The first annual meeting of the Challenger Society for Marine Science Ocean Colour affiliated group was held on 11th September at UK Oceanography 1998. The Ocean Colour group provides a forum for ocean colour activities in the UK and operates via the Internet (http://www.npm.ac.uk/rsdas/csms_ocolour). Other linked groups are The Remote Sensing Society Ocean Colour Special Interest Group and the Institute of Physics Underwater Optics Group. The meeting was attended by about 40 people, some of whom attended UK Oceanography 1998. There were twelve talks during the day, several poster presentations and a discussion session. The themes of the talks ranged from the subject of processing ocean colour satellite imagery to research in ocean optics. The first talk was given by Samantha Lavender who described the work of the Remote Sensing Group at the Centre for Coastal and Marine Sciences (Plymouth) in processing ocean colour imagery from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) which was launched in August 1997. The imagery is received by the NERC receiving station and transferred over the Internet to CCMS for processing to biological and geophysical products. The data is also used to support research cruises. Steve Groom, also from CCMS, described how imagery from the SeaWiFS instrument were used to monitor the spatial and temporal distributions of ocean colour and derived phytoplankton pigment concentration in the Northeast Atlantic between April and August 1998. The data showed the development of the Spring bloom in relation to the continental-shelf, slope and oceanic environments. Colour composites of normalised water-leaving radiance provided information on the backscatter characteristics of the water and showed enhanced scattering, believed to be due to coccolithophores (Emiliania huxleyi). Simon Boxall, from Southampton Oceanography Centre, described the Colors programme, an EC MAST project. The project focuses on 3 European areas, the Adriatic Sea, the English Channel and the North Sea and will gain bio-optical data for validation of products from satellite ocean colour data. Initially there has been an extensive intercalibration phase to ensure coherence of measurements made at different sites by different scientific groups. A database is being developed and maintained by the Irish Marine Data Centre. Models are also being developed to test and simulate the effect of the atmosphere and the seawater on the incoming light. Gerald Moore (CCMS) presented an analysis of optical data collected during the Atlantic Meridional Transect (AMT) during its passage through the English Channel and the North Sea in late April 1998. A comparison was made of the in situ data (pigments, suspended particulates and CDOM) and data obtained from a series of concurrent SeaWiFS images and algorithms for chlorophyll were developed. In Case 1 regions the NASA algorithm was found to perform well. In Case II waters where the suspended sediment concentration was high the chlorophyll algorithm was found to respond to SPM. Also the atmospheric correction also failed in the region of high SPM so an iterative procedure was developed to provide a simple retrieval algorithm for regions of high sediment, which is now routinely being used. Matt Pinkerton (CCMS) presented the results from data acquired by the Plymouth Marine Bio-Optical Databuoy (PlyMBODy) in the western English Channel, which uses similar sensors to those on the Aqua Alta Tower in the Venice Lagoon. The bio-optical sensors from both platforms were intercalibrated. The problems of making optical measurements from platforms was described: e.g. inclination from the vertical, instrument and platform shading of the sensors, descent rates, and bottom reflectance effects. John Siddorn (UCNW) presented bio-optical data from the Zambezi river plume, where a useful management tool would be the ability to map the river plume from space, using colour as a marker. In April 1998, a research cruise was carried out in the plume to make detailed optical, water quality and salinity measurements. A good negative empirical relationship was found between the salinity (S) and the concentration of yellow substance, represented by the absorption of filtered seawater at 440 nm (g440). The high levels of yellow substance in the river water colour it to the extent that the river discharge is visible to the naked eye as green water when viewed from the deck of a ship, and there is a marked front between this and the blue Indian Ocean water. Empirical relationships between salinity and upwelling radiance ratios were established to enable the visible band remote sensing of the plume. Chlorophyll levels were low everywhere, but suspended sediment loads were enhanced in the plume. Underwater irradiance measurements combined with water quality measurements were used to establish specific absorption and scattering coefficients. These were used to calibrate an optical model of the water colour. Good agreement was found between this model and the observed colour. Alison Weeks(Southampton Institute) described some initial results from the EC MAST3 Biocolor project which aims to use multispectral in situ bio-optical data to determine optically active constituents, particularly phytoplankton, in the sea. Among other sensors, measurements were made using a novel in situ radiometer which aims to measure inherent and apparent optical properties simultaneously at high spectral resolution. This radiometer, the Southampton Underwater Multi-parameter Optical-fibre Spectrometer System (SUMOSS), was used to measure upwelling (Eu) and downwelling (Ed) irradiance and the beam attenuation coefficient, at high spectral resolution (< 5nm). Data were presented from a cruise in August 1998, off the south and south west of Ireland where a large phytoplankton bloom was observed. Several distinctive water types were observed, each with distinct optical signals dominated by several different assemblages phytoplankton, and of open ocean water. Miroslaw Darecki (Southampton Institute and Polish Academy of Sciences, Gdansk) presented data from the Biocolor project comparing the optical characteristics of the Baltic and Irish coastal waters where the chlorophyll concentrations were similar .The Baltic has brackish waters, and its water optical characteristics in the coastal region are mainly modified by river inflow, where the presence of yellow substances is marked by high absorption coefficients in the blue region of the spectrum. The Irish coast has a tidal regime, with horizontal mixing with Atlantic water, and minor river inflow. The parameters compared were the diffuse attenuation coefficient (kd), yellow substances absorption, pigments and detritus absorption, remotely sensed reflectance at 10 wavelengths including the SeaWiFS visible channels. Optical algorithms were developed for chlorophyll concentration and the results of these showed that the statistics are improved in the Baltic when the numerator of the reflectance ratio is shifted to longer wavebands. This is thought to be due to the high quantities of suspended sediments and yellow substances in the Baltic. Val Byfield (SOC) described how optical remote sensing can provide information about surface and dispersed oil, which complements that of other remote sensing instruments used in the monitoring of oil pollution. Differences in optical properties (refractive index, absorption coefficient and fluorescence properties) allow oils to be classified into broad groups using spectral ratio analysis. Dispersed oil scatters light in a similar manner to suspended particulates, but the light absorption properties of oil micelles are slightly different, making it possible to detect dispersed oil if concentrations are sufficiently high. The ability of optical sensors to detect surface oil and determine its relative thickness is documented in field experiments and analysis of images from the Sea Empress oil spill. Absolute thickness determination and oil type classification depends on accurate estimates of several environmental parameters, and is still in its early stages. This also applies to detection and concentration estimates for dispersed oil, which may be difficult without a knowledge of the optical properties of the water column in a given area. Sandrine Charrier (Southampton Institute) described how the fluctuation of underwater irradiance is caused by the focusing effect of waves on incoming solar rays, which causes high intensities of light in small areas of water. The frequency and intensity of the light flashes created depends upon the water optical properties, the diffusivity of the solar illumination and the wind velocity (i.e. surface roughness). This phenomenon may have significant biological implications, affecting photosynthesis and primary production. However, there has been little work done in the field either to explore the extent and magnitude of this effect or its implications for photosynthetic organisms. Measurements of downward irradiance and upward radiance have been carried out in coastal waters. Results were compared with published oceanic data, and conclusions and perspectives for further research were drawn. Daniel Ballestero (SOC) described the problem of using satellite sensors which measure the water-leaving radiance, from which only the near-surface pigment concentration can be obtained. These observations can be used to estimate plant biomass and, by means of theoretical and empirical models, the rate of photosynthetic primary production. Satellites can estimate the pigment concentration averaged over the upper few metres of the water column, the penetration depth, whereas the depth at which photosynthetic activity takes place, the euphotic depth, is about four times larger. This limitation, intrinsic to optical remote sensing, can lead to serious misinterpretations of satellite data. Vertical profiles of phytoplankton very often show a non-homogeneous distribution which can result in errors in the estimation of the depth of the euphotic zone and its biomass. Two approaches have been suggested to address this limitation in the past. The first is to characterise the by seasonal and regional provinces where the shape of the vertical distributions of phytoplankton is predictable. The second is to establish statistical relationships between the satellite estimation and the vertical distribution can be obtained and are proposed to complement satellite observations. A database of vertical distributions of chlorophyll in the upwelling system of Western Iberia were used for the analysis. The observed profiles have been fitted with a Gaussian model and the range of variability of the parameters defining the Gaussian curve derived. The pigment concentration as would be measured by a satellite was calculated numerically for the range of observed profiles and a new interpretation of the satellite estimation has been obtained. This new approach provides a physical interpretation of the satellite estimation as an increasing function of the maximum pigment concentration within the vertical distribution. A short presentation by Raul Aguirre Gomez (University of Mexico) described the methodology for processing SeaWiFS data using the SeaDAS software programmes, and he described some of the particular problems encountered in acquiring and processing data for the tropical regions of the world. A discussion followed in which a number of issues were raised which affect the ocean colour community in the UK. The two main questions which were raised were firstly the question of future funding of the Remote Sensing Group at CCMS, Plymouth, to enable a continuing service. The second question was about the viability and desirability of developing a UK bio-optical data base for developing algorithms for UK and European waters. Further discussion of these subjects will continue via the webpages and at subsequent meetings, along with other ocean colour issues. Alison Weeks (convenor (Alison.weeks@solent.ac.uk))
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