Crater Lake, Oregon -589 m deep and possibly the clearest lake in the world, Transparency up to 90 m. Thermocline very deep for its size No rooted plants. Mud doesn’t accumulate on the bottom till > 90 m depth Some of the most spectacular tectonic lakes are formed in volcanic craters. Why is this lake so different from most lakes?

Physical features of lakes that determine habitat characteristics inflow from the watershed/Catchment Water residence time Morphometry, Mean depth and volume Thermal stratification and physical mixing wind./currents/wave action Sediment deposition Light extinction

Assume runoff coefficient of 0.15 m Drainage area =7.9 km 2 Lake area =0.9 km 2 How much water would you expect flows into this lake /yr? How much water flows into lake Beauvais lake in a year from its watershed? Evaporation from lake surface exceeds precipitation by mm/yr How much water flows out of the lake?

Assume runoff coefficient of 0.15 m Drainage area =7.9 km 2 Lakearea =0.9 km 2 How much water would you expect flows into this lake /yr? P ─ E on lake surface = ─ m/yr Q i = r * DA = 0.15 m/yr * 7 x 10 6 m 2 = 1.05 x 10 6 m 3 /yr What is the net evaporation in a year? (P-E)*A = ─ m * lake area = ─ 0.085m/yr * 9 x 10 5 m 2 = x 10 4 m 3 /yr How much water flows out of the lake in a year? Q o = Q i + (P-E)*A = 1.05 x 10 6 m 3 /yr + (─ 7.65 x 10 4 m 3 /yr) = 9.75 x 10 5 m 3 /yr

Definition of water residence time and flushing rate Chapter 4

Lake Area = 0.9 km 2 Mean depth= 4.3 m Lake Volume = 3.8 x 10 6 m 3 Water residence time= Mean renewal rate=

Water residence time Mean flushing rate Lake Area = 0.9 km 2 Mean depth= 4.3 m Lake Volume = 3.8 x 10 6 m 3 Water residence time= Mean renewal rate=

How much of the water flowing into this lake from its watershed could you allocate for irrigation before the lake would gradually begin to disappear? Answer Over 92%

Lake management—the water inflow budget or what happens when you over allocate? The Aral Sea in the former Soviet Union—mismanaging the river water inflow Allocation to desert irrigation > inflow minus evaporation Fig. 5.19

. Effects Ecosystem collapse, loss of biodiversity, worsening of water-salt balance in the agricultural areas, pollution of rivers and drinking water, changing of the regional climate – all these are new environmental developments in Central Asia.

Calculating volume and mean depth Mean depth = Volume/surface area The hypsographic curve Area under the curve = volume Fig. 7.1 in text

The thermocline occurs deeper in large lakes because wind energy is transmitted to greater depths Wind energy increases with fetch In small lakes convection also plays a role in determining thermocline depth In deep lakes only the surface layers are well mixed and quite warm, whereas the deeper parts remain cold. Lakes partition themselves into temperature zones Thermal stratification in lakes Fig in text

The seasonal pattern of thermal stratification in a deep temperate zone lake Depth-time graph of isotherms Vertical thermal profiles During spring turnover the entire Water column is 4 o C—why 4 o C Same thing happens again in the fall

Fig in text Heat diffuses much faster down the water column in large lakes—wind mixing Hence the thermocline is deeper in large lakes Table 11.2 in text

In small lakes mixing is more determined by convection currents driven by solar heating and is determined by how deep light penetrates Top of the thermocline Middle of thermocline

In very large lakes horizontal thermal shear zones occurs at river mouths A thermal bar Important habitat feature for many fish species in spring.

Waves- the gravitational response to wind disturbance The bigger the wind fetch the bigger the wave oscillation Wave energy and slope together determine the depositional zone boundary The velocity in these oscillations attenuates sharply with depth At depths > depostional Boundary depth fine mud accumulates Fig in text after Rasmussen and Rowan (1997) Log DBD(m)= ─ Log F (km) slope (%)

An undisturbed sediment core containing varves from the deposition zone of a deep lake The varves can be used to calculate dates along the core profile

Paleolimnology--Pollen stratigraphy in lake sediment cores

Cores can be dated with radioisotopes 137 Cs (half-life 30 yr) is found in fallout from bomb tests The most commonly used isotope is 210Pb, half-life 22 yr

210 Pb 218 Po 222 Rn 226 Ra 238 U The Uranium 238 decay series

The littoral zone, what determines its outer boundary?

The transparency of lake water is measured by its extinction coeficient The extinction coefficient k increases with: the concentration of organic matter (colour) of the water the amount of suspended matter eg, phytoplankton, fine suspended particles, eg clay

Light extinction --Light enters from above and its intensity (I) is sharply attenuated with depth (z)—absorption by water or solute molecules or scattered by particles IzIz z Photic zone z 50% z 1% z 10% Section 10.6 Page 144 in text

In general Photosynthesis exceeds respiration above the 1% light level and rooted plants can grow down to about the 10% light level IzIz z Photic zone z 50% z 1% z 10% Page 144 in text

The extinction coefficient k increases with: the concentration of organic matter (colour) of the water the amount of suspended matter eg, phytoplankton, fine suspended particles, eg clay

Differential absorption by wave length gives water colour Red light is absorbed much more than blue in distilled water Deep clean water appears blue because most back-scatter from depth is blue; shallower waters will back-scatter a mix of blue and greens so such lakes appear blue-green Organic matter absorbs blue the most—appears yellow/brown When a lake is rich in humic matter (tea) the organic matter absorbs most of the blue, and green end of the spectrum, Fine colloids of calcite in water absorb blue mostly—water looks green Suspended clay/silt scatter all wave lengths so water appears milky (no colour) A dense phytoplankton bloom appears green because of chlorophyll in the algal cells

The electromagnetic spectrum

Increasing depth Pure water absorbs preferentially the longer wave lenghts —at depth short wavelenths predominate-everything gradually looks blue Incoming spectrum—white light all colours present 5-10 m depth water greenish m depth water blue-green m water blue

Clean shallow lakes usually appear bluish-green

Deep lakes appear blue because back scattering from deep water is mainly blue Longer wave lengths have been absorbed already at shallower depths

Water from swamps like these appears brown because of its high content of dissolved organic matter which absorbs strongly at the blue end of the spectrum

Glacier meltwater full of suspended particles looks milky white since all wavelengths are absorbed or back-scattered.

This pond has a dense phyto-plankton bloom, and the green colonial algae make the water look green

The action spectrum for photosynthesis—blue and red work best green, yellow and brown are least useful

Based on the absorption spectrum for photosynthetic pigments, would you expect to find algae or plants growing near the lower boundary of the photic zone is (a)A clear lake with little organic or particulate matter in the water (b)A brown-water humic lake Consider what you know about the spectral composition at depth in each of these two types of lakes.

Where does the exponential equation come from. Another way of writing it is as a rate equation. The rate of change of light intensity with depth decreases as a linear function of the light Intensity IzIz z Photic zone z 50% z 1% z 10% Section 10.6

Practice questions Explain how flow processes contribute to habitat diversity in rivers and streams. Outline some examples of human activities that impact riverine habitats. Explain why these activities can put aquatic species at risk. What is a proglacial lake? Explain how they form and disappear on the landscape and why they are important in determining the distribution of aquatic species?

“Wind streaks” and Langmuir spirals

Fig

Larger scale gravitational responses to wind action—The seiche Fig

The oscillation of the thermocline during a seiche Fig

The oscillation of the thermocline produced by internal waves during a large seiche Fig