Water samples were taken at the surface and the bottom in transects crossing the lake or coastal site from locations with observed erosion to presumably undisturbed locations. The water sampler (1.5 L Uwitec, Austria) was attached to a graduated rope, lowered by hand and filled twice. Combined water samples were transferred to 2 L Nalgene bottles, filled to the top and subsampled at the field station for further analysis.
pH, dissolved oxygen concentration and electrical conductivity: pH, dissolved oxygen concentration and electrical conductivity were measured in the laboratory with a WTW 340i Multiprobe. Before each measurement the sensor was calibrated for oxygen and pH.
Dissolved methane: At the coastal sites, 120 ml glass bottles were filled without bubbles and capped with butyl septa and aluminum crimps. After adding 10 ml of synthetic air (Alphagaz 1), we added 0.2 ml of 8 N NaOH. For the lakes, 60 ml syringes were filled with 40 ml of water and 20 ml of air. After shaking for 2 minutes, 10 ml of the headspace (containing the methane) was transferred into 20-ml vials, which had been filled completely with saturated NaCl solution. Methane concentrations in the headspace were measured with a Shimadzu GC 2014, a Rt Alumina Bond column and at 50° for 2 min in the laboratory in Germany. Further calculations of the amount of dissolved methane were performed according to Magen et al. (2014).
Dissolved inorganic carbon and total alkalinity: Water samples for dissolved inorganic carbon and total alkalinitiy (DIC and TA) were filtered through a pre-rinsed Pall AcroPak 1000 Supor Membrane 0.8/0.2 μm filter. The filtered water sample was filled into 250 ml Pyrex borosilicate glass bottles. One end of silicone tubing from the filter was placed at the bottom of the glass bottle and the bottle was rotated while filling to ensure no bubbles accumulated inside. Overflowing was allowed to avoid the persistence of any minute bubbles. Samples were preserved by adding 100 μl of 50% saturated HgCl2 solution after removing 2.5 ml of sampled water for headspace (Dickson et al., 2007). A thin layer of silicone grease was applied around the glass stoppers and the bottles were sealed and stored in the dark for later analysis at GEOMAR. Analysis was performed using a conventional titration method for TA and coulometric titration for DIC on a SOMMA (Marianda) analyzer
Dissolved organic carbon and nitrogen: Water samples for dissolved organic carbon (DOC) and nitrogen (DON) were filtered through a pre-rinsed Pall AcroPak 1000 Supor Membrane 0.8/0.2 μm filter. The filtered water was collected directly into pre-combusted 30 ml borosilicate glass vials and 100 µl of 4M HCl was added to preserve the samples by lowering the pH of the solution values less than 2. Samples were analyzed by a Shimadzu TOC V series analyzer at Geomar (Germany).
Inorganic nutrients: Water samples for inorganic nutrients (nitrate/nitrite, phosphate and silicate) were taken from 2 L Nalgene bottles in the laboratory within a few hours of sampling. During sampling, latex gloves were worn to avoid contamination. A pre-rinsed Pall AcroPak 1000 Supor Membrane 0.8/0.2 μm filter (with pre-filter) was used to transfer samples into acid washed (10% HCl) 50 ml polypropylene conical centrifuge tubes from Jet Biofil. Samples were preserved with the addition of 100 μl 4M HCl since freezing was not an option. Samples were analyzed at GEOMAR using a Seal QuAAtro AutoAnalyzer (precision and accuracy values: ± 2.20 µmol/kg and ± 5 µmol/kg).
Dissolved organic carbon, cations, anions, stable isotopes: Water samples for dissolved organic carbon (DOC) were filtered through a pre-rinsed disposable syringe with a GF/F filter of 0.7 μm pore size. For DOC, 20 ml of filtrate was placed into a glass vial and 25 μl of 30 % HCl was added for conservation. Samples were also collected for the analyses of stable isotopes of water (unfiltered, 30 ml), major cation/total element concentrations (15 ml, filtered through 0.45 μm pore size cellulose acetate filters and conserved with 65% HNO3), and major anion concentrations (8 ml, filtered through 0.45 μm pore size cellulose acetate filters). DOC was measured with a Shimadzu Total Organic Carbon Analyzer (TOC-VCPH). Anion concentrations were analyzed with ion chromatography (IC) using a DIONEX/Thermo ICS-2100 measuring device. Cation concentrations were measured with inductively coupled plasma optical emission spectrometry (ICP-OES) on a Perkin Elmer Optima 8300DV. Stable water isotopes were measured with an Isotope Ratio Mass Spectrometer (IRMS: Delta-S (DIFE)).
