MEASUREMENT OF SOLAR RADIATION

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     SUMMARY:  Knowledge of the flux of radiant energy is 
     essential to interpret habitat variability in the water 
     column.  Solar radiation is measured with two automated, 
     independent instruments.  Solar radiation at the sea 
     surface (incident radiation) is measured with both a 
     cosine collector and with a scalar irradiance sensor.  
     Data are logged over the daylight period.  In addition, 
     continuous profiles of underwater radiation are obtained 
     from the surface to approximately the 0.1% light level.
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1.  Introduction

    Measurements of solar radiative flux are essential to interpret 
variability in primary production.  In addition, the depth distribution 
of underwater light is an important physical determinant of phytoplankton 
standing stock and species composition.  As part of our regular sampling 
program at station ALOHA, we therefore collect data on both solar 
radiative flux at the sea surface and the vertical profiles of underwater 
quantum scalar irradiance.  Our measurements of radiation are restricted to
the 400 to 700 nm waveband.  This waveband is defined as Photosynthetically 
Active Radiation (PAR) and is the waveband limits for photosynthetically 
useful light (Booth, 1976).  


2.  Shipboard Measurements

    A LicorR (model LI-1000) data logger and a LicorR (model LI-192S) 
quantum sensor are used to collect continuous measurements of radiative 
flux at the sea surface on monthly HOT cruises.  The quantum sensor 
is a cosine collector and measures PAR.  The sensor is positioned 
approximately 4 meters above the deck in order to avoid shadows from 
the ship's superstructure.  Radiance is averaged over 10 minute 
intervals and logged throughout the day.  


3.  Underwater Measurements

    Underwater irradiance is measured using a Biospherical Instruments 
Profiling Natural Fluorometer (model PNF-300).  This submersible device 
transmits measurements of temperature, pressure, downwelling scalar 
irradiance (PAR) and upwelling irradiance at 683 nm to an on-deck 
computer which allows one to obtain continuous profiles of the above-
mentioned parameters to a depth of approximately 175 m.  In addition, 
the instrument package is equipped with an on-deck 2 π deck cell which 
continuously logs surface light for comparison with the Licor system 
described above.  The data provided by the Biospherical PNF instrument 
are used to determine the depth of the 1.0 and 0.1 % light level, and 
to determine the underwater extinction coefficient for PAR.  In addition, 
the measurement of upwelled red light (683 nm) is used to estimate the 
depth distribution of chlorophyll a and primary production for comparison 
with these parameters measured at discrete depths (Kiefer et al., 1989; 
Chamberlain et al., 1990).


4.  References

    Booth, C. R.  1976.  The design and evaluation of a measurement 
    system for photosynthetically active quantum scalar irradiance. 
    Limnology and Oceanography, 21, 326-336.

    Chamberlain, W. S., C. R. Booth, D. A. Kiefer, J. H. Morrow 
    and R. C. Murphy.  1990.  Evidence for a simple relationship 
    between natural fluorescence, photosynthesis and chlorophyll 
    in the sea. Deep-Sea Research, 37, 951-973.

    Kiefer, D. A., W. S. Chamberlain and C. R. Booth.  1989.  
    Natural fluorescence of chlorophyll a:  Relationship to 
    photosynthesis and chlorophyll concentration in the western 
    South Pacific gyre. Limnology and Oceanography, 34, 868-881.