Assessing pelagic and benthic metabolism using free water measurements

Automated in situ sensors for measuring changes in dissolved oxygen (DO) at high frequency have facilitated estimates of gross primary production (GPP) and respiration (R) in aquatic systems. Lake researchers usually rely on a single sensor for these estimates, but such point measurements may miss important spatial heterogeneity in within-lake processes and may not accurately represent systemwide values of metabolism. Here we combine simultaneous measurements of metabolism using DO sensors along transects from the shore to the center of a lake with a spatial model to better understand the underlying heterogeneity in processes contributing to whole-lake epilimnetic metabolism. We use this model to achieve better estimates of epilimnetic GPP and R and to determine the relative contributions of benthic-littoral vs. pelagic processes to these estimates. We compared the spatially explicit process-based model to estimates of metabolism from both a single sensor at the lake’s center and a spatially explicit averaging of multiple sensor sites. Estimates of both GPP and R varied on average 2.5to 3.2-fold from site to site within the same lake, whereas variations were sometimes as high as 6to 7-fold. Estimates of GPP and R near the perimeter of lakes were on average greater than measurements in the middle of the lake. Our model estimates that benthic-littoral processes accounted for ~40% of epilimnetic GPP and R. A single, centrally located sensor often misses a significant component of this benthic metabolism and accounts for only ~81% of lakewide GPP and R. Acknowledgments We thank Darren Bade, Crystal Fankhauser, Emma Kritzberg, Molli MacDonald, and Kara Raymond for assistance in the field and lab. We greatly appreciate the use of the lakes and laboratory of the University of Notre Dame Environmental Research Center (UNDERC) and thank Gary Belovsky and Karen Francl for making these facilities available. Computation of the large number of bootstrap iterations was made possible through the University of Wisconsin Condor Project for distributed computing and we thank Sean Cornelius for providing valuable assistance in working with Condor. Funding was provided by the Ecosystems Studies Program of the National Science Foundation. Many of the YSI sondes used for this study were obtained through a National Science Foundation Major Research Instrumentation Program grant (#0115770) to James Hodgson of St. Norbert College. This is a contribution to the University of Wisconsin Center for Limnology and the Institute of Ecosystem Studies. Limnol. Oceanogr.: Methods 5, 2007, 145–155 © 2007, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS