Dc

w s av range return values of 0.12 to 0.2 m-1 (Reynolds, 1987b, and unpublished observations). Elsewhere, the effects of solutes contribute to higher average extinction coefficients, in coastal seas (ew > 0.15 m-1) and fresh waters generally (ew > 0.2 m-1). At the other extreme, coloration due to humic staining can be intense; Kirk (1994) tabulated some representative values from estuaries and coastal seas receiving drainage from extensive peatland catchments (Gulf of Bothnia, Baltic Sea; Clyde, Scotland) of 0.4 to 0.65 m-1, from some humic lakes and reservoirs in Australia (1 to 3.5 m-1) and from peat-bog ponds and streams in Ireland (2 to 20 m-1).

ep is the attenuation coefficient due to the solid particulates in suspension in the water. The particles may emanate from eroded soils or resuspended silts or biogenic detritus. Fine precipitates, for instance, of calcium carbonate, also contribute to particulate turbidity. The most persistent are clay particles (typically <5 ^m), eroded from unconsolidated deposits, which, by absorbing and backscatter-ing the photon flux, can impart high turbidity. In many lakes and seas, the contribution of background turbidity to attenuation may be trivial (<0.05 m-1) but Kirk (1994) cites many examples where it is anything but trivial; canals, rivers, meltwater streams flowing into lakes or reservoirs can impart attenuation coefficients of 1 to 4 m-1, with extreme values in the range 10-20 m-1. Investigating the turbidity of the tidally mixed estuary of the Severn, UK, Reynolds and West (unpublished data, 1988) found a correlation between ep and the mass of suspended clay and fine silt, such that an attenuation coefficient of 20 m-1 corresponds approximately to 1 kg suspended clay m-3, whence ep ~ 20 m2 kg-1.

Nea is the attenuation due to phytoplankton. Evolved to be able to intercept light energy, phy-toplankton can be, in aggregate, the main component to vertical attenuation. The extent is proportional to the mass of phytoplankton present per unit volume (N, in mg chla m-3); however, the chlorophyll-specific attenuation coefficient, £a, varies considerably. Most general accounts attribute attenuation coefficients of between 0.01 to 0.02 m-1 (mg chla m-3)-1, or, more simply, 0.01 to 0.02 m2 (mg chla)-1. Variations are, in

Figure 3.11

The chlorophyll-specific area of light interception by algal cells as a function of size and shape (•, spherical cells; A, non-spherical cells). The straight line corresponds to a diminishing efficiency of light interception by cells of increasing size but very small cells also lose efficiency at sizes close to the wavelength of light. Redrawn with permission from Reynolds (1987b).

Figure 3.11

The chlorophyll-specific area of light interception by algal cells as a function of size and shape (•, spherical cells; A, non-spherical cells). The straight line corresponds to a diminishing efficiency of light interception by cells of increasing size but very small cells also lose efficiency at sizes close to the wavelength of light. Redrawn with permission from Reynolds (1987b).

part, due to the size and shape of the algae (see Fig. 3.11). Among microplankters, larger quasi-spherical shapes, having larger volumes, hold absolutely greater amounts of chlorophyll than smaller shapes. Relative to the cross-section presented to the light field, however, the chlorophyll is more compactly arranged in larger units and more light passes between them than in smaller units carrying the same amount of chlorophyll. This approximates to the expectations of the 'packet effect' (or 'sieve effect'), first studied by Duysens (1956) in the context of solutions and suspensions but developed for phytoplankton by Kirk (1975a, b, 1976). However, among the free-living nanoplanktic and picoplanktic cells (<100 ^m3), the chlorophyll-specific area of projection is diminished, to as low as 0.004 m2 (mg chla)-1. This may have something to do with the size of plastids being similar to the wavelength of light. Alternatively, it might be explained by the second anticipated source of variation, the chlorophyll content of

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