spectrum radio provide power, data capture and telemetry for these units. Data are automatically downloaded every hour, loaded into an Oracle database, and are made available to researchers in near real time over the Internet. Background information about the northern Wisconsin lakes can be found at the North Temperate Lakes LTER web site at http://lter.limnology.wisc.edu.

We ask for funds to support a three year collaborative effort to study the metabolism of Yuan Yang Lake and compare it to similarly studied lakes in northern Wisconsin. These funds would support travel from Wisconsin to Yuan Yang Lake for scientists, technicians, information managers and students to help deploy and troubleshoot the instruments, and to collaborate with Dr. Chiu and his colleagues on detailed limnological studies related to lake metabolism. We plan to recruit a graduate student or senior undergraduate student each year to conduct a project for 4-5 weeks in Taiwan in each of the three summers. We anticipate that this student will partner with a Taiwanese student and that the pair will conduct studies at Yuan-Yang lake for one half of each summer, and then travel to northern Wisconsin to conduct similar projects at the University of Wisconsin Trout Lake Station for the remainder of the summer. These projects might include studying the spatial variability of metabolism within specific lakes, investigating the importance of mixing or stability in the water column in altering rates of productivity or respiration, or the studying the influence of daily weather variation on lake metabolism. We also request support for local travel to study lakes, room and board for the Taiwanese senior scientists and students when they visit the Trout Lake Station.

We anticipate an analogous proposal written by Dr. Chiu will be funded by Taiwanese authorities to purchase the buoy equipment necessary to make the observations on Yuan Yang Lake as well as to fund a stipend and travel of Taiwanese students to the Trout Lake Station. In addition, the National Center for High Performance Computing is supporting the development of a wireless connection to the instruments on Yuan Yang Lake. This connection, being developed by Dr. Lin, will allow data to stream from the instruments deployed on the lake to a computer connected to the internet. Through the NSF funded Pacific Rim Application and Grid Middleware Assembly (PRAGMA), information management specialists in Dr. Peter Arzberger’s group at the University of California-San Diego and the San Diego Supercomputing Center will develop middleware to support web publishing and other web services to make the data available from Yuan Yang Lake and the Wisconsin lakes in near real-time on the internet. Scientists at the University of Wisconsin will extract the metabolism signals from the time-series data.

A travel schedule is shown in Table 1 and a budget is outlined below. The first trips would take place in late winter or spring of 2004 when Paul Hanson and Tim Meinke would visit Taiwan to help assemble and deploy the buoy. Tim Kratz and a student would visit in June 2004 (Kratz for a week, the student for 5 weeks) to set up a student project. Dave Balsiger, an NTL information management specialist, would visit some point during the first year to coordinate and troubleshoot data handling from the Yuan Yang Lake buoy.

The impact of this project will be measured in the increased working interactions of the scientists in the international LTER community, the international project-based research experience for the students, an enhanced understanding of lake metabolism via the comparative analysis. In addition, the project will put in place an infrastructure for exchange of data, with near real-time data from sensors via wireless connections to the databases. Furthermore, the data infrastructure, once demonstrated and in place, has been designed to scale to include other lake systems with sensors with a minimum of reconfiguration.

Table 1. Travel Schedule. An “x” indicates one trip in that indicated year.

Person	2004	2005	2006
Tim Kratz, scientist	x	x	x
Paul Hanson, scientist	x	x
Tim Meinke, buoy technician	x		x
David Balsiger, information management specialist	x
Student #1	x
Student #2		x
Student #3			x

Literature Cited

Cole, J. J., N. F. Caraco, G. W. Kling, and T. K. Kratz. 1994. Carbon dioxide supersaturation in the surface waters of lakes. Science 265: 1568-1570.

Cole, J. C., M. L. Pace, S. R. Carpenter, and J. F. Kitchell. 2000. Persistence of net heterotrophy in lakes during nutrient addition and food web manipulations. Limnol. Oceanogr. 45: 1718-1730.

del Giorgio, P. A., and R. H. Peters. 1994. Patterns in planktonic P:R ratios in lakes: Influence of lake trophy and dissolved organic carbon. Limnol. Oceanogr. 39: 772-787.

Hanson, P. C., D. L. Bade, S. R. Carpenter, and T. K. Kratz. 2003. Lake metabolism: Relationships with dissolved organic carbon and phosphorus. Limnol. Oceanogr. 48: 1112-1119.

Hope, D., T. K. Kratz, and J. L. Riera. 1996. The relationship between PCO2 and dissolved organic carbon in the surface waters of 27 northern Wisconsin lakes. Journal of Environmental Quality 49:1442-1445.

Kling, G. W., G. W. Kipphut, and M. C. Miller. 1991. Arctic lakes and streams as gas conduits to the atmosphere: Implications for tundra carbon budgets. Science 251: 298-301.

Kratz, T.K., J. Schindler, D. Hope, J. L. Riera, and C. J. Bowser. 1997. Average annual carbon dioxide concentrations in eight neighboring lakes in northern Wisconsin, USA. Verh. Internat. Verein. Limnol. 26:335-338.

Prairie, Y.T., D.F. Bird, and J. J. Cole. 2002. The summer metabolic balance in the epilimnion of southeastern Quebec lakes. Limnology and Oceanography 47:316-321.

Riera, J. L., J. E. Schindler, and T. K. Kratz. 1999. Seasonal dynamics of carbon dioxide and methane in two clear-water and two bog lakes in northern Wisconsin, USA. Canadian Journal of Fisheries and Aquatic Sciences 56:265-274.