The aim of this study was to investigate the causes of optical particle backscattering (bbp) in the ocean, with the central hypothesis being that the phytoplankton is a significant contributor to variations in bbp· The question is important because of its implications on some algorithms that estimate primary productivity from space and the interpretation of ocean colour from remote sensing.
To test this hypothesis a separation into several components (i.e. the reductionist approach) was applied to two different scenarios, typical of the oceanographic studies: an open ocean cruise and a coastal time series.
In a case study at the open ocean (central North Atlantic, over the Mid-Atlantic Ridge), during summer 2007, the variation in bbp was explained by Chl-a (R2=63%, P<0.001) using a power law, which was consistent with previous studies.
The use of the "reductionist approach" on the open ocean dataset incorporated information on pico and nano plankton abundances using flow cytometry, pigments and particulate detritus absorption measurements. It was found that the concentration of phytoplankton cells between 2 and 20μm in size, explained 70% (P<0.001) of the variance in bbp· Furthermore, using a budget analysis of the contributions from different phytoplankton types and bacteria abundances, it has been shown that the nanoplankton have a higher backscattering efficiency per cell than bacteria, but that the elevated
bacteria abundances mean that, overall, bacteria dominate the bbp signal in the oceanic case study presented here (52% of bbp is due to bacteria).
A similar approach was used for analysis of a nine year time series of bio-optical and particle composition data (including phytoplankton diversity) at the L4 coastal site on the Western English Channel. Variability ofbbp was best explained (R2=58%, P<0.001) by the inorganic fraction, with no clear seasonal variation.
The main conclusion is that the "reductionist approach" allows a better understanding of the causes for variability of optical properties and should be further use for the study of the causes of backscattering in other areas of the ocean.