Introduction to Lake Surveys Field Techniques

1. Introduction to Lake SurveysField Techniques Unit 3 Module 8 Part A Lake Morphometry

2. 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.

3. 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

4. Part A Lake morphometry

5. Part A Lake morphometry Goal This module will help you: • Learn how to determine common morphometric characteristics of lakes

6. Lake morphometry • Morphometry defines a lake’s physical dimensionsand involves the quantification and measurement of lake basin shape. • These dimensions influence the lake’s water quality and productivity levels.

7. 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

8. 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

10. Creating a bathymetric map • 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) Lake mapping prior to 1950-MN DNR photo

11. 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

12. 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

13. 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

14. 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

15. A bathymetric map allows determination of these areal characteristics Areal characteristics

16. Areal characteristics • Example:Morphometric (and watershed) characteristics for Ice Lake

17. Areal characteristics Maximum length (fetch) Maximum width Several areal characteristics and measurements can be taken directly off the bathymetric map Z max

18. 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

19. 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

20. Surface area: planimetry method You’ll need: -a lake map -polar compensating planimeter ($200-$600) Image from : http://lakewatch.ifas.ufl.edu

21. scale 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 Area = # squares counted X area of one square

22. Surface area: digitized lake maps You’ll need: • Bathymetric map • Digitizing software (e.g., ArcPad) • Digitizing pad Method: • Software dependent www.remetrix.com

23. 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

24. Round Lake Lake Amoeba 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. Calculating SLD

25. 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.

26. Volumetric characteristics • Bathymetry also allows determination of several volumetric characteristics:

27. Volumetric characteristics: volume Importance • Total lake volume can influence a lake’s dilution capacity. • Allows the determination of mixed layer (epilimnion) volume. • Or hypolimnion; e.g. determining available trout habitat with temperatures from 4 to 25 oC and DO > 5 mg/L.

28. A top epilimnion A bottom hypolimnion Volumetric characteristics: volume

29. A top z A bottom z z Calculating Lake Volume Atop= the area at the top of the layer Abottom= the area at the bottom of the layer z = the distance between contour lines V = the volume of one layer

30. 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.

31. 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).

32. Lake bottom Maximum depth

33. 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.

35. 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.

36. 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”

37. 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.

38. Calculating HRT cont. • So the formula looks like this: HRT (years) = lake volume (acre-ft) / mean outflow (acre-ft/yr)

39. r A Calculating SLD BACK If A = lake area, then a circle with area “A” has a perimeter: (1) (2) (Formula for area based on radius) (6) (3) (collect terms) (4) (7) Substituting eq.(3) into eq. (1) (By definition) (5) (8) Substituting eq(6) into eq. (7)