THE NATURE OF THE UNDERWATER LIGHT FIELD IN WATERS TO THE WEST OF IRELAND

Allan Grassie, National University of Ireland Galway, Department of Earth and Ocean Science, Galway, Ireland, Robin Raine, National University of Ireland, Galway, Earth and Ocean Sciences, Ryan Institute and School of Natural Sciences, Galway, Ireland and Peter L Croot, iCRAG (Irish Centre for Research in Applied Geosciences), National University of Ireland Galway, Galway, Ireland
Abstract:
Measurements of the underwater light field were taken in three distinct regions of shelf waters to the west of Ireland. Variations in spectral distribution of downwelling irradiance were measured with depth. Depth profiles highlighted differences in the light regimes between offshore, coastal and inshore locations, with vertical light attenuation rates highest in inshore waters and least in offshore waters. Wavelength specific attenuation coefficients were calculated for each location and partitioned in order to qualify and quantify the contributions made to overall attenuation by the optically active components in the water column, namely chlorophyll, coloured dissolved organic matter (CDOM), suspended matter and the water itself. Results showed that chlorophyll plays an increasing contribution in attenuation with distance offshore and that CDOM is the predominant cause of attenuation inshore. Approximately 20% of the attenuation of photosynthetically available irradiance (400-700 nm) in offshore waters was due to chlorophyll, whereas CDOM contributed to 50-55% of the PAR attenuation in coastal waters of Connemara, western Ireland. Reconstruction of the underwater spectra using known values of the optically active components compared favourably with both measured spectra and the vertical distribution of photosynthetically available radiation (PAR).
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