Dissolved concentrations of CH4, nutrients, and biological parameters in the water column of twelve Mediterranean reservoirs in Southern Spain

We sampled twice 12 reservoirs between July 2016 and August 2017 in southern Spain during the summer stratification and the winter mixing. We determined reservoir area, perimeter, and capacity using the next open databases: Infraestructura de Datos Espaciales de Andalucía (IDEAndalucia; http://www.ideandalucia.es/portal/web/ideandalucia/), and the Ministerio para la Transición Ecológica (https://www.embalses.net/). The reservoir volume (m3) divided by its surface area (m2) will yield the mean depth (m). We also calculated the shoreline development ratio (DL) (Aronow, 1982) and the shallowness index (1/m) by dividing the shoreline development index (DL) by the mean depth (m). We performed the vertical geochemical profiles of the reservoirs using a Seabird 19plus CTD profiler and obtained the measurements of temperature and dissolved oxygen. We collected samples at different depths for dissolved CH4 analysis in air-tight Winkler bottles by duplicate, preserved with a solution of HgCl2 (final concentration 1mM) to inhibit biological activity and sealed with Apiezon® grease to prevent gas exchange. We measured dissolved CH4 using headspace equilibration and gas chromatography (2-3 replicates per bottle). We measured DIC, DOC, TN, and TDN by high–temperature catalytic oxidation using a Shimadzu total organic carbon analyzer (Model TOC-V CSH) coupled to nitrogen analyzer (TNM-1). We determined the NO3 concentration using the ultraviolet spectrophotometric method, using a Perkin Elmer UV-Lambda 40 spectrophotometer at wavelengths of 220 nm and correcting for DOC absorbance at 275 nm (APHA, 1992). We measured NH4 and NO2 concentrations by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Dissolved inorganic nitrogen (DIN) is the addition of the NO3, NH4, and NO2 concentrations. We measured total phosphorus (TP) concentration by triplicate using the molybdenum blue method (Murphy and Riley, 1962) after digestion with a mixture of potassium persulphate and boric acid at 120 °C for 30 min (APHA, 1992). We determined chlorophyll-a concentration by extracting the pigments from filters with 95% methanol in the dark at 4 °C for 24 h (APHA, 1992). We measured chlorophyll-a (Chl-a) absorption using a Perkin Elmer UV-Lambda 40 spectrophotometer at the wavelength of 665 nm and for scattering correction at 750 nm. . To obtain the integrated mean of chlorophyll-a (μg Chl-a /l), from the discrete points along the water column, we used the trapezoidal rule (León-Palmero et al., 2019). To obtain the cumulative chlorophyll-a concentration in the whole water column (mg Chl-a/m2), we summed the concentration of chlorophyll-a from each stratum using the trapezoidal rule, as we did for the integrated chlorophyll-a before, but we omitted the division between the maximum depth. We determined the abundances of cyanobacteria and photosynthetic picoeukaryotes using flow cytometry (FACScalibur) using unfiltered water.