ANALYSIS OF THE SPECTRAL SIGNATURE OF ALGAE LADEN TURBID WATERS OF LONG ISLAND SOUND

The relationship of anthropogenic effects on coastal zone waters can be well studied in Long Island Sound (LIS), where excess nutrients often cause algal blooms and lead to bottom water anoxia. In this project, we attempt to determine the extent to which algal blooms in LIS can be monitored and quantified using data from SeaWiFS and CZCS. Here we report on an introductory task in this study – our test of the correspondence of the spectral signature of LIS waters with varying amounts of chlorophyll and suspended sediments.

We have collected multispectral data in the 350 – 1050 nm range from several locations in the western and eastern Sound. Total suspended sediment (TSS) and chlorophyll-a concentrations in surface waters were analyzed for these sites. In the eastern Sound, chlorophyll-a concentrations varied from 1.4 – 4.5 ug/L. The spectra showed all of the major features of the chlorophyll spectral signature: reflectance minima between 400 and 500 nm and at ~670 nm, maxima at ~560 nm and ~700 nm. Chlorophyll concentrations were well correlated with the NIR/R ratio (705/620 nm), while the correlation with the 1st derivative of reflectance with respect to wavelength at 690 nm was poor. TSS concentrations ranged from 5 - 26 mg/L in the eastern Sound to 60 – 80 mg/L in the western Sound. These concentrations did not result in discernable spectral absorptions. Also the reflectance curves did not show a general increase in overall reflectance with increasing TSS.

The spectra have a correlation to chlorophyll-a and no correlation to TSS. Therefore, we conclude that composite spectra with this concentration of TSS are dominated by the chlorophyll-a abundance and thus it may be possible to estimate chlorophyll-a concentration from field spectra in LIS.