Estimating Benthic Light Regimes Improves Predictions of Primary Production and constrains Light-Use Efficiency in Strea
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Estimating Benthic Light Regimes Improves Predictions of Primary Production and constrains Light-Use Efficiency in Streams and Rivers Lily Kirk,1* Robert T. Hensley,2 Philip Savoy,3,4 James B. Heffernan,5 and Matthew J. Cohen6 1
School of Natural Resources and Environment, University of Florida, 103 Black Hall, Gainesville, Florida, USA; 2National Ecological Observatory Network, 1685 38th St. Suite 100, Boulder, Colorado, USA; 3Department of Biology, Duke University, Durham, North Carolina, USA; 4Present address: U.S. Geological Survey, Reston, Virginia, USA; 5Nicholas School of the Environment, Duke University, Grainger Hall, 9 Circuit Drive, Durham, North Carolina, USA; 6School of Forest Resources and Conservation, University of Florida, 136 Newins-Ziegler Hall, Gainesville, Florida, USA
ABSTRACT Light-use efficiency (LUE) describes conversion of incident light into gross primary production (GPP), combining the inherent photosynthetic efficiency of chloroplasts with light-capture ability of the autotrophic community. In lotic ecosystems, LUE is poorly constrained, in part because most studies neglect water-column attenuation. We hypothesized that rigorous quantification of benthic light would (1) improve GPP predictions and (2) constrain cross-site variation in LUE. We used a fieldvalidated light model to successively attenuate open-sky irradiance through the riparian canopy and water column to estimate benthic light at 11 sites spanning discharge and dissolved-color gradients where we simultaneously calculated daily GPP. Our results indicate substantial water-column attenuation (up to 96% of stream-surface light), implying significant underestimation of LUE using
Received 13 April 2020; accepted 17 August 2020 Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s10021-020-00552-1) contains supplementary material, which is available to authorized users. Author Contributions LK, PS, and MJC conceived and designed the study. LK collected and analyzed data and wrote the paper. BH and MJC helped with analysis. MJC, BH, PS, and JBH edited the manuscript. *Corresponding author; e-mail: [email protected]
stream-surface light alone. Benthic light dramatically improved GPP predictions, especially after considering mean-light conditions, which we suggest enumerates ecosystem light-capture ability due to biomass density. The model including meanlight effects explained 78% of GPP variation across sites and yielded a LUE identical to terrestrial ecosystems (1.9%). Interactions between daily and mean-light only slightly improved model fit (R2 = 0.80), implying higher LUE at sites with higher mean light, but notably reduced LUE variation across sites compared with individual site analyses. This suggests that better representation of benthic light regimes leads to LUE convergence. Our study supports use of a global river LUE to translate large-scale predictions of stream light regimes into expected GPP, from which disturbance and nutrient limitation effects can then be discerned. Key wo
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