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Re: light attenuation

On Mon, 17 Jul 2000, Wright Huntley wrote:
> > Nevertheless, I contend the *difference* in substrate-level available light
> > between an 18"-deep tank and a 24"-deep tank is essentially insignificant.

To which Ivo Busko replied:
> Sorry if I misunderstood, but you mean the article is wrong ? According
> to the "pure H2O" curve, which is the one relevant for this purpose, a 
> 16" layer of pure water should absob 15-20 % of the ligth at the 600-700 
> nm band, about 5% at the visible range and nothing at the blue.

Adey and Loveland (1991) gives values for the attenuation of natural PAR
(so, not specific to any particular wavelength) in various natural water
bodies.  I've taken the liberty of converting the values from 1/meter to
percent per foot:

   Oceanic Water
      Sargasso Sea                            0.9%
      Gulf Stream off Bahamas                 2.4
      Pacific Ocean, 100 km off Mexico        3.4

   Coastal and estuarine waters
      Bjornafiord, Norway                     4.6%
      Gulf and California                     5.2
      Tasman Sea, Australian coast            5.5
      Port Hacking estuary, Australia        11.3
      Clyde R. estuary, Australia            21.6

   Inland waters
   North America
      Crater Lake, Oregon                     1.8%
      L. Ontario, Canada                      4.6 - 18
      San Vicente reservoir, California      20
      Lake Minnetonka, Minnesota             21 - 85
      McConaughy reservoir, Nebraska         49
      Yankee Hill, reservoir, Nebraska       76
      Pawnee reservoir, Nebraska             88

      Esthwaite Water, England               24 - 49%
      Loch Croispol, Scotland                45
      Loch Uanagan, Scotland                 72
      Sea of Galilea                         15
      Sea of Galilea (Peridinium bloom)     101
      Lake Simbi, Kenya                      91 - 380%
      Lake Tanganyika                         4.9
      a) southern tablelands
      Corin Dam                              27%
      Lake Ginninderra                       45
      Burrinjuck Dam                         50
      L. Burley Griffin                      86
      L. George                              460
      b) Northern Territory (Magela Creek bilabongs)
      Mudginberri                            38%
      Leichardt                              51
      Georgetown                            260
      c) Tasmania (lakes)
      Perry                                   6.4%
      Ladies Tarn                            13
      Risdon Brook                           18
      Barrington                             34
      Gordon                                 66

Values of a few percent per foot can be used the way they read.  Higher
numbers are a little problem.  Math-impaired may want to skip down a ways
to where the English starts again.

The coefficients are for exponential attenuation so the coefficient always
overstates the amount of attenuation you would actually get as light
passed through a foot of water.

The equation is:

   I/I0 = exp(-Kd*D/100)

   Kd is the attenuation coefficient given above
   D is the depth in feet
   I0 is the incident light intensity
   I is the light intensity at depth D.

<English here I think> 

I think that means that light attenuation over short distances in pure
water would be insignificant -- less than 1% of the light per foot and
less than 1.3% in 16 inches.  The amount of attenuation in an aquarium
will be pretty variable but like Wright, I expect that our tanks are
usually on the clear side of natural water.  Let's say, something like 5%
per foot on a good day.  In the article I circulated last fall, Davis and
Brinson (1980) cited a value that converts to 6% per foot for clear lakes
like Lake Tahoe.  At 6%, the light penetrating to 24" inches would be
almost as bright (97% as bright) as the light at 18".  There's leeway for
that number to be a little higher and much lower.  Also, reflection and
refraction in a tank would effect the vertical distribution of light.

While this is all very nice I think it neglects the most important factor,
which is the plants themselves.  The plants adsorb red and blue light and
reflect green light, so the light in a heavily planted tank often looks
pretty green.  If you have a deep tank or you let the plants close in at
the surface then the light can get *really* green.  Our eyes are sensitive
to green light, so it still looks only a little darker than the unfiltered
light, but to plants that green light is nearly useless.

Roger Miller

Adey, Walter H. and Karen Loveland, 1991.  Dynamic Aquaria.  Academic
Press, San Diego, Ca. 643 pp.

Davis, Graham J. and Mark M. Brinson, 1980.  Responses of vascular plant
communities to environmental change.  US Fish and Wildlife Service