Light extinction in aquatic ecosystems can be an important limiting factor for organism growth, particularly for primary producers (plants and algae), since too strong or too weak light can limit their productivity. Suspended and dissolved substances and water scatter and absorb light; so radiation is attenuated with increasing depth and is limited only to a superficial layer. Since light used in photosynthesis (PAR or 'photosynthetically active radiation') is mainly in the visible range (400-700 nm), henceforth when speaking of light, it will be referred to as PAR. Anyway, it must be noted that almost all radiation outside that range is extinguished at about 1 m depth. PAR is expressed as Wm~2 or, more usually, as PPDF (photosynthetic photon flux density, which is the number ofphotons in the visible range incident per unit time on a surface unit, p,mol m~ s~ ).
Light is attenuated in the so-called euphotic (or photic) zone, often defined as the water layer where photosynthesis can take place or by the depth where 1% of surface irradiance is still to be extinguished, but more precisely limited by the depth where primary producer respiration equals photosynthesis (the so-called compensation depth). The euphotic depth depends on water turbidity and typical values are about a meter or less in very turbid waters like estuaries or some lakes, 30 m in coastal waters, and up to 200 m in clear open ocean waters.
Attenuation of the intensity of light entering water surface Iin is usually modeled with Lambert-Beer law as a function of depth kpart a ' C
where a is a constant to be determined on field. In shallow euphotic depth ecosystems or in ecosystems where noticeable algal bloom can be found, phyto-plankton concentration usually has dominant effects on light extinction, and it can be useful to express k as a function of its concentration (e.g., Chl-a concentration A)
or k — b + c ■ A + d ■ Ae where b, c, d, and e are constants to be experimentally determined. This equation models the so-called shading and self-shading in phytoplankton, meaning that primary producers attenuate light and can also self-limit their growth because of their increasing biomass blocking PAR. So, phytoplankton growth as a function of light should have a saturation behavior (or an optimum one, as too high radiation is harmful to algae).
In oligotrophic systems, usually water and dissolved matter have the most significant effects, and b, which is related to background turbidity other than algae and to water effects, is bigger than the other terms.
Extinction coefficient k can also roughly be related to Secchi disk depth zsd by the Poole and Atkins equation k ■ zsa — constant
Awat (z) — ^in ' e where Iwat is light intensity at depth z. The k extinction coefficient is associated with a given wavelength and can be determined experimentally by the use of photometers at several depths. It must be noted that most light wavelengths (especially blue and red) are filtered after a few meters and then light becomes almost monochromatic (green color).
where the proportionality constant must be empirically calculated (it ranges from 1.7 in clear ocean water to 1.4 in turbid coastal water). Attention must be paid also to the intrinsic limits of Secchi disk measure of water transparency, defined as the depth at which a standardized white and black disk is no longer visible from the surface, as it is dependent on observer visual acuity, sun altitude, presence of shadows, and refraction caused by water surface.
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