Remote sensing of the eutrophic state of coastal waters via phytoplankton functional groups

Abstract

It is unlikely that we will be able to remotely sense coastal or oceanic water chemistry comprehensively from onorbit passive or active sensors. Inferences currently are made on nitrates using sea surface temperature as a surrogate. Passive microwave sensors monitor conductivity as a surrogate for surface salinity. Suspended sediments, of course, are detectable by scattering. While Raman laser spectroscopy can diagnose constituent chemicals, issues of detection sensitivity and also eye safety are concerns especially in coastal regions. However, it might be that we don't have to pursue this challenging objective. We are primarily concerned from a marine management perspective with situations where coastal waters become degraded and the biology is disrupted. In such circumstances, the forcing on the relative mix of phytoplankton functional groups [PFGs] may well provide the important signature that identifies the impact that coastal water chemistry [or water temperature] is having on the biological systems. A number of case studies have shown that it is feasible to identify PFGs using both multi-spectral and hyperspectral remote sensing based primarily on the spectral absorption of the individual plankton species. As a general observation, it appears that as water quality degrades with an increase in concentration of pollutants, in particular, excess nutrients such as phosphates and nitrates from agricultural run-off and sewer outfalls, the diatom population decreases and flagellate population increases. If it is possible to demonstrate that the technology does deliver with acceptable accuracy the important trends over time in PFG composition then it certainly should be feasible to go back in time for at least a decade to examine temporal trends in the mix of PFGs.