Venetia Stuart

Remote sensing of phytoplankton pigments: A comparison of empirical and theoretical approaches

Algorithms that have been used on a routine basis for remote sensing of the phytoplankton pigment, chlorophyll- a, from ocean colour data from satellite sensors such as the CZCS (Coastal Zone Color Scanner), SeaWiFS (Sea Viewing Wide Field-of-View Sensor) and OCTS (Ocean Colour and Temperature Scanner) are all of an empirical nature. However, there exist theoretical models that allow ocean colour to be expressed as a function of the inherent optical properties of seawater, such as the absorption coefficient and the backscattering coefficient. These properties can in turn be expressed as functions of chlorophyll- a, at least for the so-called Case 1 waters in which phytoplankton may be considered to be the single, independent variable responsible for most of the variations in the marine optical properties. Here, we use such a theoretical approach to model variations in ocean colour as a function of chlorophyll- a concentration, and compare the results with some empirical models in routine use. The parameters of phytoplankton absorption necessary for the implementation of the ocean colour model are derived from our database of over 700 observations of phytoplankton absorption spectra and concurrent measurements of phytoplankton pigments by HPLC (High Performance Liquid Chromatography) techniques. Since there are reports in the literature that significant differences exist in the performance of the algorithms in polar regions compared with lower latitudes, the model is first implemented using observations made at latitudes less than 50. It is then applied to the Labrador Sea, a high-latitude environment. Our results show that there are indeed differences in the performance of the algorithm at high latitudes, and that these differences may be attributed to changes in the optical characteristics of phytoplankton that accompany changes in the taxonomic composition of their assemblages. The sensitivities of the model to assumptions made regarding absorption by coloured dissolved organic matter (or yellow substances) and backscattering by particles are examined. The importance of Raman scattering on ocean colour and its influence on the algorithms are also investigated.

Seasonal variations in bio-optical properties of phytoplankton in the Arabian Sea

Data collected during three expeditions to the Arabian Sea are examined to characterise the changes in phytoplankton properties between the intermonsoon and monsoon seasons. The results are used to elucidate the key changes that lead to the occurrence of phytoplankton blooms following the onset of the monsoon winds. Contrary to expectations, changes in the assimilation number and the initial slope of the photosynthesis-light curve decreased in the SW monsoon season, apparently in the wrong sense for them to account for the incidence of blooms in the Arabian Sea during this season. On the other hand, there were strong differences between the intermonsoon and monsoon values of the chlorophyll-specific absorption coefficient for the phytoplankton, probably arising from seasonal changes in cell size and community composition. The sense of these changes was such as to increase substantially the maximum quantum yield for photosynthesis during the monsoon season compared with the intermonsoon. When these results are applied in a simple expression for the daily, primary production of the water column, they can explain the rapid accumulation of phytoplankton observed in the Arabian Sea following the start of the monsoon season. Thus, the occurrence of seasonal blooms in the Arabian Sea depends on combined variations in both the optical and photosynthetic characteristics of phytoplankton in the region.

Role of pigments and species composition in modifying the absorption spectra of natural phytoplankton popultions

Absorption spectra from three cruises with diverse phytoplankton assemblages were decomposed into 13 Gaussian bands, representing absorption by the major chlorophylls and accessory pigments. The relationship between Gaussian peak height and the concentration of the pigment responsible for the absorption band reflects changes in the packaging effect, which were most apparent at Gaussian bands of high absorption but were close to zero for the Gaussian band centered around 623 nm, associated with chl-a for any phytoplankton assemblage, unaffected by variations caused by the package effect. Specific peak heights of the blue and red chl-a absorption bands were highest for the Arabesque 2 cruise and lowest for the Vancouver Island cruise, which is consistent with an increase in the package effect with increasing cell size. We estimated that 69 percent of the total variability in aph at 440 nm as due to changes in the package effect while the remaining 31 percent was due to changes in pigment composition.