Data set_Phytoplankton pigment dynamics in marine lake fluctuating between stratified and holomictic euxinic conditions

Study site: Zmajevo oko - Rogoznica Lake small karstified sea lake on the eastern Adriatic coast of Croatia (43o32" N, 15o58" E) Physico-chemical parameters: Temperature (T, oC), salinity (S), dissolved oxygen (DO, mg/L) were measured in situ during sampling using a HQ40d multimeter probe (Hach Lange, Germany). For the determination of photosynthetic pigments, one liter of water was filtered at low vacuum onto glass fiber filters (0.7 μm GF/F) and stored at -80 oC until analysis. The filters were extracted by sonication in 4 mL of cold 90% acetone, followed by centrifugation to clarify the extracts. Chlorophylls and carotenoids were analyzed by reversed-phase HPLC according to Barlow et al. (1997). In brief, the extracts were mixed with 1 M ammonium acetate (1:1; v/v) and injected into an HPLC system, which consisted of a gradient solvent delivery system (Varian Star 9010), an injector (Rheodyne, model 7125), a Hypersil™ MOS -2 C8 HPLC column (150 x 4.6 mm, Thermo Scientific) and serially coupled spectrophotometric and spectrofluorimetric detectors. A binary linear gradient was used to separate the pigments. Solvent A was a mixture of methanol / 1 M ammonium acetate (70/30), while solvent B was methanol. Chlorophylls and carotenoids were detected by absorbance at 440 nm (Spectra Physics UV 2000), while phaeopigments were detected by fluorescence (Spectra Physics FL 3000) with excitation at 420 nm and emission at 672 nm. Data was acquired and processed using Varian Star 4.0 software. The qualitative and quantitative analyzes of the individual pigments were performed by external standard calibration with authentic pigment standards (VKI, Denmark). The total reduced sulfur species (RSStotal) were analyzed by electrochemical methods as previously described (Bura-Nakić et al. 2009; Marguš et al. 2015). The DOC and POC were analyzed using a TOC-Vcph analyzer (Schimadzu, Japan). The fractions were separated immediately after sampling by filtration through a pre-combusted 0.7 μm GF/F filter and stored until analysis (at -20°C). Samples for nutrient measurements were stored at -20 oC prior to analysis. Nitrate (NO3-), nitrite (NO2-), ammonium (NH4+), orthophosphate (PO43-) and silicate (SiO44-) were analyzed in unfiltered samples using a spectrophotometer (Strickland and Parsons, 1972).