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.

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