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Introduction to Lake Surveys Field Techniques Unit 3 Module 8 Part A Lake Morphometry

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s2 Objectives Students will be able to: determine common morphometric characteristics of lakes identify characteristics of a bathymetric map. describe methods used to create bathymetric maps. use bathymetric maps to determine areal characteristics for lakes. determine the importance of lake volume and mean depth in lakes. calculate lake volume and mean depth. interpret hypsographic and volumetric curves of lakes. explain the hydraulic residence time of a lake. calculate the hydraulic residence time of a lake.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s3 Basic water quality assessment These slides focus on learning basic field techniques used by limnologists: Morphometry - estimating critical lake basin measurements Field Profiles - physical and chemical parameters measured from top to bottom of the water column Sampling – collecting water, sediments, and aquatic organisms

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s4 Part A Lake morphometry

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s5 Part A Lake morphometry Goal This module will help you: –Learn how to determine common morphometric characteristics of lakes

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s6 Lake morphometry Morphometry defines a lake’s physical dimensions and involves the quantification and measurement of lake basin shape. These dimensions influence the lake’s water quality and productivity levels.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s7 Determining lake morphometry First Step is to obtain or develop a bathymetric map -a bathymetric map is essentially a topographic map of the lake bottom that shows depth contours within the lake basin -used to estimate morphometric and many hydrological parameters -the reliability of your morphometric data will depend on the accuracy of the bathymetric map

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s8 Working with a bathymetric map Many lakes have already been mapped and a good map will contain the following: name, county, and geographic location of the water body an outline of the shoreline drawn to a known scale depth contours drawn to a known interval geographic orientation (which way is north?) date of map and data collectors

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s9

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s10 Creating a bathymetric map Lake mapping prior to 1950-MN DNR photo Making your own map requires measuring depths at precise locations: Sounding weight (through the ice works well) A secchi disk will work if weighted Fish finder, echo sounders (= sonar)

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s11 Creating a bathymetric map Bathymetric maps can be made by: -Drawing a general outline of the lake or finding an aerial photo or map -Measuring and recording water depths at a number of locations -Then connecting the depth “dots” to develop simple contour lines

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s12 Creating a bathymetric map The most commonly used method today involves using precise echo sounders, on board computers and GPS systems. There are several components to lake bathymetric mapping; -the GPS equipment which will work in tandem with -the depth sounding equipment, and -the data collection process

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s13 Creating a bathymetric map Soundings are taken during as the boat follows transects across the lake surface Location of transects and frequency of sounding measurements depends on: Map scale Basin complexity

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s14 Drawing the map Transect location and direction is recorded on a hardshell which is a drawing of the lake outline and surrounding features. Hardshells are drawn from orthorectified aerial photos or USGS quadrangle maps MN DNR photo

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s15 Areal characteristics characteristicunits elevationmeters feet surface area (Ao) hectares (ha) acres (ac) maximum depth (z max) meters feet Shoreline length (L) meters feet Shoreline development (DL) meters feet Fetch Max width Max length meters feet Littoral Area% A bathymetric map allows determination of these areal characteristics

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s16 Areal characteristics Example: Morphometric (and watershed) characteristics for Ice Lake

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s17 Areal characteristics Maximum length (fetch) Maximum width Z max Several areal characteristics and measurements can be taken directly off the bathymetric map

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s18 Areal characteristics: surface area Other measurements, such as lake surface area, require more work There are several methods for determining lake surface area: Cut and weigh method Planimetry Grid method Digitized map

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s19 Surface area: cut and weigh method You’ll need: A photocopy of bathymetric map (as large as possible and be sure to include map scale) An analytical balance

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s20 Surface area: planimetry method You’ll need: -a lake map -polar compensating planimeter ($200-$600) Image from : http://lakewatch.ifas.ufl.edu

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s21 Surface area: grid method You’ll need: Bathymetric map Grid paper Method: Count up the number of squares that fall within the shoreline of the lake Use the map scale to determine area represented by each square scale Area = # squares counted X area of one square

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s22 Surface area: digitized lake maps You’ll need: Bathymetric map Digitizing software (e.g., ArcPad) Digitizing pad Method: Software dependent www.remetrix.com

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s23 Areal characteristics: shoreline length Shoreline length (L) = circumference or perimeter of lake The linear measurement of the lake’s entire perimeter at a given water level Provides a measurement of the amount of interface between the lake and surrounding land

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s24 Areal characteristics: shoreline development Shoreline development (SLD) = a measure of how much the lake’s surface shape deviates from being a perfect circle. Important is assessing the potential habitat available For a lake that is a perfect circle the SLD = 1 A reservoir that impounds water in valleys may have an SLD > 4. Round LakeLake Amoeba Calculating SLD

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s25 Areal characteristics: % littoral area The littoral (shallow near shore) zone is the portion of a lake where sufficient light can penetrate to the lake bottom. It is also sometimes defined as that portion of the lake that is less than 15 feet in depth. The littoral zone is where the majority of the aquatic plants are found and is a primary area used by young fish.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s26 Volumetric characteristics Bathymetry also allows determination of several volumetric characteristics: characteristicunits Volume (V)Cubic meters acre feet Stratum volumeCubic meters acre feet Mean depth (z mean or z) meters feet Hypsographic curveGraph (depth vs area) Volumetric curveGraph (depth vs volume) Hydraulic retention time (HRT) years

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s27 Volumetric characteristics: volume Importance 1)Total lake volume can influence a lake’s dilution capacity. 2)Allows the determination of mixed layer (epilimnion) volume. 3)Or hypolimnion; e.g. determining available trout habitat with temperatures from 4 to 25 o C and DO > 5 mg/L.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s28 Volumetric characteristics: volume A top A bottom epilimnion hypolimnion

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s29 A top A bottom z z z Calculating Lake Volume A top = the area at the top of the layer A bottom = the area at the bottom of the layer z = the distance between contour lines V = the volume of one layer

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s30 Mean depth (z) Mean depth (z) = volume surface area Mean depth is important for the following reasons: Shallow lakes are generally more productive than deep lakes and mean depth is a quick way of assessing overall depth Also indicates the potential for waves and mixing events to disrupt bottom sediments If volume is not available you could collect numerous lake depth measurements and average them. Of course this is not as accurate and only practical for small lakes.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s31 Hypsographic curves Hypsographic curve = Area as a function of depth To estimate the amount of potential bottom spawning habitat for brook trout or bass (perhaps defined by a range of temperature and dissolved oxygen) To estimate the littoral zone area potentially available for macrophyte growth (limnologically defined as the depth to 1% of surface light). Often the epilimnion volume is used as an approximation. Often related to secchi depth by fisheries folks. To estimate the area of sediments exposed to low oxygen. This allows you to predict internal phosphorus release (Nurenberg 1985).

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s32 Maximum depth Lake bottom

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s33 Volumetric Curve Volumetric Curve = volume as a function of depth When used in conjunction with temperature and DO profiles, this curve can be used to estimate fisheries habitat.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s34

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s35 Hydraulic residence time (HRT) HRT is the time required to refill an empty lake with its natural inflow. A large deep lake with a moderate inflow will have a longer HRT than a small, shallow lake with the same inflow.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s36 HRT - importance HRT is needed to determine annual lake budgets for water, nutrients, heat, oxygen contaminants, and herbicides. It also provides an estimate of the turnover time for water in a lake, or “flushing time”

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s37 Calculating HRT A lake’s residence time is calculated by dividing the lake’s volume by its average annual water outflow. Lake managers calculate outflow on an annual basis so that seasonal variation doesn’t unduly influence results. Volume (V) is usually expressed in acre-feet, and mean outflow is expressed as acre- feet/year.

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s38 Calculating HRT cont. So the formula looks like this: HRT (years) = lake volume (acre-ft) / mean outflow (acre-ft/yr)

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Developed by: E. Ruzycki and R. Axler Updated: Sept. 2003 U3-m8a-s39 If A = lake area, then a circle with area “A” has a perimeter: (1) (2) (3) (4) A r (5) (6) (7) (8) (Formula for area based on radius) Substituting eq.(3) into eq. (1) (By definition) Substituting eq(6) into eq. (7) (collect terms) Calculating SLD BACK

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