Estimating summertime epilimnetic primary production via in situ monitoring in an eutrophic freshwater embayment, Green Bay, Lake Michigan

Abstract

Abstract Quantifying rates of primary production and respiration is fundamental to understanding ecosystem function. This study utilized high-frequency time series, buoy-based sensor data to estimate daily primary production and respiration rates during the summers of 2012–2015 in southern Green Bay, Lake Michigan. Highly coherent diel oscillations of dissolved oxygen concentrations in epilimnetic waters were commonly observed for much of the summer via 30-min time intervals from the GLOS buoy (NOAA 45014) sensor array. Corrections for air–sea exchange based upon wind speed-derived gas exchange coefficients and saturation state, when combined with mixing depth, allow calculation of daytime net oxygen production and nighttime respiration. Thermistor string observations at 1-m intervals over the 13 m water depth showed the onset of thermal stratification, development of the thermocline, and occasional mixing events. For the summers of 2014 and 2015, during which a nearly continuous sensor record exists, gross primary production (GPP) and respiration (R) were estimated to be 342 ± 117 and 318 ± 83 mmol O2 m− 2 day−1 for GPP and − 325 ± 120 and − 306 ± 66 mmol O2 m− 2 day−1 for R, respectively. These results indicate that during most of the summer, southern Green Bay tends towards net autotrophy with production on average exceeding respiration by 9 ± 6% (SD). Cumulative net ecosystem production from June through September was estimated to be 3.2 and 1.3 mol C m− 2 in 2014 (118 days) and 2015 (113 days), respectively, and is sufficient to drive a significant portion of benthic respiration, the principal cause of seasonal bottom water hypoxia.