LAKE ECOLOGY Unit 1: Module 2/3 Part 4 – Spatial and Temporal variability January 2004.

LAKE ECOLOGY Unit 1: Module 2/3 Part 4 – Spatial and Temporal variability January 2004

Modules 2/3 overview Goal – Provide a practical introduction to limnology Time required – Two weeks of lecture (6 lectures) and 2 laboratories Extensions – Additional material could be used to expand to 3 weeks. We realize that there are far more slides than can possibly be used in two weeks and some topics are covered in more depth than others. Teachers are expected to view them all and use what best suits their purposes. Goal: Provide a practical introduction to lake ecology. This is not a comprehensive limnology course. Rather, it is a “crash course” to be integrated with other tool-oriented WOW modules for initial training in technical areas related to water resource management. Lecture time: 2-3 weeks of classroom instruction with weekly lab/field experience. Slides: Divided into 6 subtopics. Note – Subtopics 4-6 use WOW data and visualization tools first introduced in Subtopic 4 – the density stratification discussion. Also, the module introduces lake biota before discussing physical and chemical data, although some instructors may want to reverse this order. Status (January 2004) Lecture – WOW staff review in progress; a few more slides are in prep; some graphic design needed Lab – in prep; focus will be on “traditional” field surveys comparing local ponds/lakes to each other and to WOW lakes.

Modules 2/3 outline Introduction Major groups of organisms; metabolism
Basins and morphometry Spatial and temporal variability – basic physical and chemical patchiness (habitats) Major ions and nutrients Management – eutrophication and water quality A 2 week-module can only highlight the basics of limnology. Students should be referred to the variety of introductory and advanced limnology texts now available. Some of these include: Cole, G.A Textbook of limnology. 4th edition. Dodds, W.K Freshwater Ecology: Concepts and environmental applications. Academic Press, San Diego, CA. USA. Horne, A. J. and C.R. Goldman Limnology. 2nd Edition. McGraw-Hill, Inc. New York. Hutchinson, G.E volumes . A treatise on limnology. John Wiley & Sons, New York. Mason, C.F Biology of freshwater pollution. 3rd edition. Longman House Publ., Essex, UK. McComas, S Lake Smarts: The first lake maintenance handbook. Terrene Institute, Washington, D.C., USA. Monson, B.A A primer on limnology (2nd edition). Water Resources Center, University of Minnesota, St. Paul, MN, USA. Moss, , B., J. Madgwick and G. Phillips A guide to the restoration of nutrient-enriched shallow lakes. W.W.Hawes, UK. NALMS Lake and reservoir guidance manual. North American Lake Management Society, Madison , WI ( Schmitz, R.J An introduction to water pollution biology. Gulf Publ. Co., Houston, TX, USA. Welch, E.B Ecological effects of wastewater: Applied limnology and pollutant effects. 2nd edition. Chapman & Hall, London, UK. Wetzel, R.G Limnology 3rd Edition. Academic Press, San Diego, CA. Wetzel, R.G. and G.E. Likens Limnological analyses. 3rd edition. Springer-Verlag, NY,NY, USA. ELAINE: I ACCIDENTALLY ZAPPED WHAT RICH HAD AS BULLETS IN ORIGINAL SLIDE. PROBLEMS WITH HIS SLIDE – BOX DIDN’T FILL PAGE – USE MASTER SLIDE DEFAULT AND IT WILL. CAPITALIZED SOME STUFF THAT SHOULDN’T HAVE BEEN ITALICS UNNECESSARY (TEXT IN ITALICS ALSO UNNECESSARY IN MY OPINION).

4. Spatial & temporal variability – basic physical and chemical patchiness (habitats)

4. Spatial & temporal variability – basic physical and chemical patchiness (habitats)
Physical structure – morphometric features Physical properties – vertical patterns of light, temperature and density Density stratification effects on chemistry O2 pH, EC25 (specific conductivity/salinity) nutrients (in section 5)

The size and shape of the lake matter
Shoreline development Habitat Aquatic plants Water movement Erosion potential Privacy for people Here’s 40 acre Ice Lake compared to 14,500 acre Lake Minnetonka Shoreline development ratio = the total length of the shoreline (DL) divided by the perimeter of the circle that is set to equal the area of the lake. Shoreline development (SLD) = a measure of how much the lake’s surface shape deviates from being a perfect circle. See Modules 8+9 for details. 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 (very convoluted shoreline)

Lakes: spatial variability 1
How might water quality vary between site 1 and site 2? How might their aquatic organism communities differ? Fish Zooplankton Algae Plants Interactive class discussion – Think about how various lake variables might differ in different parts of these systems. relative differences in hydraulic retention times Potential depth differences Shoreline to volume and shoreline to area differences Context of highlighting how the above factors might be similar or different in regard to different groups of organisms

How might water quality vary between sites 1, 2 and 3? How might aquatic organism communities differ? Fish Zooplankton Algae Plants Interactive class discussion – relative differences in water velocity as the river system opens up and the water slows Potential depth differences Differences between rivers and lakes re generation times of planktonic organisms Differences re fish…. Why is the lake shaded from brownish to pale green to darker green ? Would you probably need to sample at all 3 sites ? Why or why not ?

How might water quality vary across this lake? How might aquatic communities differ? Fish Zooplankton Algae Plants Minnesota or Wisconsin bass-bluegill lake Interactive class discussion – relative differences in hydraulic retention times Potential depth differences Shoreline to volume and shoreline to area differences Wetland influence Urban runoff influence (lawns and roads and driveways) Context of highlighting how the above factors might be similar or different in regard to different groups of organisms

Here’s a western US reality check California bass-bluegill lake Z-max ~ 4 m Area ~ 10 acres Watershed - ?? (urban runoff) Wind & water flow – westerly Interactive class discussion – Small but variable artificial lake Substantial construction job market in western cities because people like to live on lakes (this is based on a project in Sacramento, CA from the late 1980’s) Wind tended to pile up algae in certain coves (the ones with the highest housing density) Maximum and mean depth were important issues – because of temperature and oxygen stratification Shoreline to volume and shoreline to area differences as compared to our other lake shape examples Water source – initially low cost, poor quality groundwater – high in ammonium and phosphate. What effect might this have ? What are some possible management issues and potential problems What are some “solutions” to the presumed water quality and wildlife problems Context of highlighting how the above factors might be similar or different in regard to different groups of organisms What are major sources of variation for this system ? Water Quality Fish Zooplankton Algae Plants

Riverlake, Sacramento, CA
Price: $798,000 (Sep ’03) Sq Ft: 3511 Year Built: 1990 Bedrooms: 4 … an upscale community … commenced development in 1987… Currently, it consists of 11 villages comprising approximately 1,000 home sites (incl. 150 lake front lots),… Here’s what the “lake “ looks like now. You can tell how successful the project was by (1) the high cost of one of the lower value homes, and (2) the schematic shows how they excavated “more” lake at the east and west ends since the previous slide that approximated the situation in about This created additional shoreline real estate. “…Just minutes from downtown, you'll feel like you are living at a resort in the city! Dynamic architecture brings the lake view to all major rooms”.

Horizontal variations from physical factors
Duluth Persistent seasonal and short-term longshore currents in Lake Superior Upwelling and downwelling regions Sediment transport from shoreline erosion and deepwater resuspension Where do you sample ? How might water quality and aquatic communities vary spatially and temporally ? Where do stormwater and sewage overflows from Duluth go ? Key concepts: Lake is not well-mixed Regional scales are important as well as local scales Water masses with very different “quality” may remain discrete for long periods and great distances

Water movements- currents and waves
Waves consist of the rise and fall of water particles, with some oscillation but no net flow Currents consist of net unidirectional flows of water credit: Credit: Currents and waves normally occur together. Part of the wind's kinetic energy goes into the continuous formation of surface waves which lose their form and dissipate their energy as they break on the down-wind shore. Some of the wind energy is transferred indirectly via breaking waves to currents. Currents build up much more slowly than waves, depending on the forces of gravity, solar radiation, and wind, but eventually contain most of the lake's kinetic energy. In addition, wind induces internal waves in the thermocline and hypolimnion.

Surface waves Surface waves are wind-driven. Regular patterns of smooth, rounded waves are called swells. Capillary waves have wavelengths less than 6 cm and are restored to equilibrium due to the surface tension of the water Gravity waves have wavelengths greater than 6 cm and fall due to the force of gravity

Resuspension of nutrients and sediments
Resuspension important particularily in shallow lakes but also in deep lakes

Standing waves - surface seiches
Generated by steady wind Surface water driven downward Water piles up on the lee shore Water flows back due to gravity Standing wave rocks back and forth with decreasing motion = "surface seiches" Sloshes at resonant frequencies based on basin shape Can also result from landslides, air pressure, and earthquakes Credit: taken from Surface Seiches are generated when the wind blows for an extended period from one direction, driving the surface water downward. The wind piles water up in the lee shore and remains there until the wind drops, at which time the driving force is released and the accumulated water mass flows back under the influence of gravity. This produces a standing wave which rocks back and forth with gradually decreasing motion. A series of waves are produced which are called standing surface gravity waves or "surface seiches". The sloshing back and forth of the water produces a standing wave at certain resonant frequencies. Surface seiches can also result from air pressure and the pressure of rain falling.

Standing waves - surface seiches cont.
Credit: taken from

St. Louis River – Lake Superior seiches
The St. Louis River enters western L. Superior at the Duluth Aerial Lift Bridge The site is influenced not only by river water flowing downstream but also occasionally by Lake Superior water flowing upstream due to the lake's seiche See the water velocity switch direction from positive to negative by viewing the data set for the St. Louis River at the Duluth Aerial Lift Bridge Inlet that separates the Duluth-Superior Harbor from western Lake Superior.

St. Louis River – Duluth inlet data
Brown stripes are periods when water flows out into the lake Blue indicates “negative” velocity when the lake is sloshing back into the bay Which water body has higher EC ? What factors influence the turbidity plot ? See the water velocity switch direction from positive to negative by viewing the data set for the St. Louis River at the Duluth Aerial Lift Bridge Inlet that separates the Duluth-Superior Harbor from western Lake Superior. Note – Harbor water has a higher EC25 than does Lake Superior because of its higher TDS (total dissolved solids). It is also root-beer colored because of dissolved organic matter (DOM) from its tributaries that drain coniferous forests and extensive wetlands and bogs. However, we don’t have a sensor for that really shows this difference. In theory, optical turbidity probes are influence by the “dissolved” color of the water but most fluctuations are probably caused by particulate material from the river or from resuspended sediments due to natural (wind and high flow) or shipping-induced turbulence. There is a steady traffic of heavy ships, including 1000 foot ore boats passing through this narrow and relatively shallow channel. Visit and to find out more about the Duluth shipping industry..

Horizontal & vertical variability
How do light, temperature, sediments vary across these zones ? How do plants, periphyton, invertebrates, fish and algae vary ? LITTORAL ZONE LIMNETIC ZONE Major Lake Zones This diagram identifies the major zones in lakes. It is useful for leading the class through an interactive discussion of how physical, chemical and biological characteristics vary horizontally and vertically. Some key concepts: depth variation of light and temperature Sediment differences in the water from shore to open water (particle size and quality) Sediment differences on the bottom out from shore due to particle size; external versus internal sources of organic matter; effects of plants on sediment resuspension and dispersion, etc. Periphyton distribution (based on substrates and turbulence) Phytoplankton differences with depth and inshore versus offshore

Littoral Zone Littoral zone – usually shallow, nearshore region where sufficient light can penetrate to the bottom for plants to grow (~ 1% of midday surface light intensity) Often estimated as that area of the lake’s surface either <10 ft (3m) or <15 ft (~5 m) deep Where the majority of aquatic plants are found; a primary habitat for young fish NRRI image

Shorelines A "natural" shoreline An altered shoreline WI DNR WI DNR
Discuss differences in aquatic habitats and in the movement of water and materials from the landscape into the littoral zone An altered shoreline

Temporal variations - seasonality
Secchi depth Nutrients N or P Bottomwater- O2 winter spring summer fall Can you explain each seasonal pattern ? What might cause the mid-summer nutrient spikes ? Is this likely to be a stratified or unstratified lake and why ?

Lake Ecology Module – light, temp, density, O2
The following slides represent the temperature, density, dissolved oxygen, and stratification portion of the Lake Ecology introductory lecture module 3+4, subtopic 4 Additional explanatory information is available by viewing the attached Notes for each slide

Density, Thermal and Oxygen Stratification
Temperature and oxygen levels are major factors regulating aquatic organisms The layering of lake waters due to density differences is a major factor structuring the ecosystem and creating distinct habitats The seasonal pattern of turbulent mixing is also a critical determinant of ecosystem function and community structure

A Review of Some Basic H2O Physics
DENSITY The warmer the water, the better it floats, but ice floats too Water becomes less dense as it warms The difference in density per degree of warming increases as temperatures rise Thirty times as much energy is required to completely mix equal volumes of 24 and 25 oC water as it takes to mix the same volumes of water at 4 and 5 oC. This property explains how tropical and subtropical lakes can exhibit stable thermal stratification with only a few degrees celsius difference from surface to bottom water (as long as the water temperatures are in the upper 20’s or 30’s. SO ….

Density layering Bottom water is colder than the surface in summer (and a bit warmer in winter) Surface water is very buoyant because of the big density difference between it and cold bottom water (leading to stable thermal stratification)

Gas Solubility Warmer water holds less gas (warm beer goes flat)
As 100% air-saturated water warms, it loses O2 Temp (o C) (o F) O2- Sol (mg/L) 32 15 5 41 13 10 50 11 59 20 68 9 25 77 8 All gases are less soluble at warmer temperatures. One of the consequences is that in the fall and winter when the water in northern climates ranges from 0-4oC a lot more gas can be dissolved when fully saturated. In fact almost twice as much when ice-cold compared to midsummer surface water. Other gases besides oxygen act similarly although their solubilities are different

Plotting profiles - light
x Depth At the top of this picture you see a typical graph. At the bottom of the picture, you are looking at a “lake” in cross-section. Limnologists tend to graph data differently, so first of all, we need to make sure everyone is familiar with the seemingly strange way limnologists plot lake data that varies with depth. The independent variable, in this case, depth, is plotted vertically, starting with 0 meters at the top of the graph and increasing downward (the opposite of the usual graphing method). It makes sense when you want to visualize how things change as we dive down from the surface. The dependent variables (i.e., the physical, chemical, and biological variables that vary with depth) are plotted horizontally. In this case, the reduction of light with depth is plotted in typical “limnological” fashion, superimposed over the lake cross-section to help you visualize the data. x x x

Heat and Light Light intensity decreases exponentially with depth in a lake Which curve is the clear lake – blue or black ? Answer: Temperature would be expected to decrease exponentially with depth as well. This will be developed a few slides later when the effects of wind on this ideal – perfectly calm exponentially decreasing temperature profile are shown to lead to the classic thermal stratification pattern. Light intensity decreases exponentially with depth and is well described by the Beer-Bouguer-Lambert Law which states that: I(z) = I(0) * [ e-kz ] where I(z) = the intensity of light as a function of depth z I(0) = the intensity of light at the surface (depth = 0 meters) k = the vertical extinction or attenuation coefficient. The parameter k may be broken down into three components: kw = extinction due to pure water kp = extinction due to suspended particles (this is turbidity) and kc = extinction due to dissolved substances (typically this refers to the color of the water). What shape would you expect for the profile of temperature ?

Vertical light extinction
Light intensity decreases exponentially with depth and is well described by the Beer-Bouguer-Lambert Law which states that: I(z) = I(0) * [ e-kz ] Where: I(z) = intensity of light as a function of depth z I(0) = intensity of light at the surface (0 m) k = the vertical extinction or attenuation coefficient. The rate at which light decreases with depth therefore depends upon the amount of light absorbing dissolved substances (mostly organic carbon compounds washed in from decomposing vegetation in the watershed) and scattering and absorption from suspended materials (soil particles from the watershed, algae and detritus). Lakes with low k-values have greater light penetration than those with high k-values. The figure in the previous slide shows light attenuation profiles from two lakes with attenuation coefficients of 0.2 m-1 and 0.8 m-1 . Which curve is for the clear water lake ? Aquatic ecologists typically consider the depth to which enough light penetrates to support the growth of algae and macrophytes to be located where light is reduced to % of surface values. This is called the euphotic zone . A general rule of thumb is that this depth is about 2.7 times the limit of visibility as estimated using a Secchi disk (see Modules 8-9 for further Secchi disk depth and water clarity discussion). Light may be measured in a variety of ways for a number of different characteristics and the reader is referred to the reference texts. See also the International Light Corporation’s Handbook at which is an excellent primer on light and its measurement in the environment. Since photosynthesis depends fundamentally on light, significant changes in light penetration in a lake can be expected to produce a variety of direct and indirect biological and chemical effects. Significant trends in lake transparency, either increasing or decreasing, most often are the result of human activities - usually in association with land-use activities. In the watershed. In the spring, immediately after ice-out in temperate climates, the water column is cold and nearly isothermal (i.e. constant temperature) with depth. The intense sunlight of spring can now be absorbed in the water column which also heats up as the average daily temperature increases. In the absence of wind, a temperature profile with depth might be expected to resemble the basic shape of the light attenuation profile in the previous slide - and exponentially decrease with depth. However, another physical characteristic of water, density, then plays an important role in modifying this pattern.

Wind: turbulent mixing
Heat, as indicated by temperature would also be expected to decrease exponentially with depth, BUT …. The density differences between layers create buoyant forces, so mixing the warmer surface water down deep into the lake is akin to trying to hold a big beach ball under your feet while playing in a lake. Further, as shown in a previous slide, the colder the water layers are, the smaller these density differences are which is why thermal stratification becomes increasingly stable as the water surface warms. In contrast, fall overturn proceeds very “slowly” initially until the mixed layer has cooled to below ~ 10oC – then it gets easier and easier for a given strength wind to deliver mixing energy far down the water column. The fetch of the lake is also a factor in terms of allowing wave action to develop that essentially maximizes contact between the wind and the surface water. The wave crests provide surface area, kind of like a sail. It’s a frictional force as well that is dependent on surface area of contact. The lake surface is exposed to the wind, which mixes the surface water, but the turbulent energy from the wind dissipates with depth, having less impact further down. The greater the density difference (mostly from temperature) between layers of water, the harder it is to mix them together.

Wind mixing links Also see slides in Section 5 (Water Chemistry) of this module that discuss gases (O2, N2,CO2 and H2S)

Temperature – calm day Temperature Depth
The Temperature profile would look just like the light profile – at least on a perfectly calm day Temperature x At the top of this picture you see a typical graph. At the bottom of the picture, you are looking at a “lake” in cross-section. Limnologists tend to graph data differently, so first of all, we need to make sure everyone is familiar with the seemingly strange way limnologists plot lake data that varies with depth. The independent variable, in this case, depth, is plotted vertically, starting with 0 meters at the top of the graph and increasing downward (the opposite of the usual graphing method). It makes sense when you want to visualize how things change as we dive down from the surface. The dependent variables (i.e., the physical, chemical, and biological variables that vary with depth) are plotted horizontally. In this case, the reduction of temperature with depth is plotted in typical “limnological” fashion, superimposed over the lake cross-section to help you visualize the data. Note that on a perfectly calm day you might expect the temperature profile to follow the light profile with depth. Depth x x x

Temperature – windy day
But when the wind blows, it mixes the surface water with deeper water And its energy dissipates with depth Temperature x x Wind effects: Now we see that the wind acts to vertically mix the water and in doing so creates a layer of water that it the same temperature (isothermal). The depth of this layer depends on the strength of the wind relative to the buoyancy force that it must overcome. As you go deeper, the wind energy is attenuated relative to the surface and so its effect on mixing is also diminished The buoyancy force of the upper water layer relative to deeper, colder water depends on both (1) the temperature difference between the 2 layers which (2) defines the actual density difference between the two layers. Remember that the density difference per degree increases dramatically with increasing temperature. Therefore, eventually, the wind energy is entirely dissipated and so the temperature profile below this depth is unaffected relative to the way it appeared on the previous calm day. This upper mixed layer = the epilimnion (it is almost the same temperature throughout) The deeper “unaffected” layer = the hypolimnion (it is almost the same temperature throughout) The intermediate layer = the metalimnion which shows a lot of temperature variation (it has a strong gradient) The one depth where the rate of change of temperature with depth is maximal = the thermocline. x x Depth x x

Temperature 0oC winter Depth

Temperature 0oC Early spring just after ice-out Depth

Temperature 0oC Watch as the surface waters warm up through the spring Depth

Temperature 0oC 10oC Depth

Depth Temperature 0oC 10oC 20oC
A strong thermocline (shaded rectangle) is now in place. Depth

Mid-summer thermal stratification
Bottom water colder than surface in summer Surface water is very buoyant BIG density difference between surface and cold bottom water = resistance to mixing

Temperature 0oC 10oC Depth

Fall Turnover Temperature 0oC Depth

Temperature 0oC Depth

Thermal stratification sequences
Ice Lake, MN Apr 23 – Jun 3, 2003 Shagawa Lake, MN May 7 – Jun 24, 2003 Lake Independence, MN Apr 12 – Jun 29, 1999 These figures all show the development of temperature stratification from soon after spring ice-thaw to the onset of stable thermal stratification. time Temperature (o C)

Oxygen What are the sources of oxygen to a lake?
What are the sinks for oxygen in a lake? Ask the audience the questions listed on the slide. SOURCES = INPUTS SINKS = LOSSES or OUTPUTS

Two Major Sources of O2 Wind energy Photosynthesis Sources of DO:
Turbulent mixing of the atmospheric O2 into surface water via wind energy. 2. Photosynthesis from algae and higher plants Photosynthesis requires light, so it only occurs to the depth that light intensity is about % of surface values. 3. Minor sources from river inflows and groundwater.

Major Sinks (losses) Diffusion Water column respiration
1. Diffusion to the atmosphere (from water surface layer) - outgassing if surface water is saturated and water temperature increases. outgassing from water surface, if surface waters are supersaturated from photosynthesis 2. Respiration in water - mostly bacteria, but also algae, plants, zooplankton, invertebrates, and fish. 3. Respiration in the sediments - mostly bacteria. 4. ?????? Sediment respiration (bacteria and benthos)

Factors affecting dissolved oxygen levels
How far down can light penetrate ? Is the lake thermally stratified ? How windy is it ? Are there a lot of aquatic plants and algae ? How warm is the lake ? Is there a lot of organic “gunk” in the water ? Are there sources of fertilizer, ag & urban runoff, wastewater, etc. coming in ? How much organic sediment area is there relative to hypolimnetic volume ?

“Idealized” Stratification Curves
Unproductive This is how it is supposed to look for very unproductive (oligotrophic) versus productive (eutrophic) lakes. Oligotrophic Lakes: The key difference with respect to the cycle for a eutrophic lake is that the water column is assumed to be 100% saturated with DO at all times. Spring mixing (turnover) shows constant cold temperature with depth and uniform, high DO. When temperature stratifies, the epilimnion loses O2 as it warms, while DO remains high in the cold hypolimnion. Fall overturn is similar to spring. In winter, ice water overlies 4oC water of maximum density. There is high DO throughout the water column. NOTE – Although many texts define oligotrophy based on persistent high hypolimnetic DO, many smaller low productivity lakes are not deep enough to “avoid” some degree of O2 depletion in the lower hypolimnion. They may be oligotrophic by every other measure (I.e., chlorophyll, productivity, nutrients) yet still exhibit some degree of anoxia in their deepest strata. Eutrophic Lakes: The thermal cycle is essentially independent of trophic state. DO shows depletion below the thermocline in summer, and much or all of the hypolimnion may become anoxic. During spring overturn, the water column is re-oxygenated. Under ice, the water column becomes O2-depleted. During fall overturn, the water column is re-oxygenated. NOTE – If the lake thaws late in the spring during hot, calm weather, it may stratify before the water column is completely re-saturated with DO. This happened at Ice Lake in 1998, 1999, and 2001 (3 out of 4 years to date). When this happens, the hypolimnion will have been almost totally anoxic for a full year (fall to fall). Productive

Mid-summer thermal stratification - summary
Bottom water is colder than the surface in summer Surface water is very buoyant – it floats on top of the thermocline BIG density difference between surface and cold bottom water It takes a lot of wind energy to push the surface water down long enough to mix with the water below Note to teachers: No hard and fast rules but the expected thermocline depth for most small to medium sized lakes: Small lakes: 3-7m over the course of the summer Large lakes: m during most of the summer 0Surface Area: Small = <100 acres (40 ha) Medium= ACRES ( ha) Large = >500 acres (200 ha)

Annual cycle of thermal stratification - dimixis
Another simplistic cartoon of a lake that is assumed to mix completely in spring and fall (a dimictic lake).

Reality – “real” data Illustrations of Water on the Web lake data visualization tools (DVT’s): Profile plotter (all parameters vs depth) Color mapper (2 parameters vs depth) DxT (depth vs time)

Profile Plotter Strong temperature and DO stratification
West Upper Bay of Lake Minnetonka 8/31/2000 Dissolved oxygen Explain vertical axis = depth; horizontal = measured parameters (temperature, DO) Strong, stable summer thermal stratification Total loss of oxygen below thermocline Strong temperature and DO stratification Thermocline Temperature No O2 below thermocline Scales: oC and ppm O2

Anoxic below thermocline
Color Mapper Temperature Dissolved oxygen Background Scale DO Temp Line plot Scale Contrast with Profile Plotter for Same Data – Explain vertical axis = depth; horizontal = measured parameters (temperature, DO) Explain Color Mapper (red = hot, blue = cold; green = high DO, black= anoxic) Strong, stable summer thermal stratification Total loss of oxygen below thermocline (anoxia) Anoxic below thermocline

Color Mapper - Shallow Lake
What happens in a shallower lake (~8 m)? Warm water throughout the water column in summer. Seemingly no temperature stratification, yet DO is stratified and anoxic near bottom. Wind energy dissipates rapidly with depth. Q – What happens to the lake (with respect to DO) when a big wind comes up with enough energy to churn it up from top to bottom? A – The wind injects water low in dissolved oxygen from deep in the lake into the surface water. Along with DO come other substances from the bottom layer (e.g., nutrients, sediments, and organisms). DO Dissolved oxygen Temperature Temp

Seasonal Cycles of Temperature & Oxygen
Here’s how the annual temperature and DO profiles of 5 of the WOW Minnesota Lakes look as we step through the temperature profile data, a month at a time. NOTE: Temperature data are shown as a color map where temperature varies from ice (white) to blue (cold) to red (warm). Dissolved oxygen (DO) data are shown as a connect-the-dots line plot, where the discrete depth values of oxygen measured at 1 meter depth increments are plotted against corresponding depths. The error bars at each data value represent the range of values over the course of that particular day. WORK WITH THE CLASS TO IDENTIFY THE TIME PERIODS FOR THE FOLLOWING EVENTS: Mixing (fall and spring) Thermal stratification Hypolimnetic anoxia (and bottom water anoxia in the shallower lakes) Re-aeration periods for each lake

Partial Mixing in Medicine Lake, MN
8/31/2001 Temp DO Is it totally mixed ? Here are profiles of temperature and DO in midsummer for Medicine Lake, MN (a relatively shallow lake ~ 8m). NOTE: These are the same data shown in the shallow lake example using the Color Mapper 2 slides earlier, again illustrating: Warm water throughout the water column in summer. Seemingly no temperature stratification, yet DO is stratified and anoxic near bottom. Wind energy dissipates rapidly with depth. SAME QUESTION APPLIES HERE: Q – What happens to the lake (with respect to DO) when a big wind comes up with enough energy to churn it up from top to bottom? A – The wind injects water low in dissolved oxygen from deep in the lake into the surface water. Along with DO come other substances from the bottom layer (e.g., nutrients, sediments, and organisms).

Interpreting profiles – Ice Lake #1
Questions Time of year ? Explain profiles Temp DO pH Temp pH DO Questions - 1. pH is plotted also - is it as dynamic as temperature and DO? 2. What are some hypotheses for explaining the vertical and temporal patterns of pH? Answers– Temperature: Ice cold at surface increasing to ~ 4oC with depth = Winter DO: relatively high near surface and declining gradually to pretty low values below ~ 11m (~0.5 – mg/L). This shows that the DO has become depleted over time indicating that the lake has been ice-covered for quite some time since the water column is usually saturated with oxygen during fall mixing prior to freeze-up. Suggests a mid-winter set of data whereas you cannot tell from the temperature profile. pH: pretty constant from surface to bottom. Since we know from summer profiles that pH can increase during intense photosynthesis, s there is a moderate amount of water column respiration occuring, we might might expect the pH toThis suggests a THIS IS FROM JANUARY 28, 1998

Interpreting profiles – Ice Lake #2
Questions Time of year ? Explain profiles Temp DO pH Questions - 1. What are some hypotheses for explaining the vertical and temporal patterns of pH? Answers– Temperature: Warm at surface; uniform down to 3 m; sharp drop from 4~8 m; slight decrease from ~ 7 to 6oC in the deeper hypolimnion : Classic summer profile for a deep, stratified lake DO: relatively high near surface and uniform from 0 to 3m. Slight increase at 4 and 5 m (the water is colder and there is probably more photosynthesis occurring in this layer just above the thermocline) DO drops steadily from 5 m down to 10 m and it is essentially zero below there (anoxic). This shows that the DO has become depleted over time indicating that the lake has been stratified for some time. This may take months after stratification for an unproductive lake (some may never get this anoxic up this far from the bottom) OR it may happen quickly in a more productive lake when there is a lot of oxygen demand (meaning respiration) in the hypolimnion. The rate that the hypolimnion becomes anoxic, and the extent of the anoxia (how high up does it reach) is a good index for summer productivity and a good thing to monitor. If you know the morphometry of the lake you can calculate the total mass of oxygen in the hypolimnion and then calculate its disappearance over time (this is called the volumetric hypolimnetic oxygen depletion rate or VHOD rate). Could you get a DO profile that looks like this in winter ? Basically yes – but the bump at 4 and 5m would be difficult to explain pH: pretty constant from surface to bottom but not as much as in winter. The gradual decrease in hypolimnetic pH during summer is caused by an excess of respiration over photosynthesis. This leads to an accumulation of CO2 that dissolves in the water and acts like an acid, lowering the pH. However, the changes are subtle if the lake is reasonably well buffered. Respiration can be thought of as “contributing acidity.” Where photosynthesis is high, there may be a pronounce bump of increasing pH as algae deplete CO2 (and bicarbonate ions) from the water. They are in essence “removing acidity” in the opposite manner to photosynthesis. Look for more dramatic pH changes over seasons in Ice Lake and in other WOW lakes. In the more productive lakes, such as Lake Independence, Medicine Lake and Halsteds Bay, Minnetonka you may observe pH swings of as much as 0.5 units over the course of a 24 hour period since photosynthesis is turned off at darkness while respiration occurs day and night. . Since we know from summer profiles that pH can increase during intense photosynthesis, s there is a moderate amount of water column respiration occuring, we might might expect the pH toThis suggests a THIS IS FROM JANUARY 28, 1998 Temp pH DO

Seasonal Cycles in Ice Lake, MN (Profile Plotter)
Here’s the full annual cycle on a monthly time step Temp pH DO An example of the Profile Plotter for Ice Lake, MN, running on a monthly time step to show seasonality.

Seasonal Cycles in Ice Lake, MN (Color Mapper)
Here is the same annual cycle for temperature and DO for relatively deep (13m) Ice Lake, MN, using the Color Mapper tool. NOTE: Left Graph – Temperature “mapped” (color background) and oxygen “plotted” (line plot). Right Graph – Oxygen mapped and temperature plotted. Follow the progression: 1) Winter ice cover. 2) Spring thaw. 3) Stable summer stratification and hypolimnetic oxygen depletion. 4) Fall mixing. 5) Freeze-up. Q1 – From examining these data, was Spring mixing complete? A1 – Depends on which parameter you use. If you look at temperature, the data suggest that mixing was complete. If you look at DO, the data suggest that mixing was not complete (although there is partial re-oxygenation of the bottom waters). Q2 – From examining these data, was fall mixing complete? A2 – YES, for both temperature and DO. Q3 – Now go back to the slide with all 5 Minnesota lakes running monthly simultaneously and ask the same questions: Was spring mixing complete in each of the lakes? Was fall mixing complete in each of the lakes? A3 – Spring – only Ice Lake data suggest incomplete mixing and re-aeration in the spring. A3 – Fall – all the lakes appear to mix and re-aerate completely.

Interesting Summer O2 Depth Profiles
6/14/99 6/20/99 ASK THE STUDENTS THESE QUESTIONS: Q1: Are the lakes thermally stratified? Q2: Compare the oxygen profiles for the two lakes. What are the similarities and differences between and within the lakes in the red, green and blue layers? Q3: Can you think of any explanations for the differences you observe? ICE LAKE – Oxygen typically peaked and was supersaturated just below the thermocline for much of spring and summer. This layer (the thermocline), at about 4-6 m, apparently has optimal light (too much can cause "light shock") and sufficient nutrients, which may diffuse up from the hypolimnion (the deepest layer of coldest water). At the thermocline (the sharp color change from red to yellow) the water is more viscous (dense) than above. This allows algal cells to settle and accumulate there. Photosynthesis by algae in this zone leads to more O2 being produced than is consumed by respiration or lost by diffusion to the mixed layer above. This explains the “bump” in oxygen at the thermocline. What is curious is that the differences among depths are much greater than the day-night swings in DO, contrary to what limnology textbooks typically describe. DO was almost completely absent (anoxia) by a depth of 8 m in June. This is because: -The water below the mixed layers is isolated by density differences from the O2 in the air -It is too dark in this zone for much O2 to be generated from photosynthesis by algae or aquatic plants. -The oxygen that was present in this layer during the spring mixing period soon after ice-out was consumed, primarily by the respiration of bacteria in the water and in the sediment. Oxygen remains near saturation in the uppermost layer throughout the ice-free season due to turbulent diffusion from the atmosphere created by wind mixing. GRINDSTONE LAKE – Metalimnetic DO minimum! The DO profile shown above illustrates an unusual phenomenon more typical of reservoirs. The DO decreases from over 8 mg/L to ~ 5.5 mg/L below the thermocline (as it does at other WOW lakes at this time of year), but then increases from 8 to 13 m before gradually decreasing with depth to the bottom. -The reasons for this are not perfectly understood but it is likely caused by settling algae and detritus accumulating at the thermocline – leading to higher rates of bacterial activity and zooplankton feeding that exert a higher DO demand than above or below this layer. -Another explanation that often is relevant to reservoirs involves the morphometry of the lake, if there is a large amount of bottom sediment occurring at about 6-8 m causing increased oxygen demand. Below this zone of intense O2 depletion, the lake behaves like the other Minnesota WOW lakes in summer, and oxygen concentration gradually decreases with depth. The gradient at Grindstone is not nearly as steep as it is for Ice or the other WOW lakes because: -Grindstone is less productive and has less organic matter in water and sediments for bacteria to metabolize. -It is much deeper, so the sediment surface, which contains much more organic matter than does the water, has proportionately less contact with hypolimnetic water. -The result? The hypolimnion contains sufficient oxygen throughout the summer to support a trout fishery. As is true for Ice Lake, Grindstone’s uppermost layer remains near saturation with respect to oxygen throughout the ice-free season, due to wind-induced turbulent diffusion from the atmosphere. Ice Lake Grindstone Lake

Compare with Three Nearby Lakes (June 1999)
Q1: Why is West Upper temperature so different? Q2: What caused the strange West Upper O2 profile? Halsted’s Bay, Minnetonka West Upper, L. M’tonka L. Independence West Upper differences: Q1 – Why is West Upper temperature so different? A1 – Cold hypolimnion due to greater depth and early season date (June). The epilimnion was a bit cooler (it’s a larger lake) but the thermocline depth was similar to Halsteds and Independence. Q2 – What in the world caused the strange West Upper O2 profile? A2 – DO “bump” down deep is due to a deepwater, cold, low-light adapted community of phytoplankton. Within a week or two, lake clarity decreased enough to eliminate this peak. Note that there was a small pH “bump” here also, and that there was some indication of a diel cycle (light-dark) in pH and DO – indicating a photosynthetic origin.

Onondaga midsummer – color mapper
Set the color mapper for mid Aug 2003 (this profile is from Aug 22, Set EC to uS/cm (in red), DO to % sat (black) and pH in blue DO > 150% from 0-3m and then <10% down to the bottom ! pH drops >1 unit from 3 down to 5 m EC jumps up and down by 400 uS/cm ! Very dynamic data set

Depth versus Time Plotter (DxT)
Time of Year Parameter Scale Location Depth

Annual Temp & O2 in a Shallow, Productive Bay – Halsteds Bay, L
Annual Temp & O2 in a Shallow, Productive Bay – Halsteds Bay, L. Minnetonka Temp These data illustrate the annual temperature and DO cycles for a moderately shallow (8-10 meters), extremely productive bay, Halsteds Bay of Lake Minnetonka, MN. Q1 – What are the physical and biological processes that might create these patterns? Spring mixing (April – May) Uniform, cool temperatures and generally high DO. The brown, vertical band at the beginning of the record was due to the mixing of low DO water and probably surficial sediments (high O2 demand) soon after ice-out. Summer (July – mid-August) Unstable thermal stratification and strong DO stratification. Thermal stratification was variable, and, although the temperature gradients were relatively small, the temperatures were high enough to create a moderately stable two-layer system. DO shows this more clearly since the high levels of organic matter in the deeper water coupled with highly organic sediments acted to totally deplete O2 in the bottom few meters of water. However, the occasional green spikes indicate windy periods when the mixed layer temporarily deepened. Late summer storms (late August – September) Severe thunderstorms ripped through the area causing short (<1 day) periods of intense mixing. These were interspersed with calmer periods when the high oxygen demand rapidly produced anoxia in bottom waters. Fall mixing (September – December) The bay was completely re-oxygenated and isothermal (constant temperature with depth) while steadily cooling until it froze in mid-December. Winter ice cover (December – March) Inverse temperature stratification, with 0oC water overlying ~ 4oC water. DO was initially high, but gradually exhibited oxygen depletion from the bottom up to about mid-depth (~4 meters). * spring thaw and mixing - early thaw in March followed by mixing and re-oxygenation of water column O2

Compare two bays of the same lake Halsteds and West Upper, Minnetonka, MN
Comparison of the temperature and dissolved oxygen regime in Halsteds Bay of Lake Minnetonka (a shallow, eutrophic bay) and West Upper Lake, another bay of Lake Minnetonka (much deeper, less productive). NOTE: Temperature is “mapped” as the color background and dissolved oxygen is shown as the line graph. Run the animated gifs to show the high degree of variability of the mixing regime in relatively shallow Halsteds Bay, relative to the mid-lake station (West Upper) in Lake Minnetonka.

Medicine Lake - summer stratification
This DxT plot contains temperature and dissolved oxygen data for Medicine Lake, MN during August, 2001. Q1 – Is the lake stratified with respect to temperature? How about oxygen? What is going on? A1 – Medicine Lake is moderately deep (~ 13 m) and very productive (eutrophic). The RUSS unit was not in the deepest part of the lake. It was located in an area that was ~ 10 m deep. Midsummer was characterized by strong stratification the first half of August (until ~ 8/14/01) with an increased frequency of wind mixing events in the last half of August (~ 8/14/01 – 8/26/01). In late August the mixed layer deepened by a meter or two as indicated by the anoxic-oxic interface in the DO graph (identified by the depth of the abrupt shift in color from black to green). NOTE: Enough low-DO water was mixed into the upper layer in the last half of August to reduce its dissolved oxygen to <5 mg/L on occasion, as indicated by the brown “stripes.” A strong storm in early September (~ 9/6/01) mixed, and re-oxygenated, the lake to > 10 m. Calm periods were then interspersed with mixing events for the next week, after which the water column was generally mixed to a depth > 10 m.

Ice Lake, MN - Interannual variation
rate of thermocline descent Interannual variation in temperature, mixing regime, and O2 depletion in Ice Lake, MN. Ice Lake, MN is a rather typical, relatively deep, north temperate zone lake. Temperature Plot The temperature data suggest complete mixing and overturn in spring and fall during each of the 4 years that RUSS data have been collected. Work with the class to examine temperature changes in the lake. Look at the slope of the green – blue interface in the temperature plot to estimate the rate of thermocline descent (also = the rate of epilimnion deeping) in late summer. This is a rough measure of the the rate of destratification. Dissolved Oxygen Plot Although the temperature data suggest complete mixing and overturn in spring and fall each year, the DO data suggest otherwise – in fact in only 1 of the 4 years was the entire water column completely re-oxygenated by mixing and exposure to the atmosphere (plot becomes green from top to bottom). Question – What happened in 2002 and 2003 ? What would you predict and why ? Examine DO – look at the slope of the black – green interface to see a measure of the rate of O2 depletion in summer (the upward positive slope) and the rate of re-oxygenation during fall overturn (the downward slope after September). These aren’t the actual rates because we haven’t corrected for morphometry – recall that the lake is not a rectangular solid and show each meter thick stratum of water is smaller as you go deeper. These data are just for the middle of the lake near its deepest point. BUT- steeper the upward slope, the faster the hypolimnion is becoming anoxic. You can also see how under-ice depletion is less intense than summer hypolimnetic depletion (less organic matter for bacterial growth and somewhat cooler temperatures that slow bacterial metabolism presumably cause the rate of depletion to be lower). NOTE: Because we have to remove the RUSS “Ice House” well before the lake thaws, the only way to really know if the lake mixed completely is to get out there IMMEDIATELY after ice-out and get a series of dissolved oxygen profiles in the period before the lake thermally stratifies. We were able to do this at Ice Lake. Otherwise, if it thaws during a warm, relatively calm period, particularly if it is a late thaw, it can stratify within a few days. This is enough to essentially “seal off” the bottom waters from circulating with surface waters. Q1 – How will the biota of the lake be affected if the lake does not mix fully in the spring? A1 – Incomplete mixing means there will be incomplete re-oxygenation in the spring. This means the hypolimnion will already be carrying a large O2-deficit into the summer, with potentially important effects on lake organisms and and the biogeochemistry of the ecosystem. “rate of hypolimnetic O2 –depletion” No spring mixing Fall mixing Fall mixing ? Fall mixing spring mixing

Ice Lake, MN: 1998-2003 Spring: Fall: Complete Mixing No-98,99,01
Yes- 00,02,03 Fall: No-00 ?? Yes- 98,99,01,02

DO-Temperature squeeze on fish
How would a warm water discharge from a new power plant affect the fish community ? How would global climate change affect the fish ? Mid-summer, when strong thermal stratification develops in a lake, may be a very hard time for fish. Water near the surface of the lake - the epilimnion - is too warm for them, while the water near the bottom - the hypolimnion - has too little oxygen. Anoxia forces the fish to spend more time higher in the water column where the warmer water is suboptimal for them. This may also expose them to higher predation, particularly when they are younger and smaller. Eutrophication exacerbates this condition by adding organic matter to the system which accelerates the rate of oxygen depletion in the hypolimnion. Urban, and other forms of runoff, can also add to this problem very suddenly and dramatically by causing fish kills after excess soils and road hydrocarbons are washed in from intense rainstorms. Conditions may become especially serious during a stretch of hot, calm weather, resulting in the loss of many fish. You may have heard about summertime fish kills in local lakes that likely results from this problem. In eutrophic and hypereutrophic lakes, summertime fish kills can happen most easily during periods with high temperatures, little wind and high cloud cover. The clouds reduce daytime photosynthesis with its oxygen production and so the DO in the mixed layer. Or even thorughout the weater column of a shallow unstratified lake, can become critical for fish and other aquatic organisms. The same basic phenomenon can occur in winter (Winterkill) when ice cover removes re-aeration from the atmosphere and snowcover can light-limit algal and macrophyte photosynthesis under the ice. Many lakes in the upper midwest are mechanically re-aerated or injected with air, oxygen or even liquid oxygen to keep ice off of some of the lake and to add oxygen directly to prevent winterkills. Thermal stratification can be a challenge for coldwater & coolwater fish Too little DO where the temperature is optimal

Case study of density layering - reservoirs
The influent water “seeks” its own density (and destiny) Balance of temperature, dissolved salts and silt load Varies seasonally See movies in the WOW Lake Ecology Primer showing aquarium lake models at primer/page5.html This slide links to the next set of slides about Lake Mead, NV-AZ that are based on real data that illustrates the importance of understanding density in order to manage reservoirs in particular. Schematic from NALMS The lake and reservoir restoration guidance manual. 2nd edition. North American Lake Management Society and USEPA Office of Water, Washington, D.C. EPA-440/ August 1990.

Lake Mead, NV-AZ, USA USBR: http://www.hooverdam.usbr.gov/
1. Source: United States Department of the Interior Bureau of Reclamation - Lower Colorado Region 2. Lots of the VERY BEST images from Copyright © "LAS VEGAS OUTDOORS".

Lake Mead, NV-AZ, USA - features
Mainstem Colorado R. Lifeblood of SW US Largest reservoir in US National Recreation Area A: ,000 acres V: ~ million acre-feet z-max: ~ 150 m (main basins) z- max ~ m (LV Bay ) power for 500,000 homes (2,074,000 kilowatts) drinking water ~20 million irrigation ~1 million acres wastewater ~ 153 mgd oligotrophic - main basins eutrophic Las Vegas Bay municipal sewage density plumes can combine wastewater & drinking water Besides data, there’s always a lot going on in the southwestern US regarding Colorado River water rights and recreation. Go to the link below to find out about recent water rights issues: • Besides being a multiple use water resource, Lake Mead and surrounding areas form the Lake Mead National Recreation Area that is administered by the National Park Service • Hoover Dam is one of the great engineering wonders of the world:

Lake Mead, Las Vegas Bay – images
DW intake Boulder Basin- Sentinel I. DW = Drinking water intake for the City of Las Vegas, NV and neighboring communities (LINK to SNWA and LVVWD). It is important to point out that the drinking water intake for Las Vegas is ~ midway between the RUSS sites at a depth of about 55m at Saddle Island. The next set of slides illustrates how the treated wastewater effluent forms a plume that can extend far out into the bay at various depths depending on its density relative to the receiving water body. Additional sources of information about water quality issues associated with Lake Mead will be available on the WOW website and via the following agency websites: Las Vegas Valley Water District (LVVWD) Nevada Department of Environmental Protection US Geological Survey (USGS) Sediment Studies in Lake Mead USGS National Water-Quality Assessment Program (NAWQA) Las Vegas Wash Coordination Committee (LVWCC) (CHANGE TO JUST ) Las Vegas Valley EPA-EMPACT Project Southern Nevada Water Authority (SNWA) U.S. Bureau of Reclamation (USBR-Lower Colorado) CHANGED TO in October 2003) L. Mead National Recreation Area Hoover Dam

Lake Mead – Boulder Basin images
Inner LV Bay About 120 million gallons of treated wastewater flows into the inner Las Vegas Bay each day LV Bay Sentinel I. Effluent Inflow

Bottom scale: miles from LV Wash inlet
Density due to temperature vs salt load from sewage controls where the plume goes EC TEMP DO pH DEPTH (m) Bottom scale: miles from LV Wash inlet April 1996 Warm over cool Salinity (EC25) overrides temp and the water mass sinks as it moves into bay This is the approximate location of the Saddle I. drinking water intake Note: It was suggested by some researchers that wastewater effluent was inadvertently pumped in relatively undiluted form into the Las Vegas Valley Water District’s drinking water plant and may have caused a number of a number of infections and deaths from Cryptosporidium in the period Cryptosporidium is a small protozoan parasite that is more resistant to chlorination than most pathogens and may require extra filtration for total removal. It is not easily monitored, particularly at the time that the deaths occurred, and to our knowledge was never actually observed in the plant’s raw water intake or in the bay near the intake (as of 1997). The full paper is on-line at no cost: Susan T. Goldstein, S.T., D. D. Juranek, , O. Ravenholt, A. W. Hightower, D. G. Martin, J. L. Mesnik, S. D. Griffiths, A. J. Bryant, R. R. Reich and B. L. Herwaldt Cryptosporidiosis: An Outbreak Associated with Drinking Water Despite State-of-the-Art Water Treatment. Annals of Internal Medicine 124 (3): The limnology data is from: LaBounty, J.F. and M.J.Horn The influence of drainage from the Las Vegas Valley on the limnology of Boulder Basin, Lake Mead, Arizona-Nevada. Lake and Reservoir Management 13 (2): The bottom scale = miles from the Las Vegas Wash inlet. Most of the water is tertiary treated domestic wastewater from Las Vegas and surrounding communities. The entire bay is referred to as Las Vegas Bay and opens up into Boulder Basin which terminates at Hoover Dam where the lake outflow is discharged from the hypolimnion unless water must be discharged via surface overflow channels in extreme high water years. The bay is also referred to as “Government Wash”, the name of this natural feature ( a large ravine ) prior to inundation when Lake Mead was created. The vertical scale = depth in meters. Graphs from LaBounty, J.F. and M.J.Horn

Density plumes – Outer Vegas Bay, L. Mead AZ
These graphs show how LV Bay water moves into the main lake in Boulder Basin pH DO EC TEMP DEPTH High salinity (EC25), low DO plume from wastewater Low salinity, narrow stratum of water is the “remains” of the Colorado River flowing down lake for over 70 miles This set of graphs shows how the plume from Las Vegas Bay gradually dissipates at moves out into the Bay and towards the main portion of Lake Mead (Boulder Basin immediately above Hoover Dam). However, the signature lower salinity (EC25) of the Colorado River can still be observed almost at the Dam. The high silt load that characterizes Colorado River water as it exits the Grand Canyon to enter the far upper basins of Lake Mead (Greg’s Basin) has settled out but the water mass continues to flow subsurface because of its cold temperature and lower salinity. Questions to ask are: Q1: Why might the river be so cold in August in one of the hottest climates on earth ?The river’s col A1: It is hypolimnetic water discharged from Glen Canyon Dam that impounds Lake Powell upstream from the Grand Canyon. Q2: The Colorado River, shown most often in association with whitewater rafting expeditions, is always extremely brown and muddy. Wouldn’t this add to its density ? A2: It would but in fact the secchi depths in Boulder Basin (not shown here) are extremely deep year round ranging from meters. As the river channel widens into the Lake Mead basin, the water velocity slows and its silt load drops out rapidly – probably almost entirely gone some 60 miles up-lake. (This would be a good student lab project for further investigations) Drinking Water intake Hoover Dam– 10 miles out Graphs from LaBounty, J.F. and M.J.Horn

Lake Mead – Las Vegas Bay RUSS DxT data
Data from ~ mid Bay for April – August 2003 T DO EC pH Explain the pattern for each parameter T: Summer stratification with occasional deep mixing from either wind or floods DO: Anoxia in bottom water beginning ~ July; correlates with intensified thermal stratification; very high DO in surface waters in spring and mid summer – if supersaturated (must view the actual data and set to % saturation) must be due to algal photosynthesis. EC: The darker blue appears to define a plume of water that rises and sinks. Observe how the mixing “event” in early June (right after the gray data gap) affects the various parameters. pH: Recall that high rates of photosynthesis increase pH and that high rates of respiration decrease pH. How does pH vary in relation to DO ? Turbidity: these data aren’t shown here but can be accessed via the data section. Most of the turbidity at this station is probably due to algal blooms although there may be silt during floods.

Lake Mead – Las Vegas Bay DxT scaling
An example of how playing with the scale adjustment on the DxT tool can highlight the behavior of a stratum of water

Lake Onondaga – also has a density layer
South Deep site DxT: start 6/21/03 for 85 d Corresponding DO (% saturation) Inquiry lesson. What’s causing the high salinity layer that is ~ 10 m thick ?

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LAKE ECOLOGY Unit 1: Module 2/3 Part 4 – Spatial and Temporal variability January 2004.

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LAKE ECOLOGY Unit 1: Module 2/3 Part 4 – Spatial and Temporal variability January 2004.

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