General Methodology

Lake water samples are collected at specific depths with a five-liter Niskin bottle during normal LTER limnological sampling. In a darkened environment, water is decanted through tubing directly from the Niskin bottle into three-150 mL borosilicate glass bottles with Teflon screw cap liners (2-light, 1-dark). Each sample is inoculated with an appropriate volume of 14C bicarbonate (activity of working solution should be between 100-121 uCi mL-1). Sample bottles are attached to a depth-calibrated cable and incubated in situ for 24 hours. At the end of the incubation period, the bottles are removed from the lake and placed in a darkened box for transport to the field lab. Each sample bottle is filtered through a 25 mm Whatman GF/F filter, and placed at the bottom of a 20 mL scintillation vial (organic matter up). 0.5 mL of 3N HCl is pipetted into each vial and dried slowly on a heating block (60C) in a fume hood. Once the filters are dry, the scintillation vials are recapped and stored until scintillation counting. 10 mL of Cytoscint cocktail is added to each vial and counted on a calibrated 14C liquid scintillation counter. Primary production (ug C L^-1 d^-1) is calculated using the following equation (updated 2022-02-04; corrects an error in the previously listed equation):

PPR (mg C/m³*d) = [(DPML - DPMD) * DIC (mg/m³) * 1.06]/[DPM of the 14C added * incubation time (days)]

where DPML is the average dpm of the light bottles, DPMD is the dpm of the dark bottle, and the constant (1.06) is used to adjust for isotopic discrimination of 14C radiolabeled carbon. PAR is measured at one depth during primary production experiments.

See the "Photosynthetically Active Radiation (PAR) logged during PPR experiments" dataset for these measurements as well as notes about the weather during PPR experiments. 

Notes on PPR 2006-2007 season data and methodology The IRGA malfunctioned during the 0607 season yielding DIC data that were highly suspect. Alkalinity titrations were done according to standard methods and DIC was estimated based on Wetzel R.G. and G.E. Likens. 1991. Limnological Analyses. 2nd Edition. Springer-Verlag New York Inc. pp. 111-118. Note that hydrogen acceptors other than the carbonate system exist in the lakes yielding an overestimate of DIC. Hence PPR data at depths of high ionic strength (deeper depths) which used DIC values obtained from alkalinity may be overestimated. We also computed average DIC values from the 05-06 and 07-08 seasons. DIC data changes relatively little over time; A paired t-test revealed that there was no significant difference between DIC for these 2 years (p>0.05). We report in the database PPR data calculated using DIC values computed from alkalinity titrations, and provide a link in the metadata to PPR data calculated using DIC values computed from averages of 05-06 and 07-08 DIC. The file containing PPR values calculated using DIC values computed from the average of previous and next season DIC values can be found in the following link: