Methods

Data included in this package include output, driving data, and supplemental calculated information for a modeling study.<br/>

Methods

Data were collected from two lakes, Sparkling Lake (46.008, -89.701) and Peter Lake (46.253, -89.504), both located in the northern highlands Lake District of Wisconsin and the Upper Peninsula of Michigan over a 10 day period on each lake in July and August of 2007. Refer to Van de Bogert et al. 2011 for limnological characteristics of the study lakes.Measurements of dissolved oxygen and temperature were made every 10 minutes using multiple sondes dispersed horizontally throughout the mixed-layer in the two lakes (n=35 sondes for Sparkling Lake and n=27 sondes for Peter Lake). Dissolved oxygen measurements were corrected for drift.Conductivity, temperature compensated specific conductivity, pH, and oxidation reduction potential were also measured by a subset of sensors in each lake. Of the 35 sondes in Sparkling Lake, 31 were from YSI Incorporated: 15 of model 600XLM, 14 of model 6920, and 2 of model 6600). The remaining sondes placed in Sparkling Lake were 4 D-Opto sensors, Zebra-Tech, LTD. In Peter Lake, 14 YSI model 6920 and 13 YSI model 600XLM sondes were used.Sampling locations were stratified randomly so that a variety of water depths were represented, however, a higher density of sensors were placed in the littoral rather than pelagic zone. See Van de Bogert et al. 2012 for the thermal (stratification) profile of Sparkling Lake and Peter Lake during the period of observation, and for details on how locations were classified as littoral or pelagic. In Sparkling Lake, 11 sensors were placed within the shallowest zone, 12 in the off-shore littoral, and 6 in each of the remaining two zones, for a total of 23 littoral and 12 pelagic sensors. Similarly, 15 sensors were placed in the two littoral zones, and 12 sensors in the pelagic zone.Sensors were randomly assigned locations within each of the zones using rasterized bathymetric maps of the lakes and a random number generator in Matlab. Within each lake, one pelagic sensor was placed at the deep hole which is used for routine-long term sampling.Note that in Sparkling Lake this corresponds to the location of the long-term monitoring buoy. After locations were determined, sensors were randomly assigned to each location with the exception of the four D-Opto sensor is Sparkling Lake, which are a part of larger monitoring buoys used in the NTL-LTER program. One of these was located near the deep hole of the lake while the other three were assigned to random locations along the north shore, south shore and pelagic regions of the lake. Documentation: Van de Bogert, M.C., Bade, D.L., Carpenter, S.R., Cole, J.J., Pace, M.L., Hanson, P.C., Langman, O.C., 2012. Spatial heterogeneity strongly affects estimates of ecosystem metabolism in two north temperate lakes. Limnology and Oceanography 57, 1689-1700.

Methods

Respiration of total plankton passing a 70 micron mesh, and bacteria passing a 1 micron mesh, calculated from loss of oxygen in lake water incubated at in situ temperatures. Oxygen concentration was determined using the Winkler reaction with azide modification. Final product concentration determined via spectrometry or titration with sodium thiosulfate. Titrations that may have overrun the endpoint were not included. The following equation for calculation of dissolved oxygen concentration from titration of Winkler end product with thiosulfate (from Wetzel, R. G. and G. E. Likens. 1991. Limnological Analyses, 2nd ed. Springer-Verlag, New York). Thiosulfate with a molarity of 0.20 N was used for all titrations. mg O2 L-1 = (ml titrant)*(molarity of thiosulfate)*(8000)/((ml of sample titrated)*((ml of bottle -3)/(ml of bottle))). The following equation is for calculation of dissolved oxygen calculation from spectophotometric analysis of Winkler reaction end product (from Roland, F., N. F. Caraco, J. J. Cole. 1999. Rapid and precise determination of dissolved organic oxygen by spectrophotometry: Evaluation of interference from color and turbidity. Limnol. Oceanogr. 44(4):1148-1154). mg O2 L-1 = absorbance at 430 nm (in units of cm-1) * 8.1-0.41 Sampling Frequency: fortnightly during ice-free season - every 6 weeks during ice-covered season Number of sites: 4

Methods

Bacterial respiration of bacteria passing a 70 micron mesh. Based on bacterial growth efficiency determined empirically on bacteria production and oxygen depletion on microbes passing a 1 micron mesh. All samples from integrated sample of epilimnion to thermocline or 12 meters, which ever was more shallow. Method based on Roland, F. and J. J. Cole. 1999. Regulation of bacterial growth efficiency in a large turbid estuary. Aquatic Microbial Ecology 20:31-38 Sampling Frequency: fortnightly during ice-free season - every 6 weeks during ice-covered season Number of sites: 1

Methods

Study sitesWe sampled surface waters of 31 lakes in the Northern Highland Lake district of Wisconsin and the Upper Peninsula of Michigan during July and August of 2000 (Table 1). The lakes were chosen to span wide and orthogonal ranges in DOC and TP concentrations and for their close proximity to the Trout Lake Station in Vilas county, Wisconsin. The order in which the lakes were sampled was randomized.Limnological samplesLimnological samples were collected for each lake at 0.5 m depth as follows. DOC samples were collected as the filtrate through Whatman GForF filters, and were analyzed on a Shimadzu model 5050 high temperature TOC analyzer. Color was also measured from this filtrate as absorbance at 440 nm on a Spectronic Genesys 2 spectrophotometer using 10 cm quartz cuvettes. Chlorophyll a was collected by filtering 200 ml of lake water, and then freezing filters for at least 24 hours, followed by methanol extraction for 24 hours. Fluorescence was determined before and after acidification to correct for pheopigments. Total phosphorus was analyzed on a Lachat autoanalyzer after persulfate digestion of a whole water sample. DIC and was measured on a Shimadzu GC-8AIT (TCD detector) gas chromatograph. DIC was determined from the headspace of acidified samples, which was injected into the GC. pH was measured using an Orion digital pH meter with automatic temperature compensating electrode. Temperature and dissolved oxygen profiles were measured using a YSI temperatureordissolved oxygen meter. Spot measurements of surface water DO were made on quadruplicate samples, using Winkler titrations as described in Bade and others (1998).BuoysWe deployed a buoy that sampled dissolved CO2, DO, water temperature, photosynthetically active radiation (PAR), and wind speed for 2-4 days on each lake. All water measurements were made at a depth of 0.5 m. Wind speed was measured one meter above the lake, using an RM Young model 03001, and PAR was measured 10 cm above the lake surface using a Li-Cor model 190SA quantum sensor. Electronic control and data collection were managed by a Campbell Scientific CR10X data logger. DO and water temperature were measured with a YSI model 600-XLM sonde fitted with a Rapid Pulse oxygen probe (model 6562) and temperature sensor. The sonde was attached to the buoy at the opposite end from the CO2 equilibration chamber (described below).We measured dissolved CO2 independently from DO. We equilibrated a closed loop of atmospheric gas in an equilibration chamber submerged to 0.5 m. The equilibrated gas volume was about 234 ml. We recirculated gas for the last 10 minutes of every 30 minute period, with a flow rate of about 9 ml s-1. A pump exchanged lake water every minute during equilibration. Equilibrated gas was diverted to the IRGA, equipped with a 14 cm sample cell for lakes with CO2 concentration under 2000 ppm or a 5 cm sample cell for lakes with CO2 concentration between 2000-20000 ppm. Following analysis of equilibrated gas, solenoids were activated to route atmospheric gas (taken 10 cm above the water) for CO2 analysis.Time series data are included for 25 of the lakes.Additional detail of the methods available in Hanson et al. (2003)Hanson, P. C., D. L. Bade, S. R. Carpenter, and T. K. Kratz. 2003. Lake metabolism: Relationships with dissolved organic carbon and phosphorus. Limnol. Oceanogr. 48: 1112-1119.