Measurement of Optical Properties in the Delaware Estuary

Abstract

In situ measured optical, biological and other data of Delaware estuarine water were analyzed to improve our understanding of the spectral effects of the major dissolved and particulate substances on the optical properties of case II waters. The attenuation coefficient and irradiance reflectance were calculated from the measured optical parameters. The data show that Delaware estuarine water is optically unstratified in the upper three meters or within one Secchi depth. The variation in the concentrations of the measured constituents and the correlations among pairs of concentrations were studied. The concentration of seston is highly correlated with the attenuation coefficient from 400 to 800 nm, and even more strongly correlated from 460 to 720 nm. It has good correlation with reflectance from 700 to 800 nm. Secchi depth and the average attenuation coefficient from 400 to 700 nm are inversely related.

Major light-attenuating constituents were selected with statistical methods. Specific attenuation coefficients of seston and dissolved organic materials are determined with multiple regression analysis. The results show that seston makes important contributions to the light attenuation from 400 to 800 nm. The importance of chlorophyll-a to the light attenuation is only shown between 670 nm and 690 nm. DOC (Dissolved Organic Carbon), DON (Dissolved Organic Nitrogen), or HC (Humic Acid Carbon) can all be used as indicators for gelbstoff (dissolved organic material). The attenuation coefficient of water, calculated from the attenuation model developed by statistical analysis is remarkably similar to the measured spectrum from SMITH and BAKER (1981). The specific attenuation coefficient of gelbstoff follows the exponential form from 400 to 630 nm, and has a null region from 730 to 770 nm when HC is used as an indicator. Using DOC or DON as an indicator, a similar relationship is obtained but without the null region. The relative contributions of these constituents to the total light attenuation in Delaware estuarine water were demonstrated.