Identification and characterisation of two optical water types in the Irish Sea from in situ inherent optical properties and seawater constituents

This paper examines relationships between the inherent optical properties (IOPs) of the Irish Sea and concentrations of optically significant constituents, measured as chlorophyll (Chl), mineral suspended solids (MSS) and coloured dissolved organic material (CDOM). In situ measurements of IOPs at 98 stations fell into two groups with distinct characteristics. Instances include the ratio of particulate backscattering to non-water absorption at 676 nm (bbp676/an676), the ratio of the non-water absorption coefficients at 440 nm and 676 nm (an440/an676) and the ratio of particulate scattering to non-water absorption (bp/an) at 676 nm. The two groups showed corresponding differences in the proportions of their constituent concentrations with Chl/MSS values typically above 0.4 mg g−1 for stations with bbp676/an676 < 0.33 and below 0.4 mg g−1 for stations with bbp676/an676 > 0.33. CDOM concentrations showed no significant differences between groups. The occurrence of correlated groups in the IOP and constituent concentration data indicates the existence of identifiable sub-types of coastal water within the conventional Case 2 classification whose optical characteristics were dominated either by phytoplankton or suspended minerals. By applying linear regression analysis to the two groups we derived effective material-specific IOPs for these natural particle assemblages. The coefficients obtained enabled the successful reconstruction of total absorption, scattering and backscattering coefficients for a given water body from measurements of Chl, MSS and CDOM. This procedure may be useful for assimilating measurements of seawater composition into bio-optical models in shelf seas.