Photosynthetic rapid light curves (RLC) of Thalassia testudinum exhibit diurnal variation E.F. Belshe a , M.J. Durako a, ⁎ , J.E. Blum b a The University of North Carolina at Wilmington, Center for Marine Science and Department of Biology and Marine Biology, 5600 Marvin Moss Lane, Wilmington, NC 28409, United States b The University of North Carolina at Wilmington, Department of Mathematics and Statistics, 601 S. College Road, Wilmington, NC 28403, United States Received 13 March 2006; received in revised form 1 September 2006; accepted 30 October 2006 Abstract The use of pulse-amplitude-modulated fluorometry (PAM) and rapid light curves (RLC) were evaluated for monitoring the physiological condition of the seagrass, Thalassia testudinum, at the landscape scale in Florida Bay, USA. PAM fluorometry provides rapid, non-invasive, and quantitative physiological information on the state of photosynthesis. Yet, previous studies of effective and maximum quantum yields have shown that problems arise when expanding measurements from the organismal scale to the landscape scale, mainly due to temporal and irradiance-induced changes in photophysiology. Here, the magnitude of diurnal and spatial variation of photosynthetic characteristics among 10 sample basins and between two sample years was investigated using RLCs. Because RLCs measure effective quantum yields over a range of changing actinic irradiances, we hypothesized that the response parameters might be less sensitive to diurnal light history effects. Our results indicate that the RLC parameters, alpha and ETR max , significantly changed diurnally, as was previously found for both maximum and effective quantum yields, but the diurnal patterns were variable among the 10 basins. Both among-basin and between-year comparisons were confounded by diurnal variation and statistical analyses comparing morning, mid-day, and afternoon time periods were unable to definitively discern which time of day was best suited for assessing the relative photophysiological status of T. testudinum. However, pooling RLC data at the basin scale revealed among-basin differences and landscape scale trends that were consistent with basin-level morphometric variation in this seagrass. Thus, PAM fluorometry may be useful as a landscape scale monitoring tool within certain constraints. When using this approach over large spatial and temporal scales, diurnal variability must be considered. © 2006 Elsevier B.V. All rights reserved. Keywords: Diurnal variation; PAM fluorometry; Photosynthesis; Seagrass 1. Introduction Pulse-amplitude-modulated (PAM) fluorometers pro- vide quantitative information about photosystem II (PSII) by measuring chlorophyll fluorescence. If the sample is dark acclimated, which allows all PSII reaction centers to relax, the maximum quantum yield Journal of Experimental Marine Biology and Ecology 342 (2007) 253 – 268 www.elsevier.com/locate/jembe ⁎ Corresponding author. Tel.: +1 910 962 2373; fax: +1 910 962 2410. E-mail address: durakom@uncw.edu (M.J. Durako). 0022-0981/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2006.10.056