The relative importance of CDOM, organic coatings and suspended solids of different composition to light attenuation in estuaries

The relative contributions and compositional effects of suspended particles and colored dissolved organic matter (CDOM) to light attenuation at 400 to 700 nm were studied through laboratory analysis of samples from the Indian River Lagoon (IRL), Florida. Beam attenuation coefficients (c in m-1) were determined for whole water samples (c Total) and filtered samples (cCDOM ). Attenuation due to suspended particles (cTSS ) was calculated by subtracting cCDOM from cTotal and accounted for ∼90% of c Total. To evaluate the effects of particle composition on light attenuation, suspended solids were categorized as either organic matter (OM) or inorganic matter (IM) and used in a multi-variable analysis with c TSS as the dependent variable. The coefficients determined for the OM and IM fractions were significant and indicated that IM was at least three times more effective in attenuating light than OM. The OM fraction was further categorized as terrestrial or lagoonal organic matter (TOM and LOM, respectively) based on values of δ13C. The IM was further characterized as clays or other inorganic matter (O-IM). Results showed no significant difference between the effect of clay and O-IM to light attenuation. However, results indicated that TOM was 2.5 times more effective in attenuating light than LOM.;The importance of mineral organic coatings to light attenuation was studied through laboratory analysis of montmorillonite and kaolinite clays that were suspended in reagent water (uncoated) and water from an IRL creek that contained CDOM (coated). The clay-size montmorillonite particles (all <2 μm) had a higher surface/mass ratio and were ∼1.6 times more effective in attenuating light than the kaolinite. However, the silt-size kaolinite particles (2 to 62.5 μm) were ∼4 times more effective in scattering light than the montmorillonite. Time series experiments showed that ∼90% of organic coatings were adsorbed by clays within 15 minutes of suspension. Coated montmorillonite and kaolinite were ∼1.3 and ∼1.6 times more effective in attenuating light then their uncoated forms, respectively. A positive relationship between mineral organic coatings and light scattering was found for montmorillonite and kaolinite that account for ∼9-fold increase in intercept values associated with light attenuation models for the coated clays.