Contribution of the different components of the aquatic medium to absorption of PAR

Apart from the small amount of light scattered back out of the water, attenuation of PAR in water bodies is due to absorption, although the extent of this absorption within a given depth may be greatly amplified by scattering, which increases the average pathlength of the photons within that depth. The relative contribution of different components of the system to this absorption at a given wavelength is in proportion to their absorption coefficients at that wavelength. Absorption coefficients for any particular component and the intensity of the incident solar radiation vary independently with wavelength throughout the photosynthetic range, and so the contribution of the different components to absorption of total PAR can be assessed by carrying out the appropriate calculations at a series of narrow wavebands and summing the results. In this way it is possible to calculate what proportion of those photosynthetic quanta that

Table 3.3 Calculated distribution of absorbed photosynthetic quanta between the particulate fraction, soluble fraction and water in Australian water bodies. The calculations have in each case been carried out for the euphotic zone of the water body in question. Data from Kirk (1980b), Kirk and Tyler (1986) and unpublished.

Quanta absorbed (% of total)

By dissolved By particles Optical By colour (tripton/

Water body type water (CDOM) phytoplankton)

Coastal-Oceanic

Jervis Baya

W

Estuarine

Lake Kingb

WG

Inland impoundments

Corin Damc

G

Lake Ginninderrac

G

Googong Damc

G

Cotter Damc

GA

Burrinjuck Dama

GA

Lake Burley Griffinc

T

Natural inland

waters

Latrobe Riverb

T

Lake Georgea

T

Gulungul billabongd

GT

Georgetown

T

billabongd

aNew South Wales, b Victoria, c Australian Capital Territory, "Northern Territory.

are absorbed are captured by each of the different absorbing components of the system.696,701 Table 3.3 presents the results of such calculations for 12 Australian water bodies.

It is clear that the three major components into which the light-absorbing material of the system has been divided - the water itself; the dissolved yellow substances (gilvin, CDOM); and the particulate fraction (tripton/phytoplankton) - can all be substantial light absorbers. In type W waters, most of the photosynthetic quanta are captured by water itself. In oceanic waters not affected by terrigenous material (type W, Case 1), those quanta not absorbed by water are captured mainly by the pigments of the living and dead phytoplankton.1241 In coastal waters, with low but significant amounts of gilvin, the contributions of the phytoplankton and the dissolved yellow material to absorption are likely to be small and comparable. In estuarine waters, such as Lake King (type WG), with more substantial levels of soluble yellow substances, the water and the gilvin capture most of the photosynthetic quanta. In the type G inland waters (plenty of dissolved colour, but low turbidity) described in Table 3.3, gilvin captures the most quanta, followed by water. In the type GA waters (dissolved colour, rather low turbidity, plentiful phytoplank-ton), gilvin is still the most important component but the particulate fraction now captures about as many quanta as does the water. In the type T waters (high turbidity due to tripton), most of the quanta are absorbed by the particulate fraction. The particulate fraction and gilvin between them take most of the quanta in type GT waters (intense soluble colour, high tripton turbidity). In highly productive, type A, waters, dominated by phytoplankton, we may assume that the algal biomass captures more photosynthetic quanta than the other components of the medium.

McKee et al. (2002) studied photon partitioning in the surface layer (~5m deep, of lower salinity due to river inflow) of Loch Etive, a Scottish fjord. Despite its high colour (g440 1.2-1.4 m_1), due to river-borne CDOM, it is a productive system (chlorophyll a 3.5-8.5 mg m~3). Calculations on the basis of the absorption data indicated that ~20% of the PAR in the water column was absorbed by phytoplankton, 44% by CDOM and 36% by sea water. For a marine system, using a different approach (multiple linear regression analysis carried out on a large number of samples), Pfannkuche (2002) estimated that in the waters of the Otago Shelf (East of the South Island of New Zealand) gilvin absorbed 11%, phytoplankton 39%, inorganic particles 38% and water itself 12% of the PAR. For 13 stations in Lake Biwa (Japan), the data of Belzile et al. (2002a) indicate that on average 25% of the (400-700 nm) photons were captured by water, 33% by CDOM and 42% by particles.

Siegel et al. (2002) have used satellite imagery from SeaWiFS (Chapter 7) to characterize the global distribution of light absorption due to coloured detrital and dissolved materials (CDM, equivalent to CDOM plus tripton). They conclude that, taking the world's oceans as a whole, CDM makes roughly the same contribution to blue light absorption as phyto-plankton. Field observations show that most of the CDM absorption is carried out by CDOM, the dissolved component. In a later study, using SeaWiFS data, Siegel et al. (2005) estimated that the contribution of CDM to total non-water absorption at 440 nm varies from 30 to 40% in the subtropical gyres to more than 60% at high latitudes.

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