The objective of this study is relate changes in the properties of the water column and associated successions in the phytoplankton with changes in optical properties. Models based on these relationships will be developed and validated. These models will facilitate the interpretation of measurements of ocean colour (remote or in-situ) in terms of the temporal and spatial structures of phytoplankton communities.

Biological, physical and optical data will be obtained from two areas (the Baltic and shelf seas to the West of Ireland) chosen not only because they offer good contrast in optical properties but also because there is a large body of existing knowledge about the physical dynamics of the regions and the variability in phytoplankton.

Oceanographic data will be obtained at these sites during a series of cruises. Cruises will study both spatial variability and temporal variability. Short term variability (weeks) will be studied by the use of moored instruments and close interval sampling forays. Longer term temporal (seasonal) variability will be invesigated by timing fieldwork campaigns for different seasons known to exhibit constant or changing physical conditions with resultant variations in temporal/spatial patterns of phytoplankton community structure.

The physical characteristics (temperature and salinity) of water bodies in the study areas will be examined horizontally at the surface (sensors used whilst underway) and near-surface (undulating CTD), and vertically (CTD). Discrete samples will be taken and analysed to provide a description of phytoplankton taxonomy, distribution and biomass.

In situ optical sensors will be employed, both during cruises and on moored assemblies, to characterise the optical properties of the water column. these measurements will be complemented by laboratory studies on the optical properties of discrete water samples and analyses of the particles and pigments cotained therein. The main in situ spectrographic instrument to be used in the project is an underwater high resolution spectrometer based on optical fibre technology (SUMOSS) which can measure beam attenuation, scattering at different angles and up/downwelling irradiance throughout the visible spectrum at less than 10 nm intervals. The instrument has been designed with the ultimate objective of recovering both apparent and inherent optical properties of the water in full spectral detail, making it possible to investigate the optical 'closure' problem.

Remotely sensed optical data (from satellites and, possibly, overflights) will be processed so as to provide additional data on surface-leaving radiance and place the findings of the fieldwork surveys in their spatial and temporal context.

Data sets describing the physical oceanography of the study areas, its relationship to phytoplankton community structure and the overall effect of these on the optical properties of the water column and ocean colour will be collated and used to develop models of these relationships. Two types of optical model (semi-analytical and Monte Carlo) will be developed and validated by using data sets not already employed for model training. The effectiveness of the two models will be compared, with the ultimate objective of providing validated algorithms for the interpretation of ocean colour data in terms of phytoplankton community structure.