INHERENT OPTICAL-PROPERTIES OF THE IRISH SEA DETERMINED FROM UNDERWATER IRRADIANCE MEASUREMENTS

The results of four-channel irradiance meter measurements at 41 statio ns in the Irish Sea are presented. Diffuse attenuation (K) is least an d reflection (R) is greatest in the green channel (550 nm). A very goo d relationship exists between the attenuation coefficient at this wave length and the estimated photosynthetically active radiation (PAR) att enuation using all four channels: K(PAR) = 0.01 + 1.11 K(550) Absorpti on and scattering coefficients (a and b, respectively) have been deriv ed from the observations using the results of a Monte Carlo model. Sca ttering coefficients are generally (but not invariably) larger than ab sorption coefficients in these waters (0.5 <b/a<13 at 550 nm). Measure ment errors in b are particularly large in the red, but the present ob servations are not inconsistent with a wavelength-invariant b. A good linear relationship exists between a and K giving a mean underwater co sine mu = 0.69 in the green. The dependence of a and b on water proper ties has been determined by linear regression. At 550 nm, the results are: b(550) = 0.22 + 0.21 [I] a(550) = 0.03 + 0.027 [I] + 0.019 [P] 0.005 [Y] where [I], [P] and [Y] represent the concentrations of inorg anic particles (g m(-3)), phytoplankton pigments (mg m(-3)) and yellow substance (mFl), respectively. The correlation in the case of scatter ing is relatively poor R(2) = 0.56), suggesting that particle size and shape, as well as concentration, influence the scattering coefficient . This implies that a general optical model of the Irish Sea is some w ay off; however, a simple model of the diffuse attenuation coefficient , based on the above equations, predicts K(PAR) with a root mean squar e error of 0.09 m(-1). (C) 1996 Academic Press Limited