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UTILITY OF A WIDE SPECTRUM LIGHT METER AS AN UNDERWATER SENSOR OF PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR)
Ozretich, R J. UTILITY OF A WIDE SPECTRUM LIGHT METER AS AN UNDERWATER SENSOR OF PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR). Presented at Pacific Estuarine Research Society 2001 Annual Meeting, Tacoma, WA, May 18-19, 2001.
The strong attenuation of infra red wavelengths (>700 nm) in coastal waters is suggestive that some instruments with broad spectral responses might be useful, inexpensive substitutes for PAR sensors in studies of estuarine plant dynamics. Wide spectrum (350-1100 nm) light intensity loggers in water tight cases (HOBO, Onset Computer Corp.) were deployed for 7-10 days with LiCor spherical quantum sensors of PAR (400-700 nm) at two locations in Yaquina Bay, OR. Onset and LiCor sensors were attached to the two, 38 cm arms of the 0.75 m-long sensor array. The upper array arm was positioned at approximately -1 m (MLLW). The HOBOs were launched simultaneously and set to log data every 10 min; LiCors logged 15 min averages; data from only the first three days of deployment were used because of biofouling.
Each deployment provided a varying light field to calibrate the Onset sensors over a range of photon fluxes. Using the LiCor sensor to quantify flux, the Onset values were regressed against flux resulting in highly significant coefficients with slopes and intercepts between and among deployments (n=22) of the HOBO-LiCor pairs averaging 1.35 ? 0.097 lumen-sec/?E and 9.2 ? 2.26 lumens/m2 (mean ? SE), respectively. Using average response factors for individual HOBOs, the three-day integrated HOBO-computed flux exceeded the measured flux by 2% on the upper arm and 27% on the lower resulting computed attenuation coefficients lower than truely present. Comparisons of attenuation coefficients from paired HOBOs and LiCors were often significant but slopes ranged from 0.75 to 1.5 HOBO Ks / LiCor Ks. Calibrated HOBO units may be useful in assessing the flux of PAR in estuarine systems; however, calculation of attenuation is tenuous.