Physical processes in small temperate lakes

by Read, Jordan S., Ph.D., THE UNIVERSITY OF WISCONSIN - MADISON, 2012, 232 pages; 3527673

Abstract:

Small temperate lakes are numerically dominant and globally relevant to nutrient and carbon cycles, but physical processes in these lakes have been historically understudied. The primary objective of this research is to examine physical processes in small temperate lakes and explore differences between relevant physical drivers across different size classes of lakes. This research consisted of the instrumented physical monitoring of many lakes in addition to the creation of a hydrodynamic model that was designed and calibrated for small (< 10 ha) lakes. Instrumented buoy data was collected from 40 temperate lakes and used to compare the relative importance of wind versus convectively driven mixing for lakes of different sizes. This information was then used to parameterize a turbulence-based gas exchange model (the surface renewal model) from these physical observations. Convection was found to be dominant on small lakes (all 11 lakes < 10 ha in the analysis) and convection was also of increased importance on smaller lakes as a driver of the gas transfer velocity.

The numerical model (CLM) was designed for small convectively dominated lakes and was calibrated and validated on 8 small lakes. CLM simulations suggested that water column transparency is a very important driver of water temperatures and surface mixed layer depths. Darker lakes were colder with shallower mixed layers compared to more transparent simulations. Clearer lakes were found to be more sensitive to climate variability, as indicated by interseasonal variability in average water temperatures. CLM was also used to estimate the vertical diffusivity of heat in these small lakes, and the model indicated that heat transfer was at or near the rate of the molecular diffusion of heat. This finding is relevant to the vertical flux of dissolved gases in small lakes, where vertical diffusion below the mixed layer is likely to be near the molecular rate of diffusion for the gas of interest. A new method (the paired thermistor method) was devised to estimate water column transparency from time series of temperature measurements. This method can be used to follow patterns in water transparency as well as to estimate water column transparency for the purposes of numerical modeling.

An artificial destratification device, the Gradual Entrainment Lake Inverter (GELI), was designed to eliminate thermal stratification in small lakes. GELIs were used to destratify a normally strongly stratified bog lake (North Sparkling Bog, Wisconsin, USA) over the course of eight days, reducing the surface to bottom temperature difference from 19.2 to 0.2°C. The GELI method employs alternating stages of positive and negative buoyancy to move a large (8 m diameter) steel frame and geomembrane through the water column. GELIs introduce turbulence by generating internal waves, creating a large trailing wake, and via shear flows and circulation. GELIs are a more efficient artificial destratification method than bubbler aeration, and provide a potentially useful alternative management option.

Data analysis and data sharing platforms were created to aid in the analysis of environmental data from sensor networks. A simple file sharing standard, gFile, was developed for use in products of this research such as the hydrodynamic model CLM. The software package "Lake Analyzer" was designed for the analysis of instrumented buoy data using the gFile data standard. Lake Analyzer is a numerical program suite coupled with supporting visualization tools for determining indices of mixing and stratification that are critical to the biogeochemical cycles of lakes and reservoirs. Lake Analyzer provides an adaptable program structure and best practices for the comparison of mixing and stratification indices for instrumented lakes.

AdviserChin H. Wu
SchoolTHE UNIVERSITY OF WISCONSIN - MADISON
Source TypeDissertation
SubjectsHydrologic sciences; Civil engineering; Limnology
Publication Number3527673

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