The fifth shade of green: A novel approach to phytoplankton color index assessment in an oligotrophic system

Measurements of phytoplankton color index (PCI), an indicator of phytoplankton biomass, represent one of the largest datasets acquired through the continuous plankton recorder (CPR) survey. PCI has been used to describe changes in the spatial and temporal distribution of phytoplankton, especially in the North Atlantic, where the methodology was developed and primarily applied. The method functions by grouping samples, based on their color shade, into four categories. Each category is assigned a numerical value to aid spatial and temporal averaging. Recently a CPR has been operated in the Eastern Mediterranean, one of the most oligotrophic areas of the global oceans. To capture the specific characteristics of the area, an additional greenness grading is introduced. In this study, we use satellite-derived chlorophylla(Chla) data to examine whether PCI categories can be distinguished within the low and narrow range of Chlavalues found in the study area and to calculate the range of Chlaconcentrations expressed by each category. Furthermore, a newly calculated numerical system for the general use of PCI in oligotrophic areas and previously calculated PCI numerical value systems utilizing a variety of in situ, laboratory and remote sensing experiments, were tested for their applicability in the area. Present results indicate the feasibility of employing PCI measurements to provide an estimate of Chlaconcentration in the Eastern Mediterranean. This provides confidence in the implementation of a new category system for PCI, as well as the use of the new PCI numerical system for the assessment of phytoplankton in oligotrophic waters.