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

Increasing depth Water containing dissolved organic matter—organics absorb short wave lengths and the water absorbs the long wave lengths-spectrum trimmed from both ends Incoming spectrum—white light all colours present 2-5 m depth water yellow-brown 5-10 m depth water brown At depth > 10 m virtually no light

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.

“Wind streaks” and Langmuir spirals

Larger scale gravitational responses to wind action—The seiche

The oscillation of the thermocline during a seiche

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