A comparison and correction of light intensity loggers to photosynthetically active radiation sensors

Accurate light measurements are important in the analysis of photosynthetic systems. Many commercial instruments are available to determine light; however, the comparison of light estimates between studies is difficult due to the differences in sensor types and their calibrations. The measurement of underwater irradiance is also complicated by the scattering and attenuation of light due to interactions with particulates, molecules, and the bottom. Here, three sensor types are compared to evaluate the calibration of light intensity loggers to estimate photosynthetically active radiation (PAR). We present a simple calibration of light intensity loggers that agree within 3.8% to factory-calibrated scalar PAR sensors under a wide range of environmental conditions. Under the same range of conditions, two identical factory-calibrated PAR sensors showed a similar difference of 3.7%. The light intensity loggers were calibrated to a high-quality PAR sensor using an exponential fit (r2 = 0.983) that accounts for differences in sensor types with respect to the angle of incoming light, scattering, and attenuation. The light loggers are small, robust, and simple to operate and install, and thus well-suited for typical subsurface research. They are also useful for small-scale measurements, when broad spatial coverage is needed, or in research requiring multiple sensors. Many studies have used these simple light intensity sensors to estimate PAR, yet their limitations and advantages in mimicking PAR have not been well defined previously. We present these small and user-friendly loggers as an excellent alternative to more sophisticated scalar PAR sensors. *Corresponding author: E-mail: [email protected] Acknowledgments Support for this study was provided by the University of Virginia, the Jones Everglades Research Fund, and the National Science Foundation through a grant from Chemical Oceanography program (OCE0536431). Special thanks to Thomas Frankovich and Woods Hole Marine Biological Laboratory (Woods Hole, Massachusetts) for logistical support and assistance in field. The manuscript was substantially improved by comments from anonymous reviewers. DOI 10.4319/lom.2012.10.416 Limnol. Oceanogr.: Methods 10, 2012, 416–424 © 2012, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS The angle of incoming light may also affect measurements due to the sensor type or shading caused by sensor design. This may be further complicated by variances of up to 35% between light sensors from the same manufacturer (Jewson et al. 1984) and by deviations of up to 50% between sensors of the same model (Forster 1998). A study by Meyercordt et al. (1999) compared instruments from three manufacturers of high-quality PAR sensors and found a deviation of up to 188%, which was attributed to the collecting properties of each sensor. In many cases, the differences in sensor-collecting properties, spectral response, and the ability to measure diffuse radiation requires an individual calibration in order to determine accurate and comparable PAR measurements (Meyercordt et al. 1999). There are two main types of sensors commonly used to measure underwater PAR, planar, and scalar sensors. Planar sensors have a flat light collecting surface that responds to light that impinges on their surface from downward directions. Planar sensors tend to underestimate PAR because the collecting surface does not absorb upwelling radiation or light that reflects off of particles in the water and the sediment surface (Booth 1976; Arst et al. 2008). Scalar PAR sensors have a hemispherical or spherical collecting surface that functions to absorb light from 2p to 4p steradians, respectively. They are believed to record more accurate measurements of total underwater PAR, as they absorb diffuse radiation from most directions (Booth 1976). For these reasons, it has been suggested that planar sensors are insufficient for studies requiring accurate scalar PAR measurements (Arst et al. 2008), for example those involving phytoplankton residing within the water column where diffuse radiation may be a significant form of available light. Most commercially available planar PAR sensors are also cosine corrected, which consists of a block of light diffusing material that is designed to reduce errors associated with light impinging on the sensor surface from low incident angles. A cosine-corrected planar sensor will produce more accurate measurements of PAR than a planar sensor without cosine correction under light conditions that are not ideal, such as during sunset and sunrise where the angles of incoming light are small. Many authors have reported using simple light-intensity loggers, designed to measure relative differences in total available radiation, to estimate underwater PAR values through calibration (Glud et al. 2002; Boese et al. 2005; Piniak and Brown 2008; Liu et al. 2009; Fanta et al. 2010; Tait and Schiel 2010; Hulatt and Thomas 2011; Pedersen et al. 2011; Wall et al. 2011; Koch at al. 2012). However, little information on these calibrations has been provided, and the differences between simple light-intensity loggers and scalar PAR sensors have not been considered. This article compares three different light sensor types that were used to estimate PAR. We list the strengths and weaknesses of each type, as well as the quality and accuracy of their PAR estimates. We also show that light intensity loggers can be used to estimate PAR as accurately and reliably as scalar PAR sensors. Materials and procedures Evaluated sensor types Eleven Onset light and temperature dataloggers (UA-002-64 HOBO Waterproof Temperature/Light Pendant Data Logger), one Odyssey Integrating PAR Sensor (Dataflow Systems PTY Limited), and two LI-1000 LICOR dataloggers with LICOR Spherical Quantum PAR Sensors (LI-193SA calibrated May 2009) were compared. The HOBO pendant temperature and light logger (HOBO) is a small (6 ¥ 3 ¥ 2 cm), self-contained, planar sensor designed for measurement of light intensity (150-1200 nm). The Odyssey Integrating PAR sensor (ODY) is a self-contained cylindrical (4 cm diameter ¥ 16 cm long) PAR logger (400-700 nm) with a planar cosine-corrected sensor. The LICOR Spherical Quantum Sensor (LICOR) is a 4p scalar PAR sensor that must be connected to a nonwaterproof data logger, and is a standard instrument for PAR measurement. The LICOR has a reported angular response of < ± 4% error up to ± 90° from the normal axis. Whereas the LICOR receives light from all angles, its angular response is reduced at 180° from the normal axis due to shading from the sensor housing. Controlled growth chamber experiment: To compare differences between HOBO and the two PAR sensors, the sensors were placed in a growth chamber (Conviron 4030, Controlled Environments Limited) set to produce different PAR levels. The 11 HOBOs (referred to as letters B through O) used were mounted on a rotating circular plate to expose them to identical light conditions and the two PAR loggers were mounted to stands at the same height. The HOBOs were set to log light every 5 s; the ODY and LICOR were set to integrate over 0.25 h intervals. The growth chamber was set to have variable light intensities at 35, 165, 380, and 605 μmol photons m–2 s–1. Data were recorded over 76 h. Field experiments Three HOBOs (loggers H, K, and L), an ODY, and a LICOR were deployed through 10 d on a permanently submerged sand flat in outer West Falmouth Harbor, Massachusetts, USA (41° 36.22 min N, 70° 38.42 min W) during August 2009. The same logging parameters were used as in the light chamber experiment. Three HOBOs were used to reduce the variability of measurements by subsequent averaging and to provide a check for the other sensors if one was fouled. The HOBOs were mounted at the same height as the two PAR sensors on the top of PVC poles 10 cm above the sediment surface. The depth of the light meters varied from 0.4 m to 2.0 m, depending on tidal stage. The light sensors were faced upward and separated to prevent shading. The data logger for the LICOR was mounted in a dry box on a float. Every 24 h, data were downloaded from the HOBOs whereas the ODY and LICOR were capable of logging the full duration of the experiment. Six HOBOs (B, H, I, K, L, and M) and the ODY were also compared with two LICORs near Key Largo, Florida at two sites: a reef (25° 06.59 min N, 80° 18.12 min W) and a seagrass bed (25° 06.58 min N, 80° 18.14 min W) in July 2010. Long et al. Comparison of light loggers and PAR sensors